There should be a very visible screw to adjust the slow idle.

There is not. There is a screw to adjust the main throttle stop but at idle it doesn't even touch the stop screw. There is no idle speed adjustment on this carb. Apparently you do have to take the engine to a service center to adjust the idle speed. Well I'll never buy one of these engines again.

This is on a snowblower, right? And it has a throttle cable, right? The find adjustment for the high speed is on the throttle lever and the idle speed screw is near the throttle plate.

Can you provide any pics?

It's a replacement snow blower engine that I'm using for something else. It has no throttle cable, just a lever on the engine. I can provide some pics but not for a couple days.

this is a typical idle screw for a Tecumseh SK

{{gwi:126853}}

That's what mine looks like but for some reason at idle the stop doesn't touch the end of the screw. Wonder what could be wrong?

If you can gently move the plate to idle and it opens up again to a higher speed, you could have the governor spring messed up, or an air leak. At idle, there should be no tension on any spring. There is also a spring attached to that throttle plate underneath the plate. Does the plate return to idle with no springs or linkage attached to it?

I moved the throttle stop against the screw and the engine stopped. I restarted the engine and pushed the stop toward the screw just a little and the idle dropped but the engine kept running about the speed I want it. Under the carb there is a spring attached to a vertical bar with a bunch of holes in it. I'm thinking if I move the spring down a hole at a time there will be less tension on the spring and I'll be able to get the idle down where I want it. Is that correct?

That's the governor spring and lever and it controls high speed, but one of the effects is that it will either speed up or delay a response to a load. At idle, there will be almost no tension on that spring. I'm getting the impression that the idle circuit of your carb is not clean, but I'm not sure why you have high speed in the idle position.

Try moving the spring, one hole at a time.

Okay thanks very much for your help. I will try experimenting with the spring position.