Engine RPM sticking at 2000 rpm

Greyhounds_AMX wrote: Here's what we're dealing with:

 

Maybe it's just me, but it looks like you might have the carb return spring connected on the wrong place, i'd expect it to be connected as low and near to the carb body as possible.



Edited by 72AMX - Jul/22/2018 at 1:22pm

53w wrote: I 2nd that. The spring looks to be pulling throttle open? .i not a holly guy but could be a good catch.

Ok, I'm back at it. Hopefully I'll get to the bottom of this today.

a) "You also mentioned when it is doing the 2000 rpm thing there is slack in the throttle cable. Okay, with the engine running and it's doing its 2000 rpm thing can you physically grab the carburetor arm and close it so the engine will idle?"

I shut it down when it was stuck at 2000 rpm and found that both primaries and secondaries were already fully closed. With that in mind the air would have to come from somewhere else, like PCV or a gasket or a crack.

b) "If a quick blip of the throttle brings the engine back down to idle, somethings is stuck or sticking on the carb. I can't think of anything else other than a carb issue that would cause the situation you are experiencing, take another look at the choke & fast idle cam or better yet put or borrow another carb on there and rule it out."

I'll put my 600 back together and give that a try.

c) "Don't see much tension in the throttle return spring at idle. Coils look mostly together.   Also doesn't always work the best with the hook in the hole where you have it. I've got a swivel bracket on mine and more tension in the springs at idle, but lighter springs so not too much with throttle open (dual springs). Quick test would be a bit more tenstion in return spring at idle."

I'll add another spring just to be sure. It seems to close fine with just the shaft spring on the Holley until I add the extra drag of the cable and pedal, so it seems that's all that the external spring should need to overcome.

d) "Sound like a vacuum leak, probably the throttle plate. You said it has a new shaft. spray something around the shaft and see how bad it leaks"

Will do.

e) "Maybe it's just me, but it looks like you might have the carb return spring connected on the wrong place, i'd expect it to be connected as low and near to the carb body as possible."

I've got it connected at the lowest point on the throttle arm, furthest from the shaft centerline. That way it'll apply the most torque possible at idle. I like to use the large hole in the Holley arm because it allows the spring to slip a little as the throttle arm rotates and prevents the end from binding and getting stressed.

d) "I 2nd that. The spring looks to be pulling throttle open? .i not a holly guy but could be a good catch."

On this carb when the throttle cable pulls the throttle arm rotates clockwise, so the spring will get stretched as that happens, and pull back counterclockwise trying to close the throttle.