carb idle adjust screw

Mon Jul 12, 2004 10:02 am

why in the world do I not notice any difference when I turn the idle adjustment screw in the top of the carb out all the way or in all the way the tractor rpm doesn't change? What is wrong?
I have tried this on both cubs. They both have the IH carb. one with the mag always runs the best and never have to do a thing to it. the other one always needs some fine tunning and never runs right.

What is this idle adjusting screw suppose to do? what is the correct way to set it?

Mon Jul 12, 2004 12:21 pm


I'm assuming you're referring to the idle mixture, not the idle speed. If nothing happens when you try to adjust the mixture, chances are you have some serious air leaks within the carburetor (like a warped throttle body casting), the carburetor is running too rich (internal fuel leaks or float issues) or the tiny holes for the idle circuit are plugged up. Chances are several things contribute to this problem with idle adjustment.

Rebuild both carburetors! Do yourself a favor and splurge on the IH rebuild kit. Unless you need a float or idle tube, it includes just about everything for a proper rebuild. And include the correct instructions.

Lurker Carl

Mon Jul 12, 2004 1:09 pm

I do know that the Flat isn't working properly becasue the gas will leak out the carb if I do not shut the gas off at the tank.
Maybe this is part of the problem? Some times if I play with the choke it will help keep it running. I will plan on getting an ih carb kit for it.
what is the best way to get the carb kit? is there a serial number for the carb?

Mon Jul 12, 2004 2:31 pm


There is a part number on the carburetor but you don't need it to get a carb rebuild kit since the IH carb is basically unchanged over the years. You can get the kit from any CaseIH dealer. Make sure you have the IH carb, not the Zenith! It is a simple carburetor to repair, but please be gentle with it. She doesn't hold up well to being manhandled.

HL Chauvin has some great carburetor tips at Best of H.L. Chauvin. Previous Cubfests featured carb repairs and I got fed up with the bizzare problems my Cubs were having. Here's a link to a write-up of my trials and tribulations last winter when rebuilding and repairing a trio of IH Cub carbs: Lurker Carl's Carb Fixes.

These links should answer most of your questions. Good luck!


Mon Jul 12, 2004 10:08 pm

Chad. If you can not locate a carb kit locally, try "Carter & Gruenewald". Check this site out and look for P/N 354454R92. . Hope this helps.........

Tue Jul 13, 2004 6:00 am

thanks guys for the help!

Tue Jul 13, 2004 1:05 pm

I think the first thing I am going to try is to set the float level and make sure that the gasket is not messing up the float. I know the previous owner put a kit into it befoe I bought it and I can bet it was not an IH kit so I bet the gasket is to big.
will let you know.

Tue Jul 13, 2004 6:22 pm

One additional thought, if the ineffective screw is the idle mixture screw. It will have no effect if the throttle plate isn't pretty well closed. Having the idle speed set too high will do this. If there is some other problem that requires setting the idle too high, you get this. This could be rooted in anything from a vacuum leak to ignition timing.