The Stovebolt Site Forums The Shop Area Engine Shop Another YF question

I may have missed it somewhere in the last discussion of the YF, but here is another. I have have four on my bench right now. On one of my two 964S carbs to be used on a 216, I cannot get it to idle down to the 450 to 500 RPM range. Runs well eat 6 - 800 but that is as low as it will go. Accelerator pump system works well, have idle mixture functioning, runs smooth but won't idle down. I checked the mechanicals all seem to be assembled correctly.

Would this be a main jet and rod adjustment or the wrong size opening on the jet?

More likely, you've got a vacuum leak somewhere. Does the engine surge , stall, or just quit running when the idle speed is lowered? Is there a hole in the throttle plate? Sometimes it's necessary to assure that the edge of the throttle plate contacts the bore above the port with the mixture needle in it, and doesn't expose the "part throttle'" port that isn't adjustable. Also check to be sure the ignition timing isn't too late- - - -that will make it necessary to raise the idle speed to get a little centrifugal advance dialed in. Try closing the choke plate a little to see if you can drop the idle speed without stalling when the mixture is richened up a bit.
Jerry

Thanks. I will go after the vacuum leak possibility first. I noticed a seep around the gasket. Probably going to have to enlarge and tap out the screws holding the top of the carb down. It doesn't appear to surge and doesn't stall out. I have turned the idle adjustment screw out so that it doesn't even touch the carb body and throttle linkage. After learning of the correct float drop setting it runs great at the higher RPM.

"Idle mixture functioning."
Are you saying that as you turn the idle mixture screw all the way clockwise the RPM goes down until the engine wants to die?
Are also saying that as you turn the idle mixture screw all the way counter clockwise the RPM also goes down and the engine stumbles?
And that you then adjust the idle mixture screw until you reach the highest RPM?
The reason I ask is that when there is a vacuum leak large enough to prevent lowering the idle speed with the the idle screw (as you have indicated), one generally finds that one does not have the ability to get much to happen to the RPM by adjusting the idle mixture screw.
Do you have a throttle cable connected to the carb? Is the throttle cable stop preventing the throttle plate from closing all the way with the idle screw all the way out?
If that is not the problem, I would check to see if there is any daylight between the throttle plate and the throttle bore with the idle screw all the way out. You may need to take the carb off the engine and shine a light from the bottom of the carb. This can be corrected by loosening the throttle plate screws slightly, allowing the throttle plate to self center, then tighten the screws.

Some throttle plates have a hole drilled in them to allow some air to pass even with the throttle closed all the way. If you're sure the blade is aligned properly and there are no vacuum leaks, try plugging the hole (if it's there) with a pop rivet. That will restrict the airflow so you'll need to open the blade slightly with the idle speed screw.
Jerry

Carl,

The idle mixture screw does cause some change in RPM, but not a great deal. I have the cables right. I will check all this on the other 964 I am working on and switch this one out sometime today. I will also look for a hole in the throttle plate.

Thanks

You may want to consider using carb or brake cleaner to spray around the base of the carb while running.

Gas from an unlit Propane or acetylene torch is more controllable and not as messy when looking for a vacuum leak.
Jerry

Thanks Jerry. Don’t know where I’ve been, but I don’t remember ever hearing that. Definitely will do that from now.
Victor

Any flammable liquid or gas can be used to pinpoint the location of a vacuum leak. Most shops have either propane or acetylene handy. Even natural gas would work if it's used carefully.
Jerry

I like to get the unlit propane torch near the top of the carburetor to train my ear what to listen for before I start looking for the leak. Gives me a bit of a confidence booster.

I will get back on this tomorrow. Thanks all. More to come