YT6800 surging "pulsating" RPM

[headcase]

Cannot believe I just found this site.Ive had this machine since new.Love Yamaha equipment.This lawn tractor has developed this surging thing over the last two weeks.Runs fine when cold.Which is only a few minutes.When blades are engaged it's OK.As soon as blades disengage the rpm ramps up and down like crazy.Can hold the linkge and it smooths out.Can also deliver small amount of Wd40 into intake and it smooths out.Seems carb related for sure.Been thru carb twice and it does not appear to have air leak.Any thoughts please.Thanks

[rattosh51]

headcase,
 Welcome to the forum!!
This is the only place you'll find any info on these tractors. You being an original-from-new
owner will be a valuable addition. Most of us have just come across these used.
Since you've been through the carb, I assume cleaned and checked float, etc., the main things
that cause hunting/surging are the main fuel jet or pilot jet being clogged or bad somehow, the governor,
or a restricted fuel line or gas cap vent. Holding the linkage to smooth it out seems odd, maybe a loose
throttle cable, or one that needs some adjustment? I know the springs for the governor and throttle are
pretty fine, and after 20 years, you may want to replace them. I have a both in a package waiting to go on this weekend myself. May I also suggest reading through the older posts, as there is a lot of info you can get from other's problems. The other thing you may want to replace is the fuel lines, as they may be getting brittle and sending crap into the carb all the time. You may want to try spraying around the carb or head gasket when running, to make sure there is no leaks around them too. Let us know how you make out!

[rattosh51]

Never hurts to add some gas additive, like Seafoam, to the tank and run it through...

[headcase]

Been thru the carb.I'll take another lap thru it just to be on the safe side.Iv'e tried the spray around the carb thing that's when I discovered that a light mist in front of the carb smoothed things out.Which confirmed a carb problem.I thought I might try adjusting the valves again since it runs fine when cold.Also might add again that when there is a load on the engine it seems to straighten out.Whether it's just by engaging the blades or with the blades disengaged and you are just riding it around when, you start up a grade it smooths out.Thanks for the feedback.I'll grab my manual and make sure of all other related issues.I probally shouldn't say this for fear of murphy's law but It is only on it's third battery and has all the original belts.I treat this thing very well and has proved once again when Yamaha puts their minds together you can't own better equipment.Thanks again.Keep thinking this thru and keep the idea's coming.I will let you know what I came up with.

[rattosh51]

Can I ask what gap you adjust the valves to? I haven't done this, as I haven't ever come across a spec for it. Ditto on the Yamaha quality!! Sure seems to be something with fuel delivery...under load = more fuel being delivered? How about the fuel filter?
Thanks

[headcase]

Intake and Exhaust have the same spec  0.0031-0.0047 at TDC of each cyl mark.New fuel filter last spring.Keep thinking.Thanks