carb guys need help

Thu Sep 02, 2004 10:33 am

Ok I am working on my newly obtained carb.
I was working on cleaning it out and I was blowing on the idle tube part #(10) that is in the top half of the carb. I was then adjusting the idle adjusting screw part # (22) and it would not change the air flow through the idle tube (10). The Idle adjusting screw (22) looks a little rounded off and not like the others I have seen.
My question is If I turn that idle adjustment screw (22) all the way in should it cut the air flow completly off or to a slight trickle through the idle tube? Or does this control something else?

If it is the idle adjustment screw could be wrong I can stop over to the barn where I have another cub and try that one.
what do you guys think?

Sat Sep 04, 2004 12:02 pm


The idle screw will not change air flow through the idle tube by testing it the way you described. The concept is hard to put into words, but here it goes. The idle screw is a pressure valve, not a flow valve. The idle tube is like a straw which fuel is sucked up from the carb bowl, with the engine supplying suction, and the idle screw changes the amount of suction at the idle tube. However, the idle screw will not directly control flow through the idle tube because they are not connected to each other. The idle screw changes air pressure within the idle system. It does not function as a flow valve for the idle tube. That's why the air flow didn't change when you blew into the idle tube whether the idle screw was open or shut.

Your idle screw should not have any grooves around the beveled portion, grooves will not allow the 'fine tuning' necessary to adjust the idle mixture. I don't think the tip being pointed or blunt will affect anything. It wouldn't hurt to compare idle screws or see if switching them makes a difference in your Cub's idle performance. Closing the idle screw reduces the suction which reduces fuel flow to the engine, therefore the idle screw is turned in to lean the mixture (no suction at the idle tube = no fuel) and out to make it richer (lots of suction = lots of fuel).

I hope this helps and not just confuses it more.

Last edited by Lurker Carl on Sat Sep 04, 2004 12:07 pm, edited 1 time in total.

Sat Sep 04, 2004 12:07 pm


Well Done :!: :!: :!:

Can I prevail upon you to put it in Word format and send it to me as a Chapter in a Carburetors - 1,2, 3 or would it be Carbs for Dummies ??? :lol: :lol: article :!: :?:

Be kind of neat wouldn't it.. ??

Sat Sep 04, 2004 12:15 pm


Thanks. I'll add it to my "to-do" list behind the picture CDs I owe you and the governor rebuild I haven't started yet. Don't have much lurking time right now!


Sat Sep 04, 2004 12:19 pm


I hear you. I have been busy as a beaver lately too. Got the potato digger to rig up for next week, finally got that disc harrow that was at Alton's - it is gonna do fine as soon as I loose a little excess weight from it. Gotta figure out a way to bring home the McCormick 100 Manure Spreader I got from Alton as well, so Em doesn't get cranky! That should be a trick.

Working on the grader blade right now and trying to locate, identify, catalogue and package up all the parts I got hanging around in the shop! There is a job and a half. Gotta make room to get the honey-do list done before it becomes too long and she gets too uptight about it.