I bought a 646 with a vanguard 18 hp. It suddenly started surging. Has anyone had this problem. I see the governor linkage moving back and forth. I wondering if I have a bad coil or does someone know what to check oin the governor?
So, I have two 18HP B&S Vanguard engines in my 446's. I have found that running them on Ethanol gas will do exactly what you have described and more. The carbs of these engines do not do well with ethanol and will cause major carb bowl damage very quickly. Do what Bob has suggested but change to non ethanol gas too..
Ethanol also causes a lean running condition. I’ve had to change the main jet to a slightly larger size in order to get some engines to run right. We don’t have Non ethanol fuel in my area. The marine mechanic’s have recommended running a 150:1 to 100:1 gas outboard oil mix to help combat the corrosion issues from the ethanol.
This used to happen to me. Debris in the carb sucked into the jet or idle circuit. Some debris coming from the fuel line fixed by a -proper- filter; made for fuel pump delivery. Some debris was from a cheap knockoff fuel solenoid that was deteriorating into the fuel bowl. Buy genuine parts! There is a small hole in the intake body , near the butterfly and a shot of carb cleaner into that via the red tube of the cleaner can would clear it temporarily . Real fix is clean the bowl & jet and prevent debris.
Also note that some fuel lines are not ethanol compatible, and can be dissolved. Don't here it much anymore, but when back when all this started there was a lot of issues, with pugged carbs. ........Curt
A bit of advise on the Briggs and Stratton Vanguard engines, esp. the conversions. On the carb there is a bowl vent nipple (1/4") that is part of the vapor system required on newer engines. On the older tractors that run this engine that nipple is left open for proper bowl venting. The problem is it also allows debris into the float bowl which eventually cause plugging of the very tiny jet bleeds. The fix is to take a piece if 1/4" fuel line and attach one end to the nipple and the other end to a quality metal 1/4" line fuel filter leaving the other end of the fuel filter open. Even something as tiny as the "hair" strand of a grass seed will block the bleeds causing severe fueling problems.