Need some advice please. This is a 1987 Fourtrax 350 4x4 that spits gas/air back through the carb at wide open throttle. I noticed this when the air cleaner was off and saw the drops of gas on the rear plastic. It seems to run fine except WOT and there it runs rough and has the spitting problem. I checked the intake valve lash, thinking it was too tight, but it was ok. Could the timing chain be that loose or maybe jumped a tooth on the cam sprocket? I would think that the lash check would have showed me if the cam timing was off. Any suggestions?

 

Ok, checked cylinder leak down and it was good. That makes me think the valves and seats are good. Cam timing is dead on and chain is tight. Ignition timing is on the marks but still getting blow back through carb at high RPM. Could this be weak intake valve springs allowing the valves to float? Anyone know the pressure the valve springs should have and how to check it? This problem only exists at high RPM, low RPM runs great.

 

That could be the problem, valve float. A Clymer should give you a dimension for the length of the spring in the relaxed state. Unfortunately, you will need to remove a spring to get this. But, you can do this without removing the head. Remove the valve cover, put the piston at TDC and stuff a bunch of string into the cylinder through the spark plug hole. The other method is to pressurize the cylinder with compressed air. Then you should be able to compress the spring and take the retainer off and then take the spring off. You should only have to do one. If it's short, the spring needs to be replaced and chances are all the springs will be short.

 

Checked springs last night, thats was not it. Been through carb 3 times and found nothing. Do you think spark could be the problem? Coil checks right across terminals and with plug cap off but way too high with cap on. Reading is supposed to be 75 - 10.5k ohms but is reading a little over 300k ohms. Think this could be the issue?

 

That's over my head. I can tell you that I had a problem with spitting back through the carb but that was on start up and idle, not at WOT like yours. I worked & worked on that thing. In the end, even though I was getting spark, it was the plug. It would spark in the open so I thought it was good. But it was very weak under compression. Someone told me "before you go any further, spend $3 and get a new plug." Well I'll be if that wasn't the problem. I never knew that a plug can be bad even if it sparks in the open. I learned a very valuable and money saving lesson.

 

Yeah, new plug was the first thing I did but no luck. I think I am going to change the coil and see if that helps. It will be much easier than taking valves springs out. LOL