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Old 03-06-2019, 05:25 PM   #1
Mulletwagon
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Default Stalls When Braking

The old four banger starts and idles fine, pulls through the gears like it should and does not miss at all. When braking in neutral, especially aggressively, the engine shudders, loses rpm, and usually stalls but sometimes recovers back to normal idle rpm. Mixture set to a quarter turn and timing about 25 btdc. Thinking a carb problem, maybe float level, but don't know. Looking for a plan forward.
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Old 03-06-2019, 05:49 PM   #2
160B
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Default Re: Stalls When Braking

http://model-a.org/symptoms.html

Engine dies when vehicle comes to a stop
Common Causes
* Float is set too high or a pin hole absorbs gasoline into float.
* Compensator Jet Orifice contains dirt or rust, or is too small.
* Idling Jet Orifice contains dirt or rust, or is too small.
* Idle Air Adjust Screw is mis-adjusted or too short, collapsing spring before it seats.
* Throttle Idle Adjustment Screw is mis-adjusted.
*Upper Casting Idle Jet Passage is restricted to Idle throat port.
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Old 03-06-2019, 05:53 PM   #3
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Default Re: Stalls When Braking

Slosh forward creates over rich flood.
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Old 03-06-2019, 05:56 PM   #4
WHN
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Default Re: Stalls When Braking

What carb are you running?

I had same sounding problem many years ago with a Zenith. Rebuild corrected problem.
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Old 03-06-2019, 06:10 PM   #5
kimlinh
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Default Re: Stalls When Braking

I had the same problem with my car when I first put it on the road. It turns out that the Idle jet was too small (or blocked) and I could not get it to idle slow enough so my engine was using the main jet for Idling.( Causing a higher engine idle speed) When I braked the gas would flow forward in the carb, lowering the fuel level at the jets and the engine would stall. Fixing the idle jet solved the problem. Hope this helps.
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Old 03-06-2019, 07:33 PM   #6
Mulletwagon
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Default Re: Stalls When Braking

Quote:
Originally Posted by WHN View Post
What carb are you running?

I had same sounding problem many years ago with a Zenith. Rebuild corrected problem.
Running a Zenith. Seems like the way forward is a rebuild. Have redone many a Ford and Holley carb over the last 50 years but never a Model A. Any special concerns or cautions ? Great feedback - thanks to all.
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Old 03-06-2019, 07:33 PM   #7
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Default Re: Stalls When Braking

Float level has to be set with a gauge.Ford fixed the issue with the B carb,the float in the A Zenith hinges forward,so when you hard brake it rocks forward..with the B they side mounted the float,so it hinges port to starboard instead of fore and aft Get one of these and set the float accurately.
https://www.brattons.com/carburetor-...vel-gauge.html
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Old 03-06-2019, 08:30 PM   #8
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Default Re: Stalls When Braking

You probably have a small intake air leak. File the intake manifold flat. Clean it up until it is all shiny. Do the same thing with your carb flange, then use a new gasket. That small air leak is enough to lean out the system at a time when the fuel is pushed forward causing a little less fuel in the jets. This can also be caused by an air leak anywhere in the intake. Manifold or vacuum line.
Use a mirror and light and file the manifold flange while still on the car. Easy fix.
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Old 03-06-2019, 08:49 PM   #9
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Default Re: Stalls When Braking

Quote:
Originally Posted by Mulletwagon View Post
Running a Zenith. Seems like the way forward is a rebuild. Have redone many a Ford and Holley carb over the last 50 years but never a Model A. Any special concerns or cautions ? Great feedback - thanks to all.
here is a web site that will help you restore a Zenith carburetor for your Model a Ford


http://model-a.org/procedures.html

* Starting Procedure
* Shut Down Procedure
* Carburetor Idle RPM and Idle Air Mixture Adjustment
* Using Carburetor to Check Engine Timing
* Adjusting Engine Timing
* Removing Carburetor From Vehicle
* Disassembly of a Zenith Model A Ford Carburetor
* Removing and Replacing Brass Passage Plugs
* Repairing Destroyed Tapped Holes in Casting for Jets
* Installing Heli-Coils
* Lower Carburetor Casting Assembly
* Upper Carburetor Casting Assembly
* Installing Carburetor Onto Engine
* Testing Newly Installed Carburetor
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Old 03-07-2019, 04:50 PM   #10
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Default Re: Stalls When Braking

I have had this same problem. Stalling while braking. I check all the normal things but still had stalling.
The fix I used was to lower the float level.
The fuel level is supposed to be set at 7/8” below the seam on the lower half of carburetor. I changed this by adding extra shims under the float valve to give me a 3/4” level of fuel in the bowl.
After this adjustment there was no more stalling at a stop when braking.
I think some people adjust to the fuel level even lower.
Fred
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Old 03-07-2019, 05:12 PM   #11
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Default Re: Stalls When Braking

Quote:
Originally Posted by Fwilkinson View Post
……………….The fuel level is supposed to be set at 7/8” below the seam on the lower half of carburetor...……………….I think some people adjust to the fuel level even lower.
Fred
FWIW

I have two books for rebuilding Zenith Carburetors, one by Paul Moller and one by Steve Pargeter.

