Carburetor doesn't like my overhaul ...

[amclass]

I have rebuilt my Zenith carburetor. Well, it's more a semi rebuilt/cleaning, since I didn't buy the complete kit with new jets. I cleaned all passages and checked the original jets. The float is parallel to the edge of the upper casing and the fuel level is 5/8" from the top edge. The engine runs at about 420 rpm, but it makes a "plock" sound every 2-3 seconds next to the usual "put put". I found that I can not only turn in the gas adjust needle completely, but also the air idle adjust valve and the engine continues to run at idle and only almost imperceptibly a little rougher. In addition, the carburetor leaks about 1 drop every 2-3 seconds after the engine is turned off. Here I suspect the float needle valve, that I have tested but will replace now with a new one just for the peace of mind. Before I disassemble the carburetor again I would like to know what could be causing these weird problems. Any ideas?

BTW: I rebuilt the engine as well since the car was stored for decades and I'm now braking in the engine. Therefore the engine only runs on idle since the rebuilt and I have no information how this carburetors is acting while driving.

PS: I used the instruction and advise of Tom Endy for cleaning the passages and to understanding the Zenith carburetor ... very informative and helpfull.

[Rob Doe]

Yes, the float valve could be leaking. There is an inexpensive external gauge that will let you determine this. The vendors sell them.

I had trouble on a thorough cleaning that I did. The main and cap jets were leaking at the bottom where they screwed in. I ordered Bratton's black washer kit and the problem was resolved. If you have a camera on your phone, you might be able to see this, but I stuck my middle finger into the carb and felt that the jets were dry at their tops but wet at the bottom. I then disassembled the carb halves and put the bottom on the bench and filled the bowl with fuel. It didn't take long to see the leaking jets.

I was reluctant to attempt to smooth the surface where the jets screwed in, but some use various methods to clean up that surface.

Good luck

[amclass]

@ Rob Doe. I will follow your advice. It sounds plausible to first check the proper seating of the jets since I'm sure that the float valve is fine. I'm very carfull when it comes to tighten the jets, but remember an old timer saying in a youtube video to tighten them up very tight. I will check tomorrow. Any idea why the car still runs with the gas and air adjuster turned all the way off?

[Y-Blockhead]

Originally Posted by amclass

I found that I can not only turn in the gas adjust needle completely, but also the air idle adjust valve and the engine continues to run at idle and only almost imperceptibly a little rougher.

The GAV doesn't really have much influence at idle.

If you can close the idle air mixture screw you may have a vacuum leak somewhere.

[nkaminar]

Here is another site that may help: https://model-a.org/default.html

[Brentwood Bob]

Sponsored Links (Register now to hide all advertisements)

Lube the throttle shaft with aviation grease. Helps the idle and reduces wear.
The fit of the shaft affects idle.

[amclass]

Originally Posted by Brentwood Bob

Lube the throttle shaft with aviation grease. Helps the idle and reduces wear.
The fit of the shaft affects idle.

Good tip. I have considered that and packed it with grease since the bushing are not available at the moment. Anyway, it didn't make any difference. I will order a few parts and try the "trial and error" method ...

[Brentwood Bob]

Hard to see your problem from here, but probably something simple.

[amclass]

Originally Posted by Brentwood Bob

Hard to see your problem from here, but probably something simple.

Well, I took the carb apart the first time just to get the engine running and I didn't gave it too much attention. But now the second time I noticed these weird problem. I was hoping that someone had similar problems and has already figured it out. Since the carbs are not really complicated I might find out myself what the problem is. I wait want to wait until the new parts arrive before I take it apart again. Thanks

[Rob Doe]

amclass, I'm thinking that I read where there are at least 2 lengths of Idle Air Adjustment screws. Could you have a short one? Others may comment on this.

The following is on how my 31 coupe behaves. She has a high compression head, likely a 6.1? Snyder's. It has 110 lb. compression:

If I fully retard my coupe's spark advance and set the idle low enough, the GAV ceases to have an effect as Y-Block says. IMHO this is because the compensating well is full / at the level of the float bowl. Further, IMHO, this is likely because the throttle plate is closed enough to stop / reduce the flow of fuel from the cap jet low enough that the compensating jet has enough flow capacity to keep the well full at that idle speed.

On my car, if I advance the spark advance lever a 2 or 3 notches, the idle picks up without moving the throttle plate and the GAV setting does affect the idle mixture. If I open it too much, the idle will lope. If I close it just below 1/4 turn, the car idles best, close it too much, the idle struggles. I've never closed it to see if it dies.

I hope this somehow helps you.

[katy]

For throttle shaft bushings go to a hobby shop and buy some 1/4" ID seamless brass tubing.

[amclass]

@ Rob Doe. Thanks for the advice. I checked the Air adjustment screw by marking the tip with a black marker and it seems that it's the correct size and fit. I will check the throttle plate and will replace it anyway when I insert the bushings. Thanks for the explanation of the set up of your carburetor ... make sense.


@ katy: Thanks for the tip. I will visit my local hardware store tomorrow to look for 1/4" bushings. Would you recommend to replace both bushings or just the lever side? do I have to drill slightly under or would you recommend to glue then in with epoxy?

[ChicagoBob]

My original cap jet had a crack near its base that allowed fuel to leak, which messed up the fuel level and caused idle problems.