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Old 09-23-2021, 07:50 PM   #1
trevorsworth
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Default Model A dies with throttle open

Hey guys, my Model A recently started - seemingly without rhyme or reason - getting into a "mood" where it will idle forever just fine, but if you even touch the throttle it immediately dies. Eventually it starts acting right again, like a switch was flipped, but nothing specific seems to cause it.

I recently had to replace the head gasket. I am currently running without a muffler (the exhaust manifold is about to be replaced with a custom unit and I didn't feel like putting the muffler back on just to immediately have to remove it a couple weeks later).

Here is some video.

https://www.youtube.com/watch?v=f00E_w28000

Shortly after this video, I was able to tease the RPM up by gently pumping the throttle. It backfired through the carburetor and then began acting normally again:

https://www.youtube.com/watch?v=6oo9soVn1SU

Any input is appreciated.
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Old 09-23-2021, 08:01 PM   #2
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Default Re: Model A dies with throttle open

Sounds like a manifold leak. Take an LP torch, unlit, and direct the gas around the manifold gasket area while idling the engine. You'll hear the RPM increase if you have a leak.
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Old 09-23-2021, 08:10 PM   #3
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Default Re: Model A dies with throttle open

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Originally Posted by eagle View Post
Sounds like a manifold leak. Take an LP torch, unlit, and direct the gas around the manifold gasket area while idling the engine. You'll hear the RPM increase if you have a leak.
this works good to find a vaccum leak.

when it backfired through the carb it must have cleaned some some crap from a jet inside the carb.
pull the carb off and make sure it is clean and assembled correctly.

backfire through the carb could be too lean.
flames out your exhaust could be retarded timing.

ou
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Old 09-23-2021, 08:14 PM   #4
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Default Re: Model A dies with throttle open

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Originally Posted by eagle View Post
Sounds like a manifold leak. Take an LP torch, unlit, and direct the gas around the manifold gasket area while idling the engine. You'll hear the RPM increase if you have a leak.
That would make sense. I reused the old gasket because I didn't like the two-piece gasket that came in a gasket set I had, and I was eager to get back on the road. I checked it with ether after installing and it was fine but that doesn't mean it hasn't since developed a leak.

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this works good to find a vaccum leak.

when it backfired through the carb it must have cleaned some some crap from a jet inside the carb.
pull the carb off and make sure it is clean and assembled correctly.

backfire through the carb could be too lean.
flames out your exhaust could be retarded timing.

ou
I'm still getting my bearings with this car and have no prior mechanical experience to draw from. I kind of thought it was running way too rich - the plugs are sooty.

As for the flames, it breathes fire seemingly no matter what I do with the timing. I was concerned about it at first but have been told it's pretty normal.
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Old 09-23-2021, 08:30 PM   #5
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Default Re: Model A dies with throttle open

the second video doesn't sound too bad can you put the pipe back on and drive it?
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Old 09-23-2021, 08:36 PM   #6
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Default Re: Model A dies with throttle open

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Maybe sticking valves. Direct some Marvel Mystery Oil into the carb intake at elevated rpm. Any sticking valves will free up immediately. Had the same problem and MMO fixed it.
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Old 09-23-2021, 08:37 PM   #7
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Default Re: Model A dies with throttle open

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the second video doesn't sound too bad can you put the pipe back on and drive it?
I've been driving it without the pipe. I realize that is maybe somewhat ill-advised. It does great - plenty of power, until it gets in the "mood". I have to coast to a stop and wait for it to decide to start working again.

The popping and cackling when revving down gets lots of mixed reactions from neighbors haha.

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Originally Posted by Mulletwagon View Post
Maybe sticking valves. Direct some Marvel Mystery Oil into the carb intake at elevated rpm. Any sticking valves will free up immediately. Had the same problem and MMO fixed it.
I think the valves are all fine. I turned the engine over by hand with the head off quite a bit and the valves never missed a beat. I did try adding a bit of MMO to the fuel.

I should note it wasn't doing this before the head gasket replacement, but I did a lot of other little stuff to the engine while that was ongoing - I had the manifolds completely off, rebuilt the distributor, rewired the ignition, replaced the ignition coil, etc... so it's hard to point at any specific thing as a possible culprit. Manifold gasket leak makes sense to me though.

It is hard to troubleshoot since this is pretty intermittent and I still don't know what's causing it or making it stop. I thought I had fixed it for good a couple days ago but it just decided to do it again. I was fortunate that this time it did it in the garage and not on the road since I was able to get video.

Last edited by trevorsworth; 09-23-2021 at 09:04 PM.
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Old 09-23-2021, 09:29 PM   #8
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Default Re: Model A dies with throttle open

Two things.
Have you taken the carburettor apart? If so, make sure the venturi is in there and not somewhere on your workbench.
You mentioned you were able to get the revs up by pumping the accelerator. Pumping the Model A accelerator does nothing - there is no pump on it. You would probably have done better by easing it open.
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Old 09-23-2021, 09:43 PM   #9
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Default Re: Model A dies with throttle open

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Two things.
Have you taken the carburettor apart? If so, make sure the venturi is in there and not somewhere on your workbench.
You mentioned you were able to get the revs up by pumping the accelerator. Pumping the Model A accelerator does nothing - there is no pump on it. You would probably have done better by easing it open.
I haven't messed with this one, I bought it freshly rebuilt - which of course does not guarantee that it will be trouble-free.

