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Zenith Carburetor on 8N Ford

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Rodney Davis

08-06-2005 18:06:50

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After setting up for a couple of years (with fuel in it) tractor wouldn't start so I removed the carb and cleaned it and replaced the main body gasket. Now I can't get it to run right. Idle setup seems fine but if I advance the throttle too far (about half throttle) or too fast, the engine starts to die. Returning the throttle to idle quickly will prevent it from dying. Once it starts to die, it never seems to fire again unless the throttle is closed. I checked ignition and it is still present. It will not start at anything but idle throttle. My guess is that only the idle circuit is functioning. Also the choke has very little effect at any speed - closing the choke while running will not kill the engine (slows slightly and no smoke). I've had the carb off and apart about ten times and can't find any obvious problem - all passages seem to be open. Can someone help save my sanity???
Thanks
Rodney

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Rodney Davis

08-09-2005 15:22:06

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Re: Zenith Carburetor on 8N Ford in reply to Rodney Davis, 08-06-2005 18:06:50

Here's the latest in the ongoing saga:
The farther out the main jet needle is (up to the point when it no longer has additional effect), the better but it only moves the shutdown point to a little higher throttle. I even tried running it with the main jet removed and it still seemed to be starving. As long as I have had it, (about 30 years) it has always run with the main needle pretty much all the way out and based on the spark plugs, that was just a little rich but very little smoke.

I put a clear tubing in the drain plug of the float bowl (like a water level tube) and the fuel level is about 3/8 to 1/2 inch below the body gasket surface. I started it with this tube in place and it ran very rich - presumably because it provided a vent into the bowl and maybe extra vapor from the bubbling through the float bowl fuel.

It seems that the vacuum on the main jet is very little more than the vacuum abouve the fuel in the bowl. I'm starting to wonder if I lost something in the carb cleaner. Does anyone have the float level specs and/or an exploded view showing all the parts and gasket washers. I assume something like this would come with a rebuild kit. If so, a scan of it might help identify the problem.

Thanks for the replys so far - I'm learning about it but there's still something significant I don't know.

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Tom in MS

08-07-2005 06:37:04

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Re: Zenith Carburetor on 8N Ford in reply to Rodney Davis, 08-06-2005 18:06:50

These instructions were given to me by the guy at the place where I bought it.... Zenith Carburetor -- Initial setting--T screw on bottom turn out 2 1/2 turns-----top idle screw out 3/4 turn Let warm up, then on 3/4 throttle, turn T screw in till engine fades, then out till smooth..(In = Rich, out = lean)
Set idle (top screw) for smooth low idle..

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Rob

08-07-2005 05:29:38

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Re: Zenith Carburetor on 8N Ford in reply to Rodney Davis, 08-06-2005 18:06:50

That adjust is the main adjust and not the idle. Main isn"t really right, it"s the mixture adjust. If you can"t get it to adjust properly somewhere around 1 turn out then you"ll probably need a kit. Out 1-3/4 turn would seem like a lot to me, out 3/4 turn might be ok. The float and valve seat is likely the problem. All that needs to be in good working order and set up right.
NAPA might have the best price on a kit, if they have it for that particular carb. Check the throttle and carb shafts for slop. You may have to replace the bushings and shafts to stop air leaks.

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Rodney Davis

08-06-2005 22:15:22

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Re: Zenith Carburetor on 8N Ford in reply to Rodney Davis, 08-06-2005 18:06:50

Thanks for the comments so far. In answer to the questions:
All old fuel had evaporated/leaked out so fuel is fresh.
In the past when the tractor was running properly, pulling the choke would flood out the engine and kill it.
The zinith carb has two needles, the idle needle and one at the bottom of the float bowl. I think this one is just a cutoff to prevent fuel from flowing from the bowl to the jets - I don't remember using it to adjust the mixture - just closed or open all the way. At any rate, it hasn't been useful in solving the current problem.
Thanks
Rodney

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Dell (WA)

08-07-2005 05:36:52

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Re: Zenith Carburetor on 8N Ford in reply to Rodney Davis, 08-06-2005 22:15:22

Rodney..... ...learn how to adjust yer Zenith carb. You've got an idle-mix needle that you adjust for MAX idle-rpms, NOT smoothest running, MAX idle rpms, understand?

Remember, the behind the carb throttle idle-speed screw should be adjusted for a SLOOOooow 400 rpms. Anything under 500 is probably good but try for SLOOOooow. Do the idlemix and throttle adjust dance 3-times before yer good.

And guess what? Yer needle at the bottom of the float bowl is, are you ready for this? its yer MAIN JET. And you are complaining that yer carb only runs on yer idle circuit. Well guess what? You've eather got yer mainjet circuit clogged. (and you claim you've cleaned all yer carb passages, I doubt it) ...OR... you've turned yer main-jet OFF and no wonder it won't run like its supposted to. Try adjusting yer now identified main-jet to 1-turn and see iff'n you can't gittchur carb and engine to run on the main-jet circuit like its supposted to. You may need to adjust yer main-jet to 1=1/2 or 2-turns before you've gottchur carb running right..... ....respectfully, Dell

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dan hill

08-12-2005 17:24:25

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Re: Zenith Carburetor on 8N Ford in reply to Dell (WA), 08-07-2005 05:36:52

You must have a basic understanding of your carb to a adjust it.When tractors are idle for long periods of time gas should not be left in the carb.Old gas turns into a varnish like substance.If it smells like old paint brushes its gone by.Turn the gas valve off and let the engine run till it stalls.Sounds like you have closed the main jet,it wont run till it gets fuel.

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jwm

08-06-2005 18:41:34

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Re: Zenith Carburetor on 8N Ford in reply to Rodney Davis, 08-06-2005 18:06:50

Since you didn't mention it, did you drain and refill with fresh fuel. James

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Rob

08-06-2005 18:37:22

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Re: Zenith Carburetor on 8N Ford in reply to Rodney Davis, 08-06-2005 18:06:50

What you got for adjusting the jets? Any specs? I don"t know, maybe you got the main out way, way rich.
Turn that main adjust all the way in and back out one turn and see if that changes things. You may end up backing the main out some. Try 1/4-turn at a time.

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Dell (WA)

08-06-2005 18:30:58

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Re: Zenith Carburetor on 8N Ford in reply to Rodney Davis, 08-06-2005 18:06:50

Rodney..... ..justta WAG, check yer carb gasket. You may be missing a "transfer-hole" or may have it on "up-side-down". I have never seen a Zenith carb up close or personal, but am famaliar with carb principals. And yes, it does sound like yer trying to run on just yer idle-mix jet. As for pulling/closing yer choke plate and NOT shutting yer engine off; bettcha theres a "flapper-plate" in the strangler-disc that the engine vacuum sucks open and prevents full strangle engine OFF.

Howsomevers; by chokeing yer engine while running, you should effectly "by-pass" yer main-jet mixture circuits and effectively enrich yer total carb-mixture and taking over from yer leaned out idle-mix circuit. But appearently, this is NOT HAPPENING eather. You've gott main-jet flow problems..... ...Dell

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