The Paul Moller book sets the float level 5/8 inch from the machined surface of the upper body.

The Steve Pargeter sets the float level 5/8 inch from the upper machined surface of the lower body.
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Old 03-07-2019, 08:51 PM   #12
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Default Re: Stalls When Braking

Thanks 160B.
You are correct. I was asleep at the wheel I think.
After I adjusted the fuel level in my carburetor to 3/4” below the seam
the stalling during braking stopped.
Thanks,
Fred
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Old 03-07-2019, 11:02 PM   #13
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Default Re: Stalls When Braking

The caper continues. Took a length of hose, put one end in my good ear and poked around the carb and intake manifold with the other. Found a small vacuum leak on the flange-to-head area of the intake manifold which was corrected by snugging up the fasteners. Also did a manifold vacuum check which came in at 22 inches Hg - pretty high. Decreased the idle to about 550 rpm which made the mixture adjustment effective. At the higher rpm the mixture needle could be closed and the engine still ran - engine probably feeding off the high speed jet. Also richened the mixture knob another quarter turn and the stalling problem went away. Still think I have a carb problem but I am on the road again. What rpm should the idle speed be ?
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Old 03-08-2019, 01:33 AM   #14
Tom Wesenberg
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Default Re: Stalls When Braking

450 to 500 RPM is a good idle speed.
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Old 03-08-2019, 10:58 AM   #15
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Default Re: Stalls When Braking

I have also given my carb a little more than the 7/8" and that fixed my same stalling problem.

Piggybacking off of that last comment...
I do not have a tach on my car, what is a good way to measure engine idle RPM?
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Old 03-09-2019, 01:03 PM   #16
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Default Re: Stalls When Braking

Use one of those rpm/dwell gauges. The older one I have works on 6 or 12 volts. As I recall you have to reverse the power leads for positive ground. The one I have worked on my positive ground 6 volt 48.
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Old 03-09-2019, 01:52 PM   #17
Ed in Maine
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Default Re: Stalls When Braking

You really don't know what you are working with in a rebuilt Zenith Carb unless you have flow tested the jets. Refer to Steve Pargeter's book for the necessary tools and procedures. If the carb is clean, with flow tested jets and adjusted, it can't help but work properly. I also agree that the level should be adjusted down 3/4 to 7/8 inches from the machined surface of the lower casting. With a level set at 5/8 in., the gas level in the Main and Cap Jets will be right at the top and if the carb doesn't leak it will smell. Ed
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Old 03-16-2019, 08:39 PM   #18
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Default Re: Stalls When Braking

I’ve been chasing this problem for months on my 1929 sedan after reading about as many of the threads on this and other forums about it as I could find.

It seemed as though it MUST be related to the fuel level in the carburetor since that is the only thing that would be affected by applying the brakes other than the brake light coming on, which is easily discounted by disconnecting the switch. But it isn’t just the float level as evidenced by different people saying their problem was fixed by raising the float level, and others saying they had to lower the float level. On one thread these “cures”were proclaimed literally two posts apart.

I found it particularly interesting on my car that it would stall only when I put the brakes on going forward. I could back up at any speed, apply the brakes, and the engine wouldn’t even stutter.

I chased the float level up and down with no effect. I found a vacuum leak at the intake manifold so I bought new intake and exhaust manifolds. There was some improvement, but not a total cure. As it turned out, this was probably a waste of money.

I had been hesitant to mess with the carburetor since the guy I got the car from told me he had put on a brand new one, NOT a rebuilt one. And he hadn’t driven the car enough in the last few years to even mention.

But I got to the point where I had to check it out for plugged passageways etc. I followed some of the leads that had been posted on this forum to find the proper methods and the jet sizes. Interestingly, I found the idle jet had a burr from manufacturing that was partly blocking the orifice in it. I thought that I had finally found the root cause of my problem.

Clearing the jet did improve the situation some more, but it still wasn’t just right. So I bought a set of number drills on eBay for $8 so I could size the jets. Sure enough the compensator jet was a couple thousandths undersize. I opened it up to spec. (.035”), put two washers under it so it couldn't be hitting the secondary reservoir, and put it all back together again.

I think I have finally chased the demon from my car. I can now stop without the engine stalling. My assessment is that the comp jet was not allowing fuel to fill the secondary reservoir fast enough when the gas sloshes forward upon braking. This would explain why it wouldn’t stall when backing up. It’s hard to imagine that a couple thousandths of an inch could make that much difference, but it amounts to about 5% of the jet size.


I hope this helps someone else.
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