Problem Prevention

RC Boat Problem Prevention 

This page summarizes tips, information and tests to prevent the occurrence of problems in R/C Model Boats or to correct them when inevitable.

Engine

Engine stalls or doesn't start

Fuel:

·             No gas draw even choking the carb.

·             Dirty screen filter inside carb. Remove the cover and clean the filter.

·             Tank pickup or fuel lines clogged. Clean.

·             Tank pickup loose from the fuel line. Fix.

·             Leaky fuel lines that suck in air. Replace.

·             Tank ventilation blocked. Motor stalls after 1 or 2 runs. Even when the primer bulb is able to pull gas from the tank, the carburetor fuel pump will not be strong enough to work with a blocked vent tube. Eventually, if the pump overcome this resistance, the tank collapses. Clean the ventilation tube.

·             Check if the pressure hole on the intake port is aligned with the manifold and gasket; this hole is responsible for transfer the crankcase pulses to the carb, to operate the gas pump.

·             Defective diaphragm. Replace

·             Stuck regulator needle/seat. Clean.

·             Plugged internal fuel passage. Clean with compressed air.

·             Air in the fuel lines (bubbles in the system)

·              Check the gas lines looking for cracks or loose fittings. If necessary, change them. If the fuel system has a quick fueling valve, check for proper operation.

·               Test the system without the valve. Fix or replace the valve, if it is defective.

·               Disassemble and check the carb gaskets. Replace if necessary.

Poor fuel delivery while running. Leans out after a while

·             Check all of the above first.

·             Old age - can have a leaky diaphragm in the carb or the needle is worn out.

·             Fuel flow can be surging. Rebuild carb.

·             Not properly vented tank

Water in the gas tank

·             The water will be a lighter colored bubble at the bottom of the tank. Remove the tank, turn upside down & squeeze out the water through the vent tube. NOTE: Draining and refilling the tank may not remove all water.

Too lean carb 

·             Open H and L needles in small increments.

Carb breathing water at turns or by the rooster tail of other boat

·             Protect the carb

From the Walbro site, technical tips:

·             Pressure testing a Walbro carburetor is an excellent way to determine if there is a possible leak in the inlet needle and seat area. Walbro diaphragm carburetors have a pop off pressure range between 10 - 16 psi. The range is due to variations in the metering spring, inlet needle, and seat combination. The reseat pressure should be no lower than 5 psi. The easiest way to pressure test a Walbro carburetor is while the carburetor is still on the engine. Simply remove the fuel supply line and attach the Walbro pressure tester onto the carburetor inlet fitting. Walbro's pressure tester part numbers are - 57-11 (0 - 60 psi) and 57-21 (0 - 15 psi) for float feed carburetors.

·             The height of the metering lever on a diaphragm carburetor is very critical. A metering lever set too low may create an over rich condition. While a metering lever set too high may create a lean condition. The correct procedure to set a metering lever is to use a Walbro metering lever gauge (part number 500-13). Apply light pressure with a small screwdriver to the head of the inlet needle to keep it stable. Lift or lower the opposite end of the metering lever, run the 500-13 gauge across the lever so that the lever moves slightly, but does not restrict the gauge from moving across it. Each Walbro carburetor is clearly stamped on the metering lever gauge.

·             If your engine has sluggish acceleration, is difficult to start and seems to have low wide open throttle speed, check the exhaust port for carbon deposits. Also, check the spark arrestor screen for carbon build-up. This is a very common problem with engines that have a large number of hours of use.

·             Dirt is the most common cause of carburetor problems. When servicing your Walbro carburetor, be sure to check the inlet needle tip area as well as the low and high-speed adjustment needle tips for dirt. Compressed air is the best method to clean those areas without leaving fibers from commonly used items such as a cotton swab and pipe cleaner.

·             The best way to assure your Walbro carburetor will perform acceptably next season is to run the unit completely out of fuel. This will help to assure that the rubber components are not harmed by sour gas or that deposits don't form in critical fuel passages of the carburetor.

Electric

Kill switch grounded

·             Disconnect the wires from the kill switch and test.

