Need Help with Model B Carb
 

I’ve been trying to adjust the idle on the Model B carb on my ‘28 roadster. However, no adjustment to the idle air mixture screw will cause it to stall. In fact, when I near all the way CW it seems to rev up a little. According to Les Andrews’ book this indicates an air leak in the intake system. All I can find is by spraying carb cleaner around the throttle shaft the rpm will pick up slightly. What is the best way to remedy this? The engine requires very little choke to start cold but when I do choke it I find a little fuel on the bottom of the carb around the drain plug.

Re: Need Help with Model B Carb
 

Just some ideas. Pull the drain plug on the carb. and make sure the main jet is tight. Check the throttle shaft and check if it's not too loose. Then check the wiper vaccum line all the way back. Easiest to disconnect it and plug. Lots of places to loose vaccum. Completely closing the idle mixture screw should cause the engine to die. Do you have a good gasket on the carb. to manifold mount?

Re: Need Help with Model B Carb
 

I pulled the drain plug and the main jet is tight. Throttle shaft does have some slop in it, can pull it back and forth (or in and out) probably 1/8”. Don’t have a vacuum wiper line, plug in manifold is tight. Engine doesn’t die when idle mixture screw is closed. I did take it out though and it has a small score all the way around it about eighth inch from tip, but it seems minor. I had it set 1 1/2 turns out before trying to adjust idle. There’s a good gasket on carb to manifold mount.

Re: Need Help with Model B Carb
 

One of my B carbs had a clogged idle passage, you may want to check it.

Re: Need Help with Model B Carb
 

If you over choke the carb, gas can flood the carb and it can come out the intake/weep hole, drain down the body of the carb, and drips off the drain plug or carb bolt head.. If it happens again use your small finger on the inside bottom of the intake and see if there is gas. This does not tell you what is wrong, just how you are getting the gas drippage.

Re: Need Help with Model B Carb
 

I’ll check the idle passage, haven’t tried that. I’ll bet that the leak is coming from weep hole, I can’t find anything else leaking. It isn’t much but it shouldn’t be dripping at all and it’s only at start-up.

Re: Need Help with Model B Carb
 

My motor cold starting procedure is set spark retard lever all the way up, idle lever set to where it just starts to pull the accelerator pedal down, set the GAV 1/2 to 1 turn open. Ignition on, pull choke out all the way, press the starter and on 2cnd revolution push choke full in/un-choked. Motor immediately fires. Set spark lever 1/4 down. Adjust idle and GAV for smoothest running. Continue to adjust GAV and idle lever as motor warms up.


In the beginning of owning my A I had a tendency to pull the choke out until the motor was running. That is not the corrrect way. This has a tendancy to flood the motor/carb. The purpose of full choke is to load Intake manifold/motor with rich fuel mixture, sort of "primes" the intake manifold. Then when the choke is released there is an increase of air/leaner fuel mixture which helps push fuel/air into the motor, and then the motor fires. Continuing to hold full choke tends to floods the carb/Motor.


Do not know if this is your issue, perhaps you are already aware of this info.

Re: Need Help with Model B Carb
 

My cold start procedure is very similar to yours, I only choke 1 or 2 revolutions and it starts fine. My problem began in attempting to get to a slow idle. It doesn’t respond to the idle air mixture screw. I don’t know if the slight carb leak at startup is related to that or not. Does seem to be a slight air leak at the throttle shaft and there is minimal movement when I pull out on it. I do appreciate yours and the others’ responses.

Re: Need Help with Model B Carb
 

The idle mixture screw is set at very low idle, below 500 rpm. You can tell you are idleing slow enough by turning the GAV. at very low idle GAV is ignored/does not do anything.




My point is are you able to idle that slow?
You can try this procedure


https://www.model-a.org/installing_carb.html
There is a lot more really great info on this site.

Re: Need Help with Model B Carb
 

Just realized that your is a Model B carb, most info is for the model As Zenith 1 and 2 carbs I do not know if they adjust the same way.

Re: Need Help with Model B Carb
 

This is what the June 1932 Service Bulletins has to say about the GAV on the B carb.
"The valve should be turned back (clockwise) as soon as the engine has
become warm. Advise owners the car should never be operated with this
adjustment open."


Bob

Re: Need Help with Model B Carb
 

Good Morning all...Dave Renner at Renner's Corner 734-428-8424 has a good reputation for providing correct jets and often, over the years, the wrong jets have found their way into these carburetors and they don't run as they should. Give him a call and see if he can help you...also you could have a bad float valve or a heavy oversize float...bad floats will definitely make you say bad words and think bad thoughts! Ernie in Arizona

Re: Need Help with Model B Carb
 

Check the idle port, very small and easily plugged. I straighten out a large fish hook and use the sharp point to clean the port. Then blow it out from the
throat side

Re: Need Help with Model B Carb
 

Jerry in Shasta, is the port in the upper casting? Is this the same thing Sparky mentioned as the idle passage? I haven’t identified it with my drawings. There is an opening in the throat just above the throttle plate but it’s closed with a small brass screw.

30 closed cab PU, it will idle slow enough that GAV doesn’t affect it. And Bob C I had read on here to leave GAV closed, it even starts well closed.

I have a good Model A Zenith but would really like to get “B” adjusted properly. I bought it on Ford Barn and had it professionally rebuilt locally. It ran well on his dynamometer.

Re: Need Help with Model B Carb
 

Does the throttle shaft move up and down, or back and forth. ( not in and out) which it really shouldn’t? You saying spray around the shaft picks up rpm tells me you need to repair. 1 way is to solder it up. Another is to use an oversized shaft, or drill and bush. But it won’t idle right with a worn shaft

Re: Need Help with Model B Carb
 

I think it’s the throttle shaft. Put my Model A Zenith on this afternoon and it runs fine. Which is the better repair? I don’t have a drill press.

Re: Need Help with Model B Carb
 

You might be able to tighten up the throttle shaft by adding some solder to the the brass shaft. Then carefully file it around for a snug fit. Also, you should check the idle jet flow rate to be at 43-47 ml/min. per Rex Reheis, Restoring Model A & B Carburetors, page B-15. Ed

Re: Need Help with Model B Carb
 

It's been a while since I had a B carb apart.But,I think the throttle shaft can't really be soldered up and filed like the A.I think it has a step,or cut out in it,and a passage in the body that lines up with it.When it is in a certain position it allows fuel to run through it from the passageway that lines up with it.You can't just bush it,because you can't plug up that passage.I can't remember if that passage is for idle,or off idle enrichening.

Re: Need Help with Model B Carb
 

Doesn't say much for the rebuilder.

Re: Need Help with Model B Carb
 

Seems it may be time to contact the rebuilder, politely tell him the symptoms and what you have found . Get him to repair it properly. Approach it right and see if he will make things right. If you start working on it, gives them an opening to say it is your fault.