3F Carb intermittent high idle

[g-man]

At this point it may be best to swap out with a good carb just to determine if the issue is within the carb or outside the carb.

when you say "loose linkage" I mean is there axial play? or torsional play? Does the return spring bring the linkage back and the primary throttle plate back when you let off the accelerator linkage at the back of the carb? Look down the bore and actuate the linkage with your hands at the back of the carb and watch the primary throttle valve in the bottom of the carb to see if it's returning all the way when you let go. Make sure your choke plate isn't loose and moving around independent of the choke cable. Make sure your secondary throttle valve is seated.

 

[g-man]

When you check your linkage in relation to the throttle plate position, move and release it slowly and first then try and get it to return quickly. See if moving in back slowly does not seat the valve and quickly does. When you say you use the gas pedal to get it move the idle back, I assume you are doing this quickly, revving the motor quickly for a second. Take a long screwdriver and check the tension on both throttle valves when they are closed and see how easily they move. If you want to move the secondary throttle plate you will need to move the throttle linkage forward a bit at the back of the carb. Do this to unlock the secondary at the cam on the linkage. Once it is unlocked the screwdriver can push it open the rest of the way w/out holding the linkage and should return snuggly if the secondary diaphragm is good.

Also, I'm thinking about the diaphragm in your vacuum advance on the EDIT: dizzy. If it were worn or the linkages/return springs inside the dizzy were failing or gummed up or trash in there then your timing would not be retarding at idle causing a high idle condition. I guess you could rule this out by simply removing the vacuum line from the dizzy (primary) and plugging the vacuum line, then try to get the idle to act up again.

 

[Cruisunder]

At this point it may be best to swap out with a good carb just to determine if the issue is within the carb or outside the carb.

when you say "loose linkage" I mean is there axial play? or torsional play? Does the return spring bring the linkage back and the primary throttle plate back when you let off the accelerator linkage at the back of the carb? Look down the bore and actuate the linkage with your hands at the back of the carb and watch the primary throttle valve in the bottom of the carb to see if it's returning all the way when you let go. Make sure your choke plate isn't loose and moving around independent of the choke cable. Make sure your secondary throttle valve is seated.

I’m going to provide some photos here to help the discussion, one in engine before, rest after overhaul.

You can see there is some slop in the choke breaker linkage but again can’t see that’s related to the issue.

Valve seating looks good to me but I don’t have much reference so may be wrong.

I’ll have a closer look again this weekend.

 

[Cruisunder]

Checked movement of throttle plates, linksges and springs, again. Double-checked settings on carb as per FSM. The bush with the lever at the base of the choke linkage has about 1mm play on the primary throttle shaft, see photo.

It catches a bit as well. If the primary throttle plate is opened and then catches on this bush, I can see how this could keep the plate from closing when getting the foot off the pedal, explaining the high idle. Not sure how to prove this further though. After some good lubrication I had a perfect test drive, good return to 650-700rpm idle. Hope it can be managed this way but it’s not a solution of the problem.

Planning an LS swap further down the track, does anyone have experience with the cheap aftermarket carbs being sold on ebay? They all seem to be of the type that has a “throttle position” linkage which I think is the A/c kick-up? My carb has a separate diaphragm for that but the aftermarket ones are what the FSM refers to as the Arabian model? Thinking if it fits it could be a bridging solution until the swap.