Preventing and Correcting Problems

*This information posted with permission from Carlos Andrade (Shark Racing)

This chapter summarizes tips, information and tests to prevent the occurrence of problems or to correct them when inevitable.


Engine stalls or doesn't start:


1. No gas draw even choking the carb.

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

b. Tank pickup or  fuel lines clogged. Clean.

c. Tank pickup loose from the fuel line. Fix.

d. Leaky fuel lines that suck in air. Replace.

e. 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.

f. 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.

g. Defective diaphragm. Replace

h. Stuck regulator needle/seat. Clean.

i. Plugged internal fuel passage. Clean with compressed air.

2. 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.

3. Poor fuel delivery while running. Leans out after a while

a. Check all of the above first.

b. Old age - can have a leaky diaphragm in the carb or the needle is worn out. Fuel flow can be surging. Rebuild carb.

c. Not properly vented tank

4. 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.

5.  Too lean carb. 

Open H and L needles in small increments.

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

Protect the carb.

7. 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.


1. Kill switch grounded

Disconnect the wires from the kill switch and test.

2. Kill wire touching ground


3. Fail safe 

a. 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.  

b. Disconnect the fail safe and test.

4. Coils

a. 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

b. Check the correct gap between the coil and the flywheel

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

d. Check if the coils have a good path to ground

e. Check the spark produced by the coil. 

5. Spark plug

Try a new spark plug

Poor off idle to WOT transition

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

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

3. 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

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

b. 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.

c. 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.

1. Autolite #425
2. Champion RDJ7Y
3. NGK BMR 7A or BM6A (1 degree colder)

Gap: .015" ( 0,4 mm)

Exhaust pipe:


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:

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

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

3.      keep some flexibility at the pipe mounting;

4.      use a tad of grease when mounting the O-rings;

5.      check the seats for roundness.



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 � BTW what I always recommend � 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. 


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.


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



Be sure the transmitter and receiver crystals are matched pairs.



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:  

1.Take off the receiver and servos cases.

2. Dip the electronics into isopropyl alcohol for 1 hour.

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

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

5. 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.

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

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

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

9. 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: 

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

b. 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.

c. 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. 

d. 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. 

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

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

g. throttle and rudder cables and rods: if feasible, use non metal rods for throttle and rudder or, at least, non metallic connectors at the tips. 

h. too long antenna wire inside the hull: if the antenna wire inside the hull is too long, it will pick up noise from the engine more easily. Keep it as short as possible. Close the receiver to the exit point for the antenna.

i. try soldering a 10 microF/16v capacitor on servos terminal, to filter noise on the wires.

j. never use a multistrand cable to run the servos and battery wires.

l. if you are using a fail safe and a kill switch keep the wires far from each other.

Some time ago, Jim Nissen offered a tip on his board on how to check if your servos are suffering from interference problems. I couldn't find it, so I am not able to use 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)


1. 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  

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

b. 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.

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

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

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

f. if both are, replace the receiver.

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


2. Problem only with a running engine:


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


a. non resistor spark plug: substitute immediately;
b. spark plug with inside cracks;
c. 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);
e. low battery voltage, mainly if you are using a fail safe (Note: use a 5 cells pack
whenever possible);.    
f. pipe supports and motor mounts.
g. a
s 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.  
h. unhook the wires that go to the kill switch, if the came into your radio box.  
i. If using a micro switch, by pass it.  
j. check if the coils have a good connection to ground.
l. check the gap between the coil and flywheel, to prevent contact.  
m. if necessary, solder a 10 microF/16V capacitor on the servos terminals, to filter noise on the wires.
n. do the tests for item 1 above.  

Let�s see what Jim Nissen has to say about this:  

Range difficulties...  

Unhook the wires from the motor going into the kill switch. Tie them together and away from the motor. The test is to see if you have ignition noise migrating into your box from these wires. A lot of electrical ignition noise flows right up the wires and if the switch/failsafe is anywhere near your receiver you can have trouble. I layed out my box to place the failsafe on one side of the box (closest to the motor) and the receiver and associated servo wires/power are on the opposite side.

EMI (electromagnetic interference) can be a real pain the in the boat. The single largest contributor is the high voltage spark cable. You should check to make sure you magneto has good clean grounds where it attaches to the motor. Verify the magneto is not hitting the flywheel. Clean grounds are important, as the electrical energy will try to find a path to ground somehow. Lets hope it's not through the kill switch wiring!

That all said it could be you just have a bumm radio. Was the radio new or used? Dropped? Anytime you drop a transmitter you run the risk of damaging the crystal or ferrite cores in the RF section. Another reason to wrap your receivers in soft pliable foam. Good idea to have a radio expert looks it over and tests the sensitivity and tune for that channel.

Jim Nissen 


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