Kill wire touching ground

·             Isolate

·             Fail safe

·             If you are using a 4 cells pack, try a 5 cells one. On 4 cells and big servos, the load at the servos may be too much and low voltage at the fail safe can trigger it.

·             Disconnect the fail safe and test.

Coils

·             Disconnect the wire between the low voltage coil and the ignition coil. Connect a 6v low amperage lamp at this wire (low voltage side). The lamp's other pole should be connected to an engine ground. Pull the starter a few times, the lamp should glow. If not, the low voltage coil is the culprit, otherwise, look for trouble at the high voltage and connections

·             Check the correct gap between the coil and the flywheel

·             In an engine with 2 coils (primary and secondary) check the correct connection between them.

·             Check if the coils have a good path to ground

·             Check the spark produced by the coil.

Spark plug

·             Try a new spark plug

Poor off idle to WOT transition

·             Low side needle to far in. Back out 1/8 of a turn each time and try again.

·             Blocked low needle circuit inside carb. Clean and rebuild carb. Clean with carb cleaner - Not gasoline!

High side blocked. 

·             Same procedure as above.

A tip from Jim Nissen:

Transition problems typically indicate that the low end needle is not out enough. I typically will loosen the low 1/8 of a turn per adjustment. For each 2 adjustments (1/4 turn out) on the low end you have to compensate and turn the high side in about 1/8 turn.

So if your in the ballpark - out on the low - in on the high. This will help balance the transition for your setup so that it will not stumble. 

Poor WOT performance

·             Check ignition system: spark plug and ignition coils. Check spark and ignition timing. Correct as necessary.

·             High side needle to far out . Check plug for color. Should not be dark or sooty looking. Close needle in 1/8 increments each time.

·             High side needle to far in. Boat will start to run well then dies suddenly. Open needle 1/8 turn each

Excessive vibration

·             Out of balance engine. Check specially the flywheel.

·             Out of balance propeller. Check and correct as per the instructions bellow.

·             Flex cable with to much play. Use washers to reduce play.

·             Offset or bent square drive or collet. Replace

Resistor spark plugs

A gas engine uses, as in you car, a spark plug to provide the spark that ignites the air/gas mixture that runs the engine. To avoid the plug interfering with the radio system, use only resistor type plugs in your engine. They normally have an "R" on the identification number. All our boats are assembled with resistor type spark plugs. The following plugs are appropriated for our engine. Use the one you can easily find. 

·             Autolite #425

·             Champion RDJ7Y

·             NGK BMR 7A or BM6A (1 degree colder)

·             NGK BPMR6F

Gap: .015" ( 0,4 mm)

Exhaust pipe:

Leaking

Dry pipes (the ones that there is no water injection into them) use O-rings to seal against the engine. Those rings need to be protected against excessive heat e with this purpose the flange between the pipe and the engine is water cooled - exception made to Quick Draw engines, with a water cooled cylinder and no need O-ring additional cooling. Sometimes, those O-rings leak and 2 problems occur: water inside the hull and/or inside the pipe. Inside the pipe, water hampers engine performance, inside the hull may even sink the boat.. An interesting discussion on Jim’s board offered some solutions to the problem: 

·             the obvious step is checking if the O-rings are in good shape and correctly mounted on the seats grooves;

·             check for correct alignment between the pipe, pipe coupler and engine;

·             keep some flexibility at the pipe mounting;

·             use a tad of grease when mounting the O-rings

·             check the seats for roundness.

Electronics 

Batteries 

·             Check your batteries. The voltage and capacity must be within specification. If in doubt, use the biggest capacity available, especially if using a 1/4 scale servo.

·             If using a fail safe use 5 cells packs for a total voltage of 6 volts.

·             Always use rechargeable batteries. As the name implies, they can be recharged after use and, in a short time are much more cost effective. However, they are not eternal and must be well treated to offer the best performance. After use, you should discharge them completely, let them cool, charge and store the pack for next use. We advise reading your charger manual for better details.

·             But, if for any reason you are using non-rechargeable cells, fix then to their support with elastic bands, duck tape, whatever is available. Remember: engine vibration tends to loose the cells from their support. And should this happens, hope you have a fail safe with the new Battery Control device.

