idle screw useless

karl f · Mar 29, 2009

the idle mixture screw on our 400 gas does not affect the idle at all. it runs fine, you can drop it down to about 300 rpm if you want. However, it idles pretty smoky, smells bad too. If you try to start it at lowest throttle even when speed set correctly to about 450, it will overchoke and flood. Also, if you partially remove idle mixture screw, the tractor all of a sudden will idle fast. But, if you completely remove screw it will die.
This was a running tractor (always hard starting, rich idle, throttling up even one notch when cold would kill it). I partially disassembled the carb to check float adjustment and other things, and then synchonize governor/carb. I suppose it is a passage i didn't make time to ream out, but which one? I think it is the correct carb for the tractor, the main discharge jet is removed with a nut inside the bowl vs. a screw head on the outside.

thanks for your input.
karl f
ps i found one float set higher than the other.

Janicholson · Mar 29, 2009

A high float level will do it, as will an idle jet (where the screw goes in) that someone has plugged up. It should be richer turned in leaner turned out. Thus a thought might be that the passage that supplies air to the back side of the needle might be plugged or blocked by gasket material. JimN

Red Mist · Mar 29, 2009

Karl: Well, for one thing, don't try to start with throttle (governor) set at low idle. As per manual, open about 1/3 of way. Beyond that, sounds like you need a thorough carb rebuild which would include getting everything clean and all passages "blowed" out.
mike

karl f · Mar 29, 2009

took the bowl off again and poked with welding tip cleaners, sprayed with carb clean, blew with a hose. everything seems to let air move. the only thing left would be to remove factory plugs for even more thorough reaming. could it be the wrong screw? I am using a caseih gasket and everything seems to be lining up.

does the carb for the 400 use the throttle shaft and bushing for an economizer circuit like the e12 carbs on earlier models?
I forgot to get the carb number off the tag..

karl f

Red Mist · Mar 29, 2009

Also, if you partially remove idle mixture screw, the tractor all of a sudden will idle fast. But, if you completely remove screw it will die.

The idle mixture screw on these carbs is an air (rather than fuel) adjustment. So, if you open the screw it is acting correctly when it starts idling faster - since it is getting leaner. Similarly, if you completely remove it, it would die, since it would be completely leaned out. So, I'd say the idle screw was behaving correctly.
Are you turning the screw 1/4 turn at the time while giving it a few seconds to adjust to each 1/4 turn? That is important when fine tuning.
mike

Janicholson · Mar 30, 2009

The idle passages are in the top half (main casting). The idle discharge slots are at the edge of the throttle plate where it hits when closed. To assure they are actually clean, and open it is necessary to have it in your hand. I have found that covering the idle ports with a finger and spraying cleaner in the screw hole with a little rubber (homemade) seal on the spray tube) will push solvent in the direction the air and fuel must come from. Remember this issue is lack of air getting to the needle, not lack of fuel. Best of luck, JimN

karl f · Mar 30, 2009

thanks for all the input. The carb had been rebuilt at the dealer about 20 years ago and I figured it was done right and with low number of hours on it just needed a quick refresh/check. I'll check it out again more thoroughly next time i am able. and get the carb # too.
should i be looking for passages in the throttle shaft bushing?

karl f

D Slater · Mar 30, 2009

Off the top of my head I think the carb # for a gas is 362173 R92. They first used R91 carbs but all those were supposed to be changed over to R92 carbs. Unless you have a kerosene carb Those carbs don't have a econimizer slot or a hole in throttle shaft bushing. The idle air adjusting screw is smaller on the end than the ones for earlier carbs. If someone put one in from a rebuild kit like most sold that could be a problem. If there's another problem with engine you may have to have the throttle stop screw in farther so that butterfly is open enough to have fuel comming in from off idle fuel circut causing air adjustment screw to have less effect. Sometimes if timming is off it will make the idle air adjustment have less effect.
Unless you have a kerosene carb or other, the throat in the carb sould have a 2 holes. One were butterfly closes and a real small one oppisite air adjusting screw.

karl f · Mar 30, 2009

the idle air screw is practically in the carb flange. I think i saw 3 tiny holes. one at the throttle plate when closed, one where the tip of the idle screw goes in, and the other may have been inside the dead area where the venturi goes but maybe there was another in the throat? i'll start a new post when I am actually near the tractor again.
karl f

idle screw useless

karl f

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Red Mist

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karl f

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Red Mist

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