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C Zenith carb question

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Topic: C Zenith carb question

Posted By: tbran
Subject: C Zenith carb question
Date Posted: 15 Feb 2010 at 3:38pm

Guru's, working on a restored C. Zenith carb. Will not idle when it warms up unless choke is pulled out about a 1/3d and the idle jet closed. Lean condition but cannot open up the idle jet over 1/4 round round w/o killing it at this setting. Can turn the idle bleed screw in and seat it and it runs better. More choke and it dies . At 1.5 turns out on the idle it will not lower below 850 rpms w/o dieing but as rpm is lowered and choke is pulled it out slightly it idles smooth as silk to about 750 rpm. Below 750 the idle has to be turned in. Runs great at high idle and main load works fine. Float level is ok, have shut off draincock to lower bowl level with no effect. Removed carb and rechecked the orfices in throat - all clean as are jets. Checked for air leaks with ether, no effect. Kind of puzzled. With all this knowledge.....anyu ideas

Replies:

Posted By: Jeff Z. NY
Date Posted: 15 Feb 2010 at 3:41pm

Sounds like ther is still some internal blockage in a passage way somewhere.

Send me a private message and with your regular email and I email you a service manual with good breakdown of the internal passages.

Posted By: TomMN
Date Posted: 16 Feb 2010 at 10:04am

This is what I found in a Zenith carburetor manual:

The position of the idle adjusting needle controls the suction on the idle jet and thereby the idle mixture.
Turning the needle in closer to its seat results in a greater suction with a smaller amount of air and therefore a richer mixture.
Turning the needle out away from its seat increases the amount of air and reduces the suction. and a leaner mixture is delivered.

Posted By: Dick L
Date Posted: 16 Feb 2010 at 10:16am

Have you adjusted the idle mixture in conjunction butterfly stop screw on the back?

They can not be adjusted independently, they have to be adjusted starting at a higher idle with the stop screw holding the butterfly open and then working back and fourth closing them a very little at a time. You can get to the lowest idle after the engine is warm without the choke being closed.

Posted By: Dick L
Date Posted: 16 Feb 2010 at 10:38am

I doesn't matter what make of carburetor I am adjusting, I start by running the engine at what ever slowest RPM it will run without choking. I then adjust the butterfly screw to hold that RPM and move the throttle all the way back in the slow idle position. I then adjust the idle mixture screw in slightly then move back to the butterfly screw and close it slightly to the point it almost quits. Then back to the idle mixture screw and clost it slightly. It takes me several trys back and fourth and several restarts of the engine.

It sounds like the butterfly is closed to much and your trying to over come it with the choke.

Posted By: Ken in Texas
Date Posted: 16 Feb 2010 at 10:50am

I have been messing with 6 tractors and 6 carbs for a week. The only one I haven't messed with is on my CA. It is what I would call a NICE Carburator.

Just swaping a carb from one engine to another can produce a remarkable change from how that same carb runs a engine.

I took a carb that ran perfect with the choke wide open off a smooth running C and put it on a hand crank B thinking it would be easier to start. When I finally got it started it blubbered with the choke open. It would not smooth out unless the choke was more than half closed. Put it back on the C and it ran perfect only with the choke wide open like it should. Whats going on?

Took another carb off a good running B and put it on the hand crank B and it ran perfect with the choke wide open just like it did on the B I took it from. Go figure.

Posted By: tbran
Date Posted: 16 Feb 2010 at 11:46am

found the problem. On a Zeneth the enrichment idle curcuit works as you described. The more the screw is turned in, the more vacuum air flow is allowed to flow by the idle jet tube and thus draw in more fuel to the idle slot - (thanks Jeff four your help!) The more I turned in the idle the better it ran until I bottomed out the screw. Pulling out the choke made it purr, so it was still to lean. Removed the idle brass jet tube and drilled out 2 jet sizes larger, could use 3 but it runs great now. I think the end of the jet/tube was squashed by a previous experimenter as the screws to remove the body were about worn out. Thanks for all . Oh by the way there is a suction channel that is bored into the throttle shaft area and lines up with a slot on the throttle shaft, then turns down to the bleed area.