Antennas 

·             Never cut or coil the receiver antenna wire. It should be inserted in the antenna support in a vertical position, to allow for better reception. A 75 MHZ receiver should have an 18" (45 cm) antenna wire. This length is calibrated for best signal reception.

·             Turning the radio ON

·             Transmitter is always turned before the receiver and turned off after the receiver has been turned off.

Wires

·             Avoid long wires from the battery pack to the receiver; the resistance of the wire reduces battery performance.

Protecting the radio 

·             Keep the receiver and batteries in a plastic bag or balloon, closed with tape. Seal the radio box. Remember: water, especially salt water, is enemy of your electronics.

·             Protect the receiver from engine vibration by wrapping it with soft foam

Crystals 

·             Be sure the transmitter and receiver crystals are matched pairs.

Servos 

·             If your servos work erratically, or oscillate, check for water in the servo. Open and dry, or use the procedure outlined above.

·             If they buzz or hums at full travel, reduce the travel, if your radio has this option. Otherwise, connect the rod to the carb/rudder at the inner holes of the servo arm and/or do just the opposite at the carb/rudder end.

Repairing a wet receiver

·             Take off the receiver and servos cases.

·             Dip the electronics into isopropyl alcohol for 1 hour.

·             Blow off the alcohol with compressed air and let electronics dry for 2 - 4 hours.

·             Put a fully charged pack on the receiver and leave it turned on until the batteries discharge completely.

·             Check the receiver for corrosion or some other liquid material in or around it's internal components. If this is the case, the receiver must be replaced. Remember: it is expensive, but is not more expensive than your boat.

·             Connect the servos, a fully charged battery pack and test for operation.

·             If the receiver does not work, it should be replaced.

·             If the servos don't work or oscillate, repeat the process

·             Protect all components with anti corrosion product such as Corrosion X.

Radio range & Interferences 

AM radios are more prone to interference than FM ones. Interference may even cause loss of control of the boat and an accident with personal injuries. Besides all the measures you take to prevent interference, use a fail safe. This site has a chapter on how to install a fail safe that will give you the certainty that you did everything possible to prevent an accident.

There are 2 kinds of interference: external, caused by sources out of the control of the R/C boater, and internal, for what the main culprit is the engine. Not much can be done to external interference, except, maybe, look for another spot to practice your hobby. A lot of those interference are caused by radio transmission on the neighborhood and/or other cause out of your control.

Regarding the interference caused by your equipment, steps can and should be taken. The most usual interference and the solutions for them are: 

·             coil scraping the flywheel. The obvious signal are scratches on the rotor. Adjust the coil gap as recommended above on Motor segment.

·             loose and unprotected receiver into the radio box: Protect the receiver from engine vibration by wrapping it with soft foam, to prevent engine vibration to be transmitted to the receiver. Hold it to the radio box.

·             spark plug: the spark from the spark plug is the main source of interference on radio. Always use a resistor plug, to minimize the problem.

·             half charged batteries: batteries not fully charged, by themselves, to not cause interference but make the radio more prone to them. Always use fully charged batteries when putting your boat on water.

·             interference picked up by antenna or servos wires: keep those wires away from the coil and as short as possible.

·             engine vibrations: any metal vibrating is a potential source of noise (interference). Always use rubber mounts to fix the engine

·             carb/rudders cables and rods: whenever possible, use non metallic rods to carb and rudder or, at least, non metallic connector at the tips.

·             too long the antenna wire inside the hull: if the antenna inside the hull is too long, it will catch engine's noise more easily. Keep the segment inside the hull as short as possible. Put the receiver closer to the antenna outside.

·             try soldering a 10 microF/16V capacitor to the servo terminals, for filtering noise at the wires.

·             never use multiple wires to connect servos and battery together

·             if you are using a Fail Safe and a kill switch, keep each wire far from the other

Bellow, a tip from Jim Nissen for checking your system for vibration problems. I couldn't find the post, so I am not using his words:

Wrap the receiver on a Dremel, wire it to the servos and turn the Dremel on at maximum speed. If the servos vibrate, change crystals. If vibration continues, change the receiver. 

