Hello. My son and I are fixing a father son project but we are both stumped on this one. So what’s it’s doing is… can be hard starting at times then the next time it will idle for a minute or two then all of a sudden will start idling out of control meaning it will rev high. Has backfired a few times but seems not to be doing that anymore? It will not absolutely idle unless the screw is near fully turned in. Plug is sooty black but it acts like it’s running lean???

what we have checked cleaned and replaced
compression 115
leak down tested valves and have very minimal loss less than 5%
new spark plug
cleaned the carb 3 times with ultra sonic cleaner rechecked jets every time. (Carb was clean when it was originally taken off) air screw is at 2 turns out jetting is stock. Choke seems to be holding.
replaced need and seat with genuine Yamaha and test it for vacuum
replaced carb orings
checked float level at 22mm
replaced carb and intake o rings and sprayed intake for leaks
checked valve lash
checked timing that was good
ohmed out staror was within specs
I’ve been test running it off a hanging test gravity bottle.

sorry for the long message just wanted to list all I could. ANY other help/ideas would be awesome 👍thanks

 

There can be a few reasons for a sooty plug, not getting enough air through carb, air cleaner fault. Too much fuel, faulty jet or needle setting, float level wrong or stuck open choke as it is an excess fuel type choke. Finally it can be a misfire due to faulty ignition. Remove air filter to eliminate that. Check choke is working properly, check carb is rebuilt properly I have known item19 on this picture https://www.yamahapartshouse.com/oem...810/carburetor to be put in upside down, throws everything off. Jets and needle should be OK if it was working OK with them before the problem. Float level can be checked according to the manual, setting seems surprisingly high on Yams if I remember right. If nothing cures it, the cheapo Chinese carb substitute is a way of eliminating carb problems. I have however, found the ones for Yams can provide problems of their own. Tracing an intermittent ignition fault is usually a case of substitution and that gets very expensive, so last resort stuff.

In passing, I once got a Timberwolf that hadn't run for years, opened carb and liquid inside was bright green, cleaned float bowl and jets, noticed the idle mixture screw was missing and, from gunge in threads, had been for some time. Thought it is never going to idle but may just start, so fired it up and it ran and idled fine with just a hole where adjuster should be. On the "if it aint broke don't fix it" principle, I left it like that.