Motorcycle Questions? Ask a Mechanic.
Hi and thank you for your question.
I am happy to help you and symptoms like your ATV has can be frustrating and confusing to diagnose. Basedx on that, we want to make sure that we are not violating the cardinal rule of not assuming anything when it comes to diagnosis. If we can't prove it, we don't really know. My first suggestion is to put a compression tester on the engine and get definitive proof of what your compression is. If it truly is in spec on both cylinders, we can move forward from there.When I cannot see and feel and touch the vehicle, I depend on you to assist with some of the diagnostic work to facilitate the diagnosis and repair. If your engine's compression is in spec, the next thing we need to do is make sure that the spark plug is firing under compression. An easy way to check for spark under compression is to attach an automotive style inductive timing light attached to the spark plug high tension lead, and the spark plug installed in the cylinder head. and test for presence of spark by observing the flashing light and making sure it continues to flash until you stop cranking the engine. If it is not flashing when installed in the cylinder head and under compression, try a new spark plug. It is the highest probability component to fail in the ignition system and one of the easier components to exchange/replace. If you do not see spark, throw a new plug in, NOT A CLEANED REPLACEMENT PLUG but a bran new spark plug and not just a recently cleaned of spark plug.
Check that out and we can go from there,
That is exactly where I was going with my diagnostics. A fouled plug will fire when it is not under compression but will not fire when installed in the head because of the compression. It is great that you replaced the plugs and got it going, however, I suggest keeping a close eye on the new plugs to make sure the plugs weren't a result/symptom of the problem and not the root cause of it. Because it is fuel injected, it should not be fouling spark plugs. Carbureted machines are much more susceptible to plug fouling that fuel injection. Whenever I come across a FI unit with fouled plugs, I check it very closely to make sure the problem is actually resolved and I have not just put a band aid on it.