Check the range of your radio before putting your boat on the water: Turn on the transmitter and collapse it's antenna, turn on the receiver and start the engine - for gas boats - and move away 30 yards (15 for FM radios). Ask a friend to check the operation of the servos while you operate the transmitter with the antenna collapsed. Be sure the servos are not vibrating or making strange noises. Don't launch your boat if your radio is erratic. If you don't have enough range, check first if there is humidity at the receiver. If this is the case, dry it and use Corrosion Block or Corrosion X and test again. 

Note 1: FM radios have a shorter range
Note 2: Just collapse the antenna, don't unscrew & remove it. Some radios don't appreciate not having a load on the output transistor(s). You should get over 50 yards with a collapsed antenna. (Ron Frank tip) 

Problem even with the engine turned off. 

If there isn't enough range do the test again with the engine turned off. If the problem remains, it's eliminated the engine interference as the source of the problem. Take those actions, which presume all the above points were already verified. 

Note: when, during the test, the replacement of a component is recommended, this doesn't mean you have to take the older component from its place and put the new one there. For instance, the recommendation of replacing a serve just means: disconnect the older and connect a new one, leaving the old one in its place 

·             if using a fail safe, by pass it, just to be sure that it is not it the culprit.

·             visually check all electrical connections, from the battery on. Use a multimeter to check the voltage at the battery, at the receiver and servos. Bad or corroded connections cause voltage drop and can be responsible for the problem.

·             with a multimeter check the antenna electrical continuity. Antennas that are externally in good shape may by broke inside.

·             replace the crystal from the receiver and transmitter for another matched pair. Sometimes, crystal apparently good is damaged.

·             if only one of the servos is erratic, replace it.

·             if both are, replace the receiver.

·             replace the receiver and the transmitter by a set you know is OK.

Problem only with a running engine 

If the problem only occurs when the engine is running, check first: 

·             non resistor spark plug: substitute immediately;

·             spark plug with inside cracks;

·             drive line: flex cable into a non lined stuffing tube (although we recommend it)

·             header/pipe coupled with less than 1/8" gap (Note: don't use more than 1/4" gap, otherwise you may blow out the coupler);

·             low battery voltage, mainly if you are using a fail safe (Note: use a 5 cells pack whenever possible);.

·             pipe supports and motor mounts.

·             as a general rule, avoid any metal to metal contact, especially on control rods, it will create interference. If the carb and rudder cables are metal, use plastic links or kwik connects to isolate the metallic elements and eliminate interference.

·             unhook the wires that go to the kill switch, if the came into your radio box.

·             If using a micro switch, by pass it.

·             check if the coils have a good connection to ground.

·             check the gap between the coil and flywheel, to prevent contact.

·             if necessary, solder a 10 microF/16V capacitor on the servos terminals, to filter noise on the wires.

·             do the tests for item 1 above.

Bellow, a copy from Wayne Rathbun's post at Jim's board about this subject. 



Electric noise

The drawing bellow shows how to wire a radio box with a Shark Racing fail safe and micro switch to turn off the engine. Sometimes, it could happen that the kill switch wire introduces electrical noise into your radio. Unhook the ms, take off the wiring and test. Confirmed that the ms is the source of noise, try to twist the wires from the ms to the kill switch and/or using a coil at the wire (Jim Nissen's tip). Next step, use toroidal ferrite core on the wires as per the drawing. Another try: transfer the ms out of the radio box, away from the receiver. Last alternative, because the mechanical link to the servo becomes very complicate. As final solution, if nothing else helps, take the ms out. 


Real life cases: 

At this segment I present some questions posted at Jim's board and the tips offered as possible solutions. When the boater that asked the question informed which tip was the solution, I quote this. No information doesn't necessarily means that no tips were useful; all them are technical reasonable and on a similar case may be the ticket.

