Idle speed too high

cactusjacktx · Mar 28, 2015

I just installed a carb kit on the Marvel Schebler carburetor on my TO20. I can't get the idle speed down enough. The wingscrew spring has no tension left when the rpm gets close to sounding slow enough and I think the screw would work itself out from vibration. Any ideas?

Tony in Mass. · Mar 28, 2015

The main jet, the throttle linkage, the timing, hey you can spend all day tomorrow playing with different do dads!! we all been there. Good luck...

Steve@Advance · Mar 28, 2015

Several factors can cause high idle. A few things though:

Did it idle too fast before the carb kit? Was the throttle plate removed? Anything about the throttle shaft, linkage, arms, etc. disassembled or changed?

Possibly there is a mechanical problem with the throttle plate or shaft not letting it close properly. The governor linkage provides the closing force at idle, if it is adjusted too short, it will run out of travel before closing the throttle.

Are there any other issues, like missing or rough idle?

Could be a vacuum leak.

If all else checks out, and there are enough threads left engaged when the idle is right, just shim the spring with some small washers.

cactusjacktx · Mar 29, 2015

(reply to post at 20:21:36 03/28/15ote] I did not have a problem with the idle speed before. I did remove the throttle plate in order to install a new shaft that came w/ the kit but was very careful to orient it back correctly. The new shaft seemed to fit snuggly enough but I wonder about what might have been a seal supplied that I couldn't figure out how to make it work into the carburetor body. Could that create a vacuum leak you mentioned? I didn't change anything else but I'll have to study up on what the "governor linkage" is, but that sounds like what is happening. I was just thrilled that it even started after installing the kit! Never attempted anything like this. Thank you Steve, you have been most helpful!

Jason S. · Mar 29, 2015

(quoted from post at 13:10:04 03/29/15)

(reply to post at 20:21:36 03/28/15ote] I did not have a problem with the idle speed before. I did remove the throttle plate in order to install a new shaft that came w/ the kit but was very careful to orient it back correctly. The new shaft seemed to fit snuggly enough but I wonder about what might have been a seal supplied that I couldn't figure out how to make it work into the carburetor body. Could that create a vacuum leak you mentioned? I didn't change anything else but I'll have to study up on what the "governor linkage" is, but that sounds like what is happening. I was just thrilled that it even started after installing the kit! Never attempted anything like this. Thank you Steve, you have been most helpful!

Click to expand...

You may want to readjust the governor. John(uk) always recommended adjusting the governor if the carb had been removed. He always said removing the carb would change the governor settings. In your case you installed a new throttle shaft. That will most definately change the governor settings.

Steve@Advance · Mar 29, 2015

The seal that came with the kit may or may not have been used in your carb. If there was no place for it,
and there wasn't a seal before taking it apart, it probably wasn't used. Unless there is significant wear
in the housing where the shaft goes through, that wouldn't cause enough vacuum leak to be concerned with.

Most likely there was a slight difference in the new shaft assembly causing the problem. When reassembling
the shaft/throttle plate, the plate must be in the fully closed position when the screws are tightened.
This centers the plate in the bore. If it's not centered, the plate will bind and not let the throttle
fully close. The shaft/plate assembly must move freely from fully closed to full open without any binding.

The governor linkage is the link that attaches the throttle shaft to the governor assy. To check it, start
the engine, set the speed control hand lever to idle. Locate the governor link, hand push it to slightly
open the throttle, and release it. The engine should quickly return to idle and the linkage should hold
tension against the throttle arm, holding it against the idle stop screw. If there is slack in the linkage,
(engine still running), adjust the linkage length so it holds the throttle closed.

Grandfatherjim · Mar 30, 2015

Recently did a M-S carb rebuild and had the exact same problem. If you still have the old throttle shaft, compare it closely with the new. One mine, the right angle bend at the adjuster screw is in the wrong place, and puts the screw farther away from the stop. I took the screw out, bent that tab back even with the rest, and then re-bent it so the tab was longer (and closer to the stop). I cut off what was now the excess, right where the original screw hole was, and then drilled and tapped a new hole where the original bend was, and re-inserted the screw. I was lucky; the screw is now much closer to the stop; it worked.
At first glance it doesn't look like the old and new are much different but when you really stare you can see it.
Now I just have to get the governor story straightened out. Haven't had much time to deal with it but will need to soon.
Next time I was back at the MF dealer I mentioned this and he said that nobody else had complained, but I can't see how mine was a one-off error.
It would be nice if they made stuff good-as-new anymore.

Idle speed too high

cactusjacktx

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Tony in Mass.

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Steve@Advance

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cactusjacktx

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Jason S.

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Steve@Advance

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Grandfatherjim

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