Zenith Carb Q: High RPM

[Pilotdave]

I've read a lot of the posts and articles referenced therein on here but haven't found material that addresses my latest Zenith carb question.

I replaced the Zenith we have been running on the roadster with another Zenith. The engine, which idles very nicely with the first carb, runs at high rpm with the second - with a clear gap between the idle speed screw and the stop. With the throttle linkage attached I can't fully close the throttle plate. The throttle plate shaft on the second carb has been replaced. After removing carb #2, I noticed the the throttle plate when fully closed is positioned so that the idle port is below the edge of the plate. Seems to me that the throttle plate/arm may be no good; at least that the arm is not the right shape.

I'd love to hear the collective Barn wisdom on this - thanks much.

[BRENT in 10-uh-C]

It sounds like the throttle plate needs to be re-adjusted to close tighery. First, ensure the throttle shaft is tight (no slop or play) in the upper housing bore. If not, this is the first area that needs to be addressed (throttle shaft bored for oversized shaft or re-bushed back to proper size). Next, verify the throttle plate has the beveled edges correctly oriented where they seal against the throttle bore in the housing. If it is correct, loosen the two screws where the plate can be slid around to seal better. Then re-tighten the screws.

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[Pilotdave]

Thanks, Brent. While taking another look at the plate just now, I see some gouges on one edge. Bet this could a contributing factor. Will try another plate, adjust as you suggest, and give it another try.

[BRENT in 10-uh-C]

The way the engine speed is controlled is by shutting off the air. That is why leaks around throttle shafts affect the quality of the carb's operation.

[Pilotdave]

I checked the throttle plate/shaft as Brent suggested. The shaft is snug in the bushings. Loosening the plate screws didn't seem to move the plate at all. If I adjust the idle speed screw to expose half of the idle port, I can see a very small gap between the plate and the walls of the throat. I haven't retried this carb as I haven't changed much - if anything. I did compare the throttle shaft to another repro to check the relationship between the shaft slot and the arm - they seem identical, but that may not be meaningful as the parts likely came from the same store. I'm attaching a photo of the plate in the throat - not a great picture but maybe good enough to tell a story.

Thanks again.

[Tom Endy]

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Suggestion:

Back the throttle shaft idle screw off so that it does not touch the stop. loosen the two screws on the throttle plate. Now close the throttle several times sharply. This will center the plate. Now tighten the two screws. Screw the idle screw in until it touches the stop. Screw it in more until it starts opening the plate. In the throat of the carburetor is a small hole. Keep closing the plate so that it is in the middle of the hole.

Screw the air mix screw in until it bottoms. Back it out one and half turns.

You should be able to install the carburetor on the car and it should start up at a high idle. Back the throttle stop off and the idle should decrease as the plate closes. The idea is for the idle to be controlled by the small hole in the throat.

It is possible the air passage in the throat is plugged. Remove the idle jet and blow air into where the idle jet screws in and at the same time place your finger over the hole in the throat. If it is open you should feel a blast of air come out.

Tom Endy

[Pilotdave]

I applied all of suggestions from Tom and Brent. Put carb #2 back on and started engine. Ran at a fast idle. The linkage won't allow the shaft to rotate enough to slow down the idle, though I can force it by hand.

While inspecting original carb (#1) top, I noticed that the throttle plate was on upside down. The engine ran well that way, but I flipped the plate so the bevel, rather than a sharp edge, closes against the wall of the throat. Put carb #1 back on the engine and could tune the idle better than before.

I remain puzzled by the carb #2's behavior, though happy that #1 works even better!

[Tom Wesenberg]

You may have a bent arm on the crosslink. This is the long thin arm that the carb linkage clips to, and it can be easily bent while cold.

[Pilotdave]

I will investigate. Thanks.