Carburetor:

Q-1: After about two WOT laps the motor warms up and starts running nice and clean. After @ 3-4 minutes the motor starts rhythmically cutting out, just like I am blipping the throttle every 3-4seconds. At first it is only a stumble, then it gets progressively worse and after about a minute the motor quits. Restarts fine on shore, carb adjustments seem to make no difference. Plug is tan/gray. Stock Homie..... Boat ran fine for 2 full tanks 3 days ago. Any ideas?
Thanks 

A-1: it may not be the carb we had the same problem and it was radio interference

A-2: Engine or ignition is getting hot. Check to see that you don't have either too small fuel line or that the fuel line is restricted any place either. Possible run lean condition. Make sure that you don't have the fuel line to close to the exhaust making the fuel hot If you have a velocity stack, remove it. 
Is it water cooled and do you have good water flow?
Richen the you-know-what out of it.
Make one change at a time...........

A-3: Sounds like a fuel supply problem, check your inline filter then your screens inside the carb itself.

A-4: Check to see if your gas tank is properly vented. 

A-5: If this is an air cooled engine it may be over heating. Try putting a little more oil in the gas and if you have the cover on the boat take it off. 

A-6: Heat or improper fuel flow (clunk, lines, screen, etc...). What oil ratio are you running? 

A-7: Check to see if there is water in the gas tank. 

A-8: If your fuel tank uses a screw-on cap, the vent is a one-way valve in the cap. It should let air in but nothing out. Try drawing on it by mouth and see if it works. The stock Homie cap has a way of getting stuck shut after a while. Also, one guy here found his coil was half-bad and was cutting out when it got hot.

A-9: Did you accidentally cover the carb pump hole in the manifold with your gasket or perhaps installed the carb upside down on the motor? If your carb isn't properly pumping fuel it will act like this. 
I also had trouble with the clunks getting plugged and restricting fuel over time.

Q-2: At the lake today my boat was doing odd things. It had great low end, but then it would almost stall and start over. The boat didn't stall so I took a couple laps around to see if it would warm up, no luck. The carb is set about 1 1/2 low end and 1 3/4 high end. It is a Zenoah with a big bore carb. I-vee bag for a gas tank. The primer line on the carb is plugged cause the I-vee bag only has one hole. That's all the info I can think of. Any body know what is wrong. I'll try it again tomorrow and see what happens. 

A-1: If it is a big bore such as the Wt-257 set it 1 turn on Hi and 2 turns on Lo. If it is a WT-167 set it 1 1/4 turns on Hi and 1 1/2 on Lo. Check the filter screen in the carb and make sure the pulse hole is correct. 

A-2: Check the screen and set Hi at 1 1/8 turns. Lo 1 1/2 

A-3: Nothing to it but make sure you put the gaskets back on the same way, it won't run right if you flip them over. 

A-4: Don't take it all the way apart at first, just remove the pump side cover (the one with just one screw in the center) and look into the 1/4" diameter recess that has a screen at the bottom. These fill up with strange things like lint, hair, dirt, and whatever else is floating around. Flick any junk out, blow on it and put it back together. Some guys take the screen out and run without it = not recommended. 

Final: Just got back, it ran great. Thanks it was the screen I think. I cleaned it set the carb and that was it. I didn't have to touch anything at the lake. I had a blast and thanks for the help. It is a lot faster than I thought it would be. This is a cool hobby, THANKS!! 

Q-3: I have a 25cc Homelite, only mods are carb and muffler.
It starts on the first or second pull and runs like a champ.
My problem happened twice this weekend, I run for 10-15 min. and brought it in for a strut adjustment. It starts back up on the first pull, but I cannot get a transition to high speed without stalling. (tried many needle settings) When I finally get in revving, I toss it in and it stalls.
Its an open cowl, so I thought air-cooling would be fine, but now I'm working on a water jacket.
Do you guys think heat is the problem?
Thanks for any info. 

A-1: What really made my engine act weird was water in the fuel. The slightest amount would get caught in the little screen in the carb and cause all sorts of weird problems like what you are describing. 

A-2: Is the gas in the primer bulb clear or is it milky? Milky = water in the fuel. 

A-3: Make sure you have a clear vent on the gas tank.
Also I would start over with the carb adjustments
1 1/2 turns out on the low
1 turn out on the high
If it stalls- back out the low side a bit & try it again.
Also get a HS8E Bosh plug. 

