Main jet won't lean out

[J Witt]

I can turn my mixture control all the way in (clockwise) until it stops, but I still don't believe the engine is running lean enough (Zenith carb), and I certainly can't get it lean enough to stumble and die.

Is this the normal range? It wasn't in my Model T carbs.

I can rich it up some and it is immediately running a little less smooth, leading me to think it could be leaner. I removed the needle assembly and it looks OK, needle not scored or deformed in any obvious way. The mixture is not far off, judging from the way the engine starts and performs, but that is with the needle in as far as it will go.

Suggestions?

Thanks,

John

[Will N]

The Zenith carb is way different than the carbs in a Model T, which had a main jet and an idling circuit only. The adusting valve in the T carbs impacted the main and idle. The Zenith has a main and compensating jet and idle jet too. The Gas Adjusting Valve really only impacts the compensating and idling jets, which draw gas from the well that the GAV regulates. The main jet draws gas right from the carb bowl with no provision for adjustment. Do you think you're running too rich at idle or at high speed?

[J Witt]

It's not clear whether the engine is running too rich or not, however the smoothness and performance kept increasing as the GAV was turned in, until it hit the stop.

[Y-Blockhead]

Originally Posted by J Witt

It's not clear whether the engine is running too rich or not, however the smoothness and performance kept increasing as the GAV was turned in, until it hit the stop.

My engine let's me know when the GAV is turned in too far... it farts thru the pipe when I let off the gas to shift, telling me it's too lean. I have Renner's jets in my Zenith.

[Will N]

What do your sparkplugs look like? Black and sooty would indicate too rich.

[Rex_A_Lott]

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I'm not clear if you are trying to adjust the idle screw on the carburetor or the GAV in the cab. Also it isnt clear if your are trying to adjust the idle or the mixture running down the road.
The GAV, in the cab, shouldnt affect the idling of the car, unless you have the idle speed way too high to start with. This adjusts the mixture running down the road.
Good Luck

[Charles Coe]

My Zenith was running too rich in spite of everything I tried until I got a set of flow tested jets from Renners: http://rennerscorner.com/carburetor.html

[J Witt]

I may have been trying this adjustment at too low a speed. I'll try again when I can get the speed stabilized above 35 mph.

Thanks for all the input,

John

[Tom Wesenberg]

5 seconds after starting a cold engine does leaning out the GAV make the engine stumble? If not, then something isn't right with the carb. For the first minute my GAV needs to be one to one and a half turns open or the engine will dies. After a minute or two I can turn it down to fully closed and it will idle like it should. I got 22 MPG on a 500 miles trip, so the carn is pretty well set. My engine and car is all stock.

[BILL WILLIAMSON]

Originally Posted by J Witt

I may have been trying this adjustment at too low a speed. I'll try again when I can get the speed stabilized above 35 mph.

Thanks for all the input,

John

Chief taught me, to adjust the GAV: With WARM engine, run @ a constant 25MPH, in second gear & adjust the GAV for BEST & SMOOTHEST running.
Remember, the GAV is just a supplemental nozzle, to aid the MAIN jet nozzle, when the main nozzle is TOO LEAN. This usually causes COUGHING BACK through the carb on acceleration, NOT to be confused with BACKFIRING out the tail pipe. ALSO, tailpipe backfiring can be worse, IF your idle speed is set QUITE HIGH, as that causes some fuel to still be dumping out the HIGH speed jet & some from the GAV jet, IF it's open.
IF you are confused between tail pipe backfiring or COUGHING BACK @ the carb, JUST notice that you can SMELL the carb COUGHING BACK!! Ladies will ALERT you about that, in SHORT ORDER!!! They got a sniffer that rivals Buster T!
Not tying to be "funny", just tryin' to clear up misconceptions about the GAV! Henry could have thought up a BETTER name for it??
OH, IF your GAV affects your idle, IT'S IDLING TOO FAST!!! AND, that's OK, IF you have your hand throttle advanced, on a COLD start.
AND, depending on YOUR CAR, weather conditions, altitude, etc, IF your car starts OK, COLD, runs OK during warm up, runs OK cruising, with the GAV OFF, just leave the GAV alone & DON'T DIDDLE WITH IT!!! On Minerva, I only ever had to open it a little, when it was REALLY COLD.
Bill W.

[Tim B.]

If you are running an air cleaner your carb will run rich unless the body and venture are modified for pressure balancing. Renner's can modify yours if it isn't. Tim

[J Witt]

So here's the status ( no air cleaner, BTW):

Cold start: (83 degrees F) half a turn out on the GAV to stay running, momentary choke, bit of hand throttle, starts up and runs ok. GAV turned CW to hard stop after a minute or two.

Engine warm, GAV turned CW against hard stop produces best running. I can not get the engine to display any lean running characteristics at all. Gas mileage not checked, but doesn't seem to be very good.

I think my next step is to pull the carb and check float level, jet integrity and cleanliness, etc...

Carburetor is a French word that means "leave it alone"

[BILL WILLIAMSON]

Originally Posted by J Witt

So here's the status ( no air cleaner, BTW):

Cold start: (83 degrees F) half a turn out on the GAV to stay running, momentary choke, bit of hand throttle, starts up and runs ok. GAV turned CW to hard stop after a minute or two.

Engine warm, GAV turned CW against hard stop produces best running. I can not get the engine to display any lean running characteristics at all. Gas mileage not checked, but doesn't seem to be very good.

I think my next step is to pull the carb and check float level, jet integrity and cleanliness, etc...

Carburetor is a French word that means "leave it alone"

Your last sentence is FUNNY!!
Dis you know that VEGETARIAN came from an OLD INDIAN word, that means, POOR HUNTER!!
Bill W.

[Mitch//pa]

next step pull the plugs and post pics so we can see how they are

get a set of renners flow tested jets before you pull the carb...
putting them in while its apart is a nobrainer

[mrtexas]

Originally Posted by Charles Coe

My Zenith was running too rich in spite of everything I tried until I got a set of flow tested jets from Renners: http://rennerscorner.com/carburetor.html

Your carb will only run good by accident unless you have a set of flow tested jets.

[Tom Wesenberg]

"GAV turned CW to hard stop after a minute or two."

You should not turn to a hard stop. Stop turning when you feel the valve come to the end of travel while turning CW. A hard turn can damge the needle. Also, I'm not aware of a stop when turning CCW. Doesn't it just keep unscrewing until it's out of the housing and can be completely removed?


I seem to recall mention of the housing turning with the needle, though I've never seen it happen. It sounds like the best thing to do is a thorough cleaning and set of flow tested jets.

[Will N]

Here's a very good and detailed explanation of how the Zenith works:
http://modelabasics.com/carb%20basics%202.htm