Yes, I do know there's not a pump in the A carburetor. Using the hand throttle to slowly open it would certainly have worked better but that's not what I did.
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Old 09-24-2021, 07:33 AM   #10
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Default Re: Model A dies with throttle open

Next time it happens try loosening the gas cap. Sometimes the vents can get plugged or the wrong cap can get put on and suction will build up in the gas tank and restrict gas flow.
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Old 09-24-2021, 07:36 AM   #11
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Default Re: Model A dies with throttle open

I WOULD SWAP CARBS JUST FOR THE..............


was having a similar problem yesterday, swapped out carb and running terrific.
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Old 09-24-2021, 09:50 AM   #12
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Default Re: Model A dies with throttle open

Yep. Seems lean. Either vacuum leak or lean carburetor condition.

These carburetors are as simple as they get, easy to fix.
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Old 09-24-2021, 09:53 AM   #13
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Default Re: Model A dies with throttle open

It runs on the idle system but dies on the transition to high speed. I'm thinking the cap or main jets are clogged. Take apart carb and clean the jets and passages.



http://modelabasics.com/carb%20basics%202.htm
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Old 09-24-2021, 10:34 AM   #14
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Default Re: Model A dies with throttle open

I have exactly the same problem as trevorsworth, except that it's continous on my car.
The engine idles nicely after warming up (choke off and GAV completely closed), but as soon as I push the accelerator, the revs drop and you can hear it sucking in air without igniting the mixture. The engine has almost no power, while it ran just fine two years ago. (when it was last driven)

If I slowly increase the gas through the hand lever, I can get the revs up, but only to about 1.5 - 2k rpm (estimated), after that the same as above happens.
Now, if I clog up 1/2 to 3/4 of the air intake of the carb with my hand, the revs climb fast and the motor seems to have more power.
Am I right in assuming that it's running WAY too lean and I either have

a) dirt / gummed up fuel in my carb

or

b) a partially clogged up jet ?


Can the float cause problems like that too?




EDIT: I forgot, there is a steady fuel stream through the gas line and I had the fuel cap removed for testing, so it's not most likely not a problem with not getting enough fuel to the carb.

Last edited by bavArian; 09-24-2021 at 11:04 AM.
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Old 09-24-2021, 11:32 AM   #15
Chuck Sea/Tac
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Default Re: Model A dies with throttle open

Quote:
Originally Posted by bavArian View Post
I have exactly the same problem as trevorsworth, except that it's continous on my car.
The engine idles nicely after warming up (choke off and GAV completely closed), but as soon as I push the accelerator, the revs drop and you can hear it sucking in air without igniting the mixture. The engine has almost no power, while it ran just fine two years ago. (when it was last driven)

If I slowly increase the gas through the hand lever, I can get the revs up, but only to about 1.5 - 2k rpm (estimated), after that the same as above happens.
Now, if I clog up 1/2 to 3/4 of the air intake of the carb with my hand, the revs climb fast and the motor seems to have more power.
Am I right in assuming that it's running WAY too lean and I either have

a) dirt / gummed up fuel in my carb




or

b) a partially clogged up jet ?


Can the float cause problems like that too?




EDIT: I forgot, there is a steady fuel stream through the gas line and I had the fuel cap removed for testing, so it's not most likely not a problem with not getting enough fuel to the carb.
An A carb shouldn’t be run with the GAV closed. An average is about a quarter turn open. Every cars different. You’re right, you’re running lean. If that doesn’t fix it, look for vacuum leaks.
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Old 09-24-2021, 11:58 AM   #16
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Default Re: Model A dies with throttle open

Turning the GAV open 1 turn seems to not change much. I'll try tomorrow with more than 1 revolution, maybe that will fix it? (I doubt it though)
If that doesn't help, I'll check for vacuum leaks and in case I don't find any will probably disassemble the carb and check the GAV valve seat and the passageways.
Using brake cleaner is probably not the recommended method with the exhaust manifold being on the same side as the intake? Clean Water with a pump sprayer or culinary torch?
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Old 09-24-2021, 06:43 PM   #17
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Default Re: Model A dies with throttle open

Quote:
Originally Posted by Will N View Post
It runs on the idle system but dies on the transition to high speed. I'm thinking the cap or main jets are clogged. Take apart carb and clean the jets and passages.



http://modelabasics.com/carb%20basics%202.htm
Thanks. That makes the most sense so far. I cleaned my gas tank obsessively but I'm sure there's still crud kicking around in there. If it starts doing it again I know what to do.
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Old 09-25-2021, 01:02 PM   #18
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Default Re: Model A dies with throttle open

Update:

Chuck was right!
Found a vacuum leak today, the throttle shaft has some play. Spraying brake cleaner onto the shaft helped, the engine ran better but still not at all nice.

After taking the carb apart I found that the float chamber was surprisingly clean, but the entire intake area was quite dirty. Also the main jet was completely clogged from what seemed to be gummed up gas, the cap jet was basically the same.
So I took the carb off and disassembled it. To my surprise, it even came apart without much of a fight.

I cleaned everything up, polished the venturi because the inner surface was quite rough (it's a die cast venturi), changed the gaskets for new ones and put the carb back into the car.

What a change! The engine runs a lot better now, not perfect yet, but certainly way better than yesterday! It can even move on it's own now!


Over the winter I'll install new throttle shaft bushings along with a new shaft and check the idle circuit (which runs fine atm) and the seat of the GAV-valve.


Thanks a lot everyone!
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Old 09-25-2021, 01:26 PM   #19
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Default Re: Model A dies with throttle open

Try lubing the throttle shaft with dialectic grease until you can fit a replacement shaft, or rebush the casting.
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Old 09-25-2021, 02:34 PM   #20
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Default Re: Model A dies with throttle open

I already thought about using grease, but I'd guessed that it would liquify as soon as the engine is warmed up the first time?
The intake manifold becomes quite hot when the engine is running after all.


Maybe high temperature grease will work? Anyone tried it?
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