A-4: Check the diaphragm on the pump side. A bad one acts like this when hot. Note: it's not easy to identify a good one from a so-so.
Regards 

A-4: The Homelite tank cap has a one-way valve in it. It lets air in and lets nothing out. It's sometimes referred to as a duckbill valve because that's what it looks like, a tiny rubber duck's bill. Sometimes they stick and won't let air in, causing the engine to lean out and stall after a while. You can pinch the duck bill get it open, and test it by sucking on the cap (yuck - gas and oil). It would usually have a little drag inward and no flow outward. 

A-5: How much oil is in your gas? Overheating in a air cooled motor is often for lack of enough oil. You should be running at least 6oz oil to a gallon of 89 octane gas. Any less and you run the risk of severe engine damage. An air-cooled motor relies on the extra oil to seal the rings as well as COOL the engine. Less friction also help reduce heat. Let us know. 

Q-4: The motor is a Mac 22cc water cooled with Walbro 167 carb, and dual lawn mower muffle exhaust system. Motor seems to runs very hot though. Starts fine, runs great out of water...in water it bogs if I try to give too much throttle...dies down. tried x482, 280, 275 props...275 let motor rev better.. but couldn't open her up, and never got on plane. I have Sullivan gas tank with pickup, carb return and vent line...vent is fine. I only have 1 gasket and its between heat dam and carb...none between heat dam and block...so , am I sucking in air and running lean 

A-1: There should be a hole punched in the cylinder base gasket and the heat dam gasket and the carb-to-heat dam gasket. The Walbro 167's have two holes in the mounting face but only one is the pulse hole, the other is a dummy that goes nowhere. The one farthest away from the primer bulb is the real one. If you mounted the carb with the bulb facing forward and down the pump won't work. Set the needles at 1-1/2 on high and low. Should be a gasket between the cylinder and the heat dam, too. If that leaks air, it'll run poorly. 

A-2: My friend had a 22cc echo, it wouldn't pull any of the props you have tried, we went with an X670 and it liked that best, it ran well with a 270 also just not as fast. We did get it to run with the bigger props but had the same problems you are experiencing and had to keep fiddling with the needles every second run. 

Final: Made a gasket today and runs much better now...so I will take her backout for a run soon I hope...I'll try smaller props too if I keep getting the same problems, but I've got a feeling the gasket was the prob...will keep you all posted.. 
Q-5: Primer bulb not filling with gas. I have checked the gas lines, and they are ok. What is the next step in figuring out why the primer bulb is not filling with gas? Do I need to take the carb apart? 

A-1: I would. Check the screen filter and the pump diaphragm 

A-2: check that little screen. I've had this happen, did I say check that little screen?.. oops, guess I did. 

A-3: Just a Thought! Is your tank vented? Check the (check the Vent check valve to make sure is isn't letting pressure out! 

A-4: Is it pumping no gas or is it ending up with some small amount of air in it even as it moves gas? The first is a problem, the second is not. I had the regulator needle (we would call it the float valve needle if it had a float, it's under that see-saw thingie) in my weeder get stuck and it would not move any fuel. The problem came from using leftover race gas (that contained some Blenzall castor oil). The castor didn't like to sit around that way and would make the needle stick. 

A-5: I realize this is obvious and you already probably checked it but do you have a pin hole in primer bulb? I know I know very basic but it has happened to me. 

A-6: You didn't mention if this was a first time use, or if you've used it previously and now all of a sudden it's not working...?
If you're using a non-vented "IV bag" tank, the bulb won't fill, and will get hard after a partial pump. Simply cover the carb with your finger and S-L-O-W-L-Y pull the starter cord a couple times, just until your finger is wet, and it should pop right off.
If you're using a vented tank, check the vent line along with the other guy's suggestions. 

Final: Thanks for all your reply's guys you really helped me out, The screen in the carb was dirty! 

Water pump

Q: At idle it will pump water fine but when I get the boat moving the water pump stops pumping water but at idle it will pump fine.
Sometimes it will just stop pumping for no apparent reason tried several water pumps and the same thing happens 

A-1: double check your connections;
try larger diameter tubes;
how is the pump connected? - from where is the pump taking pressure?

