Compression & cylinders tests
Good compression and firing cylinders are vital for a motor because cylinders compress air and fuel before ignition. Not enough compression means not enough force to move the piston and crankshaft. No firing in the cylinder means no piston movement. This could be why your engine stops when in gear.
Test
1
Is your engine compression normal and even on all cylinders?
Normal? Two strokes recommended compression: 90-130 PSI. Four strokes recommendation: 160-220 PSI. You’ll need to do a compression test with a compression tester. It will tell you if your engine can compress fuel and air properly in the cylinder. There is a tutorial below, but in short: you’ll insert a fitting into the spark plug's threaded hole inside your engine’s cylinder head, then you’ll crank the engine, and see what the gauge on the hose says.
No
All good, next test
Your engine is not creating the right amount of compression, so the pistons are not moving properly/enough. This could be due to stuck rings or a dirty carburetor.
Your engine is not creating the right amount of compression, so the pistons are not moving properly/enough. This could be due to stuck rings or a dirty carburetor.
Need further testing
Test
2
Are each cylinders firing properly?
Warning: do not use a metal tool for this test, you could catch a spark. To do this, remove each spark plug wire individually and listen to the engine as you remove each one. If you notice a difference (a sound or a little shake), it means this plug is working properly. So do this until you find one that does not generate a sound or shake change when the wire is removed.
No
All good, next test
The cylinder you have pinpointed is not firing, meaning the explosion is not happening and therefore the piston is not moving. This could be caused by 3 different things: no spark, not enough air, or not enough fuel.
The cylinder you have pinpointed is not firing, meaning the explosion is not happening and therefore the piston is not moving. This could be caused by 3 different things: no spark, not enough air, or not enough fuel.
Spark plugs tests
A bad plug can cause your engine to stop when put in gear. The controlled explosion could be prevented because spark plugs are responsible for generating the spark inside the chamber. You can detect an ill-working plug or its wiring by doing a spark test or a color test.
Test
3
Are your plugs generating a spark?
This is done through a spark plug test using a spark plug tester, we have a 7 min tutorial below. It goes as follows. Disconnect the spark plug wire and connect that to the spark plug tester. Then, place the tester’s tip against a clean, unpainted, bare metal part of the block (ex: bolt head). Then, turn the engine over and see if there is a spark in the tester. Finally, replace the plug where it does not spark and test again to see if the missing spark has changed location (this will tell you if your issue is plug or wire related).
No
All good, next test
The spark plug that does not spark is not doing its job. This can be for a couple reasons, so we are gonna do a couple more tests.
The spark plug that does not spark is not doing its job. This can be for a couple reasons, so we are gonna do a couple more tests.
Test
4
What color are your spark plugs?
This will need to be done for each plug. Remove a plug, remembering its position, and compare it to a plug color chart. We have resources below, but generally, you are looking for signs of carbon or oil deposits, is it wet, burnt, are the electrodes worn or broken.
Whitewashed, Burned
All good, next test
Your plug is running too hot. The cause could be that the heat range is not correct for the plug or that you have an air/fuel mixture that is too lean (too little fuel).
Your plug is running too hot. The cause could be that the heat range is not correct for the plug or that you have an air/fuel mixture that is too lean (too little fuel).
Need further testing
Black (dry or oily)
All good, next test
If it's dry, it may be due to a dirty air filter or too rich of a fuel/air mixture (too much fuel). If it’s oily, it means oil could be leaking into the cylinders (through pistons or valve guides).
If it's dry, it may be due to a dirty air filter or too rich of a fuel/air mixture (too much fuel). If it’s oily, it means oil could be leaking into the cylinders (through pistons or valve guides).
Need further testing
Test
5
Are your spark plug wires fouled or corroded?
To be safe, start by disconnecting the negative battery terminal to remove power to the engine. Then, take one wire off at a time (put it back before removing another one, their exact position matters a lot), and check it. You are looking for signs of age such as cracking, bare wiring, burn marks on the insulation, or brittleness. Then, inspect the metal connectors inside the boot areas (the connecting tips) for corrosion and check if they are bent out of shape.
Yes
All good, next test
The spark plug is not properly connected and therefore does not spark properly.
The spark plug is not properly connected and therefore does not spark properly.
Need further testing
Fuel tests
It could be that your engine is stopping when put in gear because there are issues related to the quality or quality of the fuel that your motor gets. Let’s check that now. Not getting enough fuel would mean not the right ratio air-fuel, therefore improper firing.
Test
6
Is the fuel filter clean with no water or impurities?
To do this, you need to pour some gas that has been through the filter already into a clear container. Then inspect it for water or impurities, they could be the cause of your engine problems.
No
All good, next test
It can be pretty bad if something other than fuel is getting inside your engine. On top of preventing proper combustion, impurities could damage your engine.
It can be pretty bad if something other than fuel is getting inside your engine. On top of preventing proper combustion, impurities could damage your engine.
Need further testing
Test
7
Is there gas in the carburetor or injector?
To check for that, first you need to remove the cover of the carburetor or injector, then look inside. If there is no fuel, you might be preventing your engine from firing.
No
All good, next test
You might have a fuel supply issue, meaning the gas from the tank is having trouble getting in the carburetor. These issues can come from different parts of your engine.
You might have a fuel supply issue, meaning the gas from the tank is having trouble getting in the carburetor. These issues can come from different parts of your engine.
Need further testing
Test
8
Is your gas pump working properly, and is its pressure at the right level?
To check if your gas pump is working, inspect the relay, fuse, fuse holder, and harness connector pins. For the fuel pressure, you should aim for something between 4 to 7 psi.
No
All good, next test
Your fuel pump is most likely not pushing the correct amount of fuel inside the carburetor/engine. As a result, your engine cannot stay running and stops.
Your fuel pump is most likely not pushing the correct amount of fuel inside the carburetor/engine. As a result, your engine cannot stay running and stops.
Need further testing
Test
9
Is the inside of the fuel line clean?
Shine a light inside the tube while keeping it straight, or pass a sponge or cloth through the line thanks to a string. The inside of the fuel line is susceptible to deterioration, if you don't take care of it, it can cause stalling.
No
All good, next test
You are preventing the right amount of fuel from going into the engine. As a result, combustion cannot happen properly and your engine is stopping.
You are preventing the right amount of fuel from going into the engine. As a result, combustion cannot happen properly and your engine is stopping.
Need further testing
