Model A Runs rich after Carb rebuild

[Canton_Model_A's]

I recently rebuilt the carb on my '29 Fordor and now it appears to run real rich and sounds like it is missing. I put all the jets back where they belong. The one odd thing was with the rebuild kit from Synders, the gasket that goes between the 2 halves of the carb the side that has the venturi was a little smaller than what the original was. I did not take out the venturi when i cleaned it. Ideas on why it would be doing this? It also appears to surge at times and I can't keep a smooth idle when parked and or driving her.

Thanks everyone!

[Rob Doe]

A rebuild of the carb without removing the venturi???? More info needed here. How was the car running before you disassembled the carb? etc.

[Canton_Model_A's]

Car was running smooth, albeit a little rough.

[Y-Blockhead]

I was just reading an great article by Lynn Sondenaa in the March/April 2022 issue of the Restorer magazine on Zenith Carburetors (Six Overlooked Problems...) and one thing he mentioned that I found interesting is the thickness of the gasket between the carburetor halves.

Lynn states the the gasket in the kits is ~ ⅓ the thickness of the original gasket and that will causing idling problems because it affects the height of the idle jet.

Also check the float level. I check my actual float level with the engine running.

Checking level on my Marvel Schebler

[Will N]

Did you replace the original jets with the jets in the rebuild kit? Could be the new jets are a little oversized.

[CA Victoria]

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Just finished flow testing my box of mostly used jets, fewer than 10% were within spec. Some were close but most were way off. The non-original Main & Cap jets varied in their length. Renners Corner offers a rebuild kit with matched jets. IMO it is a good deal considering the time it takes to re-size & flow test jets or wrestle with an inferior kit.

[Ruth]

Originally Posted by CA Victoria

Just finished flow testing my box of mostly used jets, fewer than 10% were within spec. Some were close but most were way off. The non-original Main & Cap jets varied in their length. Renners Corner offers a rebuild kit with matched jets. IMO it is a good deal considering the time it takes to re-size & flow test jets or wrestle with an inferior kit.

You may have to Contact Dave. I don't see flow tested jets listed anymore on his website. http://www.rennerscorner.com/carburetor.html

[Canton_Model_A's]

I used the new jets provided in the kit from Snyders. I still have the original jets if I need to put them back in

[jeepguy1948]

Any of the above comments could be your problem, possibly more than one. In my limited experience virtually everybody sets the float level too high. A 90 year old gentleman in our club ran a Model A restoration shop for over 25 years and he showed me about it. If you set the float level so that it is level when you hold the top upside down you are too rich. I have found that most people who “rebuild” a Zenith really do nothing more than clean it (and usually not well enough) I am fascinated about the ideas of the gasket being too thin. What to do about it? Is anybody aware of how thick the gasket should be? Should we use 3 new gaskets?

[jeepguy1948]

If I could find a Marvel at a reasonable cost I would love to try one.

[Y-Blockhead]

Originally Posted by jeepguy1948

I am fascinated about the ideas of the gasket being too thin. What to do about it? Is anybody aware of how thick the gasket should be? Should we use 3 new gaskets?

Accourding to the article in the Restorer the original is 3/64" thick. Maybe try two gaskets.

I use 1/32" ButaN/Cork and cut my own for my Marvels.

[bobbader]

You don’t say so, but I assume this is a Zenith Carburetor. If so, the standard rebuild kits contain all four jets ............. AND A “REPLACEMENT BRASS VALVE SEAT” that installs in the Gas Adjusting Valve Chamber (commonly referred to as “GAV”) IN 28/29 CARBURETORS. This brass seat can be mistaken for the COMPENSATOR JET in the bottom of the gas bowl in the lower carburetor body. They look much the same, but the VALVE SEAT has a much larger hole in the center than the COMPENSATOR JET. If you installed the valve seat instead of the compensator jet, (or switched the two by mistake) the carb will run “extremely rich” and idle very poorly.

Good luck. Bob Bader

[jeepguy1948]

I spoke to Davis Renner this morning about the gasket thickness (BTW he’s still selling his jets). He said that .020-.030 would be correct. 3/64” = .050” so who knows? He did say that he didn’t think a gasket that was too thin would be much of a problem but a thicker than spec would be more likely to cause problems. I think more research is required....

[redmodelt]

If the jets are being flooded because the thickness of the gasket is suspect, then that can be taken care of by shimming the float valve to get the proper level in the bowl. Also making sure all old gaskets are removed before installing the jets.
When is it running rich? If at idle, have you adjusted the air/fuel valve? I start at about 1-1/2 turns open.

[Gary WA]

Jet length is important!!! and dished ends.

[Oldbluoval]

My experience (albeit limited compared to many) with repop jets has been bad!
Cleaning old jets and flow testing is the only way to go.

[Gary WA]

All jets purchased should be tested and checked for length-old ones too, for proper flow.

[katy]

FWIW, Jet sizes: The original Ford/Zenith carburetor jet number found stamped on the jet is the orifice size as expressed in 0.05mm increments. For example a jet marked number ‘18’ has an orifice diameter of ‘18’ X ‘.05’ = .09mm = .035”

[Gene F]

Originally Posted by Oldbluoval

My experience (albeit limited compared to many) with repop jets has been bad!
Cleaning old jets and flow testing is the only way to go.

I have found old jets the way to go also. I have a big vat of Berryman Chem-dip.... Stinks, but gets the job done.

I have a B carb and intake now though.

[mcgarrett]

I'll admit going in that I'm not an expert when it comes to setting up and tuning a Zenith, although I have "rebuilt" a few of them with fairly good results. It sure seems to me that they are a tad fussy to get set up correctly, Maybe my Marvel has spoiled me.