A-2: I was under the impression that because of the drop off in pressure at higher rpm's they do stop working. With the forward movement of the hull should keep water flowing. 

A-3: Its not uncommon for the pump to stop pumping at high rpms but at that time you should be moving and should have plenty of water being forced though it, as long as you have the pickups in the right place. As for the stopping at idle is your pulse line filling up with oil if so that will stop the pump. You will need to find a better spot for your pulse line 

A-4: Take the pump apart, there may be sand or debris stuck in the diaphragms

Engine 

Q-1: QD25 difficulty starting( 2 bearing). One of my fellow boater friend purchased a Stryker with the QD25. This is the early one with the 2 bearings. We tried and tried but the motor won't start. Carb setting was a7/8 to 1 on high and 2 to 2/14 on the low side. Checked carb, fuel line, switched plug and checked the spark for spark which is good. We switched the carb and checked to make sure that the hole is not blocked by the gasket. Darn motor ran after the carb replacement but dies when throttle is let go. Tried to dial the low side, but kept on dying.
Any QD owners out there had the same mishap with the earlier QD25? 

A-1: Sounds kinda like a crankcase air leak. I've heard of the silicone that is used instead of the cylinder base gasket blowing out, especially if you use WD-40 to prevent corrosion in the water jacket. You can pressure test the crankcase by applying low air pressure (5 psi) to the exhaust pipe's stinger with the piston at BDC and spraying Windex around to see where the bubbles form. Other possibilities - bad crankshaft seal, pulse hole between cylinder and crankcase blocked by silicone, badly worn piston ring, badly scored piston and sleeve from engine seizure, leaking head button. The head button is installed "dry" and held on with 10 tiny machine screws which have been known to break.) Just to rule it out, try a new spark plug first. And make sure there are no pinholes or splits in the fuel line, especially right where it goes onto the carb fitting. (Cut a 1/2" off to be sure?) 
Q-2A: Just picked up a 54" scarab with a Homie 30cc pro mod engine and a 167 carb.
Why won't it start? I set the needles on the carb at 1 1/4 out each. Still nothing.

A-1: Check for spark on plug (get plug out of cylinder and pull it, no spark no ignition, it won't run). If it sparks check for fuel. When you cover the carb with your finger and you pull the starter rope a few times, does it get wet? If not fuel is the problem. You might want to drop a few drops of fuel directly in the cylinder and pull it, should fire almost instantly 

A-2: f you have spark coming out, how about to the nipple of the plug (making sure plug is still good and not fouled as happens a lot with 2 cycle motors because of the oil/gas mix) Can you tell that you have compression? This is a must for this little suckers more so than a 4-cycle motor. 

A-3: The carb settings should be 1 1/2 out on the low side and 3/4 to 1 turn out on the high.

Q-2B: Well I got gas, I got spark, I got compression, but I ain't got an engine that will start, any more suggestions? 

A-4: It should do something ... unless your spark is too weak ... (wrong plug or mag is too far away and not generating enough field) 

A-5: Check the spark plug - gap should be about 0.020" and plug should be clean. Try a new one if you have one. Just to get it going, some other plugs will fit, like a J-8J, even though they have the ring gasket instead of the taper seal. If the engine is flooded, give FULL throttle and pull. If the plug is dry, it needs to be choked more. Sometimes the crankcase back plate (usually black plastic) will crack, the screws that hold it on can get loose, or the gasket can be shot. When that happens you get a crankcase air leak, which can keep it from starting. Does it have good compression? If not, a scored cylinder or bad piston or ring can be the problem. Anybody been messing with the carb mounting gasket? If so, could be a pulse hole problem. The tiny hole in the carb base needs to line up with the hole in the carb adapter ("heat dam") by way of a tiny hole in the gasket (make sense?). Hope you don't have a crankcase seal problem (rare) because the seal is in between the two main bearings. One more thing to check - that the exhaust port isn't clogged with carbon and that the muffler screen (if it has one) isn't plugged 

Q-2C: Would the fail safe have something to do with this engine not starting? 

