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Old 07-09-2020, 10:47 PM   #1
27touringT
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Default 1931 model a running rough.

Ok so I have a 31 model a that will run great then out of nowhere run terrible. Iím running a downdraft carb with a 97 on it. Rebuilt distributor with modern points. New wiring. Someone had mentioned that the model a coils need to have a resistor. Is this true? Iím running an old echlin ic55 coil in it right now and no matter what coil I put in it iit still has issues. IMG_3069.jpg


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Old 07-09-2020, 10:55 PM   #2
daveymc29
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Default Re: 1931 model a running rough.

6V or 12V? Sound like it could be a condenser also. If 12 Volt system, do you have the correct coil? Feel the coil when the car starts and later when it begins to misbehave. Be careful though, they can burn your hand. I prefer to just touch at it and if it feels too hot get out the infared temp gun and see how hot it is. Anything hotter than the firewall spells trouble.
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Old 07-09-2020, 10:57 PM   #3
27touringT
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Default Re: 1931 model a running rough.

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Originally Posted by daveymc29 View Post
6V or 12V? Sound like it could be a condenser also. If 12 Volt system, do you have the correct coil? Feel the coil when the car starts and later when it begins to misbehave. Be careful though, they can burn your hand. I prefer to just touch at it and if it feels too hot get out the infared temp gun and see how hot it is. Anything hotter than the firewall spells trouble.

Itís still a 6 volt positive ground system. And now for some reason I can not get the car to start back up. I had it running and it was hesitant at first then ran fine then just died and wonít start back up.


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Old 07-10-2020, 12:23 AM   #4
Brentwood Bob
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Default Re: 1931 model a running rough.

Good ground? Strong battery? Strong spark?
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Old 07-10-2020, 09:02 AM   #5
Bob Bidonde
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Default Re: 1931 model a running rough.

Go for the easy things first. Replace the condenser and make sure the points are clean. Next replace the modern wires with the OEM bare metal leads which are far superior conductors than your modern wires. Next try another coil and make sure it is a 6v coil.
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Old 07-10-2020, 12:23 PM   #6
Patrick L.
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Default Re: 1931 model a running rough.

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Have you tried to determine whether the problem is fire or fuel ?

Do you have spark ?

Is there fuel getting to the carburetor and is that fuel getting to the cylinders ?
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Old 07-10-2020, 01:55 PM   #7
30 Closed Cab PU
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Default Re: 1931 model a running rough.

Do some basic troubleshooting before shotgunning and replacing things. That typically confuses thing. Be methodical and write down what you did.

This is somewhat of a rehash of the above posts.

Did you recently work on the car and and if so what did you do? Sometimes the issue is related to the last thing you did.

If you have a 6 volt system with 6 v coil, no resistor needed. If you have a 12 v system with a 6 v coil, then you need a 1.5 ohm series ballast resistor in series with the coil. If you have a 12v system with a 12v coil, resistor not needed.

Check for spark
Check that the motor is getting gas.
Pull a spark plug and check condition
Check timing.
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Old 07-10-2020, 02:36 PM   #8
Brentwood Bob
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Default Re: 1931 model a running rough.

When you make a change, do one thing at a time.
And then test, and document in your vehicle log.
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Old 07-10-2020, 03:10 PM   #9
Purdy Swoft
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Default Re: 1931 model a running rough.

A six volt coil is rated at one point five OHM or near it . A twelve volt coil is rated at three OHM and will not need a resister .
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Old 07-10-2020, 05:56 PM   #10
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Default Re: 1931 model a running rough.

if you have a known good coil, I'd put that on. Someone near you may loan you one just to test if that is the problem. Somehow we seem to accumulate a few over the years.
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Old 07-10-2020, 06:58 PM   #11
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Default Re: 1931 model a running rough.

Quote:
Originally Posted by 30 Closed Cab PU View Post
Do some basic troubleshooting before shotgunning and replacing things. That typically confuses thing. Be methodical and write down what you did.


Did you recently work on the car and and if so what did you do? Sometimes the issue is related to the last thing you did.


Check for spark
Check that the motor is getting gas.
Pull a spark plug and check condition
Check timing.
LISTEN TO THIS ADVICE! Don't start throwing parts at it. You'll soon have more troubles. If you haven't checked the point gap, that will do as you've described.
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Old 07-12-2020, 11:59 AM   #12
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Default Re: 1931 model a running rough.

I have a coil related question, first off mine is 6 volt positive ground, I picked up a new 6 volt coil for a spare, the coil on my car is only marked (dist) for the red wire no markings on black terminal so it's almost idiot proof, new coil is marked ( + & - ) and I'm thinking positive terminal to distributor, but then I second guess myself because of the positive ground. Could someone put this O'l Nebraska corn cob on the right track.
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Old 07-12-2020, 02:43 PM   #13
Patrick L.
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Default Re: 1931 model a running rough.

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Originally Posted by ricconhan View Post
I have a coil related question, first off mine is 6 volt positive ground, I picked up a new 6 volt coil for a spare, the coil on my car is only marked (dist) for the red wire no markings on black terminal so it's almost idiot proof, new coil is marked ( + & - ) and I'm thinking positive terminal to distributor, but then I second guess myself because of the positive ground. Could someone put this O'l Nebraska corn cob on the right track.


Points are wired same as battery. Battery + ground, points + ground. So, - on coil is power lead. + on coil goes to key switch [ if wired as original] which in turn goes to distributor.

Just check that your new spare coil is not oil filled.
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Old 07-12-2020, 08:42 PM   #14
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Default Re: 1931 model a running rough.

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Originally Posted by Patrick L. View Post
Points are wired same as battery. Battery + ground, points + ground. So, - on coil is power lead. + on coil goes to key switch [ if wired as original] which in turn goes to distributor.

Just check that your new spare coil is not oil filled.
Thanks, yep it's oil filled so not ideal for upside-down use. I'll have to get one of the solid kind.
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Old 07-13-2020, 11:36 AM   #15
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Default Re: 1931 model a running rough.

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Thanks, yep it's oil filled so not ideal for upside-down use. I'll have to get one of the solid kind.
It'll be OK if you just turn the coil around and have the terminals facing up.
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Old 07-13-2020, 06:11 PM   #16
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Default Re: 1931 model a running rough.

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It'll be OK if you just turn the coil around and have the terminals facing up.
He may need a longer coil wire.
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