Still dying at idle sometimes

[rivcokid]

Good afternoon. My 1931 Fordor is having trouble with idle. Oftentimes (but not always) it will die on deceleration or at stop - it sounds like the idle is too slow. If I increase the idle, it doesn't, but the idle is too fast to shift without grinding gears.

I have cleaned out the fuel line, carburetor, and sediment bowl. I have confirmed that the float level is fine and there is nothing getting into it. I have sprayed starter fluid around the manifolds with no change in engine speed either way. The idle screw on the top of the carb has been adjusted per the blue book and it runs fine - in fact, the engine runs great other than the occasional dying on deceleration and/or stop.

I thought I had fixed the issue when I enriched the gas mixture with the GAV - it ran great for several minutes, then went back to its old ways!

Any other ideas beyond what I have tried?

 

[Ed in Maine]

Hello, I think this is a good topic. I assume we are talking about a Zenith Carb. I rebuild carburetors for my club and I created the most serious stalling carburetor after rebuilding this Zenith carburetor. All you had to do was take your foot off the accelerator and you just knew the car was going to stall. I had to leave home for vacation for the summer, so I haven't actually diagnosed the problem but this is what I think.
1. When you stop at a stop sign, you take your foot of the accelerator and the throttle butterfly goes closed. All gas from the Main Jet and the Cap Jet is shutoff immediately. The only jet that remains able to feed gas to the engine is the Idle Jet.
2. However, at the moment you take your foot off the accelerator, the Secondary Well is nearly empty. This is the source of gas for the Idle Jet. It would take a few moments for the Secondary Well to refill with gas coming from the Compensator Jet (that little jet in the carburetor bowl).
3. Now, you have a race going on between trying to re-fill the Secondary Well and getting gas to the Idle Jet and the engine speed decreasing so low that the engine stops. I think what has to happen is that the Secondary Well has to refill faster and this can be accomplished by installing a Compensator Jet with a flow rate at the upper end of its published range or about 141 -142 ml/min. It might help to also increase the size of the Idle Jet to its upper range of 47-48 ml/min. Also, increase the fuel bowl level to about 11/16 in. instead of 5/8 in.
4. Sometimes you can get a Secondary Well whose diameter closely matches the diameter of the mounting hole. It may be helpful to grind a fuel flow path on the Secondary Well circumference from where the Compensator Jet is located. This is easily done by scratching a mark on the Secondary Well that is tightened in position with a nail or sharp awl. Go through the Compensator Jet hole.

I am looking forward to analyzing the carburetor on my bench to see just what is the cause of Model A stalling at stop signs. But I think the solution is described above. Good luck, Ed

 

[nkaminar]

The gasoline in the float bowl sloshes forward when you stop. This does two things, raises the float which shuts off the gasoline supply and lowers the level where the pickup for the idle jet is. You can try enriching the idle slightly by screwing in the adjustment needle and/or raising the flow level slightly with one thinner washer under the float valve.

 

[rivcokid]

Thank you gentlemen!! I'll give those a try.

@Ed in Maine - I may take you up on that offer to rebuild.

 

[Jim/GA]

<snip>

Also, increase the fuel bowl level to about 11/16 in. instead of 5/8 in.

<snip>

Good luck, Ed

Ed-
If you go from a fuel level 5/8" (also known as 10/16ths) down from the carb seam to 11/16" on the fuel level, you are lowering the fuel level in the carb bowl.

And I can tell you from experience that this IS the best fix for a car that stalls on hard stopping. I have now adopted 11/16" down from the carb seam as my rebuild standard.

Lower the fuel in the bowl, don't raise it.

.

 

[Bob Bidonde]

Check your fuel system for the issues addressed by the following slides.

 

[Rob Doe]

rivcokid, check out Mr. Marshal Daut's post on the recent thread titled "Idle Speed Too Fast" the post was dated 09-17-2022, 11:10 AM. He describes how to make certain you have the throttle plate correctly installed and adjusted. When the plate is incorrect, air can leak past and mess up the idle speed/mixture.

 

[Rob Doe]

rivcokid, when setting the base idle, where did your Idle Air Adjustment screw wind up? How many turns out from closed? And where was the GAV when making the adjustment? How many turns out from closed?

 

[Ed in Maine]

Jim/GA, you are correct. I was not thinking properly. I tell myself gas level down 11/16 in. but not to 3/4 in. I don't want to see the Compensator Jet being starved on stops. Ed

 

[sdpet56]

Check your fuel system for the issues addressed by the following slides.

Thanks for posting this, I printed off the 9 photos and put them in my 3 ring binder with my Model A stuff

Steve

 

[cars2cool]

Check your fuel system for the issues addressed by the following slides.

Is the angle of the engine checked at the attach point of the carb. to the intake manifold?

 

[rivcokid]

Wow - even more and better advice. It turns out that the float was too low. I guess I'm still trying to figure out how something like that can happen just all of a sudden - the car went miles and miles with it being fine. then one day, the float is too low. Sure seems like that should be more of a gradual thing to occur. Thanks again, everyone!

 

[Rob Doe]

It could have developed a pin hole and leaked fuel into the float. Has to start sometime.

 

[Jim/GA]

Check your fuel system for the issues addressed by the following slides.

Bob, in your multi-page graphic carb guide, you say on Slide 6 "no vent in carburetors before mid-1930". Do you mean no "bowl vent"? because there is a side vent. It's just an indirect connection to the top of the fuel in the bowl.

Thanks!