A-6: Other fail safes just close the throttle. The Shark Racing failsafe does kill the engine, but then there'd be no spark. But you already suggested that, right? Some 2-stokes that have been seized still have a little compression but are the devil to start because they don't have enough. Likewise, sometimes a crankcase leak makes 'em hard to start but once they do start they will run - not well, but they will run. I like to pressure-test the crankcase just to get all those possibilities off the table.
BTW, another thing to look for on a Homie is a leaking cylinder-to-crankcase gasket. The screws that hold the jug onto the crankcase go up from below and when you deck the bottom of the cylinder (for compression), it leaves precious little metal for the screws to thread into. Sometimes helps to use longer screws and a nut on top.

A-7: Also, on Homies some guys take the flywheel key out and advance the flywheel a bit and just tighten the nut down without a key. If the flywheel later slips on the shaft, you still have spark but it's at the wrong time and she won't run. Considering that when you yank the rope, the starter turns the flywheel and the flywheel, thru the key, turns the crankshaft, leaving the key out seems like a risky proposition to me. 

A-8: About the fail safe:
1. if it is not a Shark Racing Fail Safe (and I believe so) the only thing he does is closing the carb in an event of faulty radio signal. The worst an out of order FS could do in this case is keeping the carb closed even when you try to accelerate - but this would be so obvious that I don't believe he didn't check this. It has no interference on the engine spark, so it could not be the culprit.
2. a Shark Racing FS would cut the spark and kill the engine in a series of events that I won't describe here. If faulty, the FS could keep the engine from starting. But in this case, he wouldn't have spark and as the owner stated it does have spark.

Others:

Q: I can't seem to empty my gas tank on my Microburst. It's a 12oz. Sullivan
It appears that the breather tube is working properly but, the engine always quits at about half a tank. All other plumbing is correct. Does it matter that the tank is lower than the carb? I checked the tubing in the tank. and it's set up right. 

A-1: If you have a copper pickup tube then make sure you have it facing down, back and to the left in the tank. If you are using a clunk, get rid of it Have you considered using an IV bag? And the height of the tank doesn't matter. 

A-2: Do you have a vent tube to equalize the pressure inside the tank? When you suck the fuel out if you don't have a vent tube it will create a vacuum and won't allow the carb to pull in the fuel. This can happen at any given fuel level, depend on the tank size. If you have to make one, make sure you use one of those little one way valves that they use on snowmobiles, small motorcycles and some weed whackers etc. on their fuel lines. I use the fiber type, not the rubber flapper type. This will allow the tank to breathe, but not allow water infiltration. 

A-3: How about the engine temperature when it quits?
Bringing the boat back to shore, the engines starts again or you must fill the tank to get it running? 

A-4: Also check for air leaks in the pickup tube, and in the fuel line & connections between the tank and the carb. Check the screen inside the pump side of the carb. Check the pump diaphragm. Make sure there isn't water in the fuel. I like to solder a short piece of the next-size-bigger brass tubing over the outside end of the pickup tube on the tank so the tygon line will get a tight seal. 

A-5; If you have the inlet stopper towards the transom, switch it around to be towards the bow of the boat. Set the tank up so the pickup is at the lowest end of the tank when the boat is on plain. Double check your vent. To add, I have always used (and still do in several boats) a filtered clunk and can suck the tank dry. 

A-6: Are you using a return line? Make sure your pick-up and return aren't backwards. 

A-7: Maybe a bad crank seal. The motor would run cold then when it got hot it sucked air went lean and quit. 

A-8: If you have to suck on the check valve to make it work, then I think you got it on backwards. That means air will only go out. The air should be able to flow IN to the tank. This doesn't sound good, because it also means that water will be able to go IN, too. How about something with a filter? 

A-9: Check for loose lines that could be sucking air around barbed inlets, clunker should be pointed toward the transom ,check vent to tank 

A-10: I had a similar problem on my nitro boat. After some head scratching I found that the pickup tube inside the tank was cracked at the bend and was allowing it to suck air from inside the tank and mix it with the fuel coming down the line. My guess is it was caused by either fatigue from vibration, or corrosive action of the nitro fuel, or both. I found the problem by using the filler pump to pull fuel out and noticed bubbles coming thru the lines with it even though the tank was half full. 

Carlos Andrade