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Old 09-30-2020, 06:54 PM   #1
Daba
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Default Carburetor backfire

I have a 31 three window four-door with a B motor the car runs great up until 45 miles an hour anything over 45 it starts to pop through the carburetor any suggestions would be grateful. Carburetor adjustment? Timing adjustment? fuel mixture adjustment? drive the car off a cliff ?? Lol
Thanks again Daba








31 three window four-door deluxe with B motor
31 slant window four-door deluxe
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Old 09-30-2020, 08:39 PM   #2
GRutter
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Default Re: Carburetor backfire

I would check point gap setting to be sure that there