RPM's way to high, not idle or low needles

svynx

Well-known member
Messages
160
Location
Wrightsville, pa
Motor: 29.5 FM LR
Pipe: Victory Pro
Oil: 912 at 25:1

A friend hadn't seen the truck run, so got it out while I was visiting. Let it warm up for a few minutes while talking. I did a few low speed passes on the street, and then a few high speed. Couldn't have been running it for more than 3 minutes. On the last pass, when I let off the throttle, the RPMs stayed higher than normal, high enough to keep the clutch shoes engaged. I could still brake, so I made the turn, shutting the motor off remotely about half way back.
Fast forward to a few days ago. I cleaned all the gunk off of the exterior of the carb, along with all the grass that had accumulated under and around the motor. Started the truck. Again, the RPMs were high enough to engage the clutch. Shut it down, and started taking things apart. No air leaks. Cleaned inside of carb, just to be sure. Changed spark plug to a new one. Reassembled and started again. Same thing. I quickly lowered the idle screw, but that didn't change anything. If for some reason the low needle got knocked while running, common sense would say that it would go out/richer because of the spring.
I'm at a loss here. Not sure what to check. Any ideas?
 

svynx

Well-known member
Messages
160
Location
Wrightsville, pa
Yup. At neutral throttle, the butterfly valve is in the right position. In neutral, I pushed on the external limiting arm (what the idle screw pushes against...not sure of the real term). Doesn't move.
 

MCRIPPPer

Well-known member
Messages
5,777
Location
carmel valley ca
the throttle valve screw could be loose. it holds the brass circle onto the shaft. if it comes loose, it gan get stuck and not return to the closed position when you let off the throttle.
 

svynx

Well-known member
Messages
160
Location
Wrightsville, pa
All the gaskets are in good shape. The pipe gasket is new, along with the one between the head and the carb spacer. I tightened all bolts just to be sure. Throttle valve screw is tight. I'll try starting it again after lunch.
 

svynx

Well-known member
Messages
160
Location
Wrightsville, pa
Check the isolator block for warping on a flat plate of piece of glass. They do sometimes warp an cause this.
Will do.

I reset the needles. Started properly. Let it warm up and did a little tuning...maybe 1/8 turn low needle (leaner) and 1/4 turn high needle. Still ran smoothly. Let it sit over dinner. Got it out again a little while ago. Started properly, ran properly, for about 5 minutes. Then, the problem came back. Set needles back to stock and the issue is still there. I'll check the isolator block next. I checked fuel lines to see if anything happened to be in them. Nothing. Checked fuel filter. Clean. I'm going to swap carbs with a brand new one and see if that helps.
 

dscjester

Well-known member
Messages
14,798
Location
NJ
Check the isolator block for warping on a flat plate of piece of glass. They do sometimes warp an cause this.
Agreed. The black Zen ones warp. Put it on a flat surface and watch it wobble. :scared: HaHa. Also make sure your spark plug is tight.
 

TeeKuang

Well-known member
Messages
89
I also think it must be leaking somewhere, it's either carb manifold, head gasket or probably crank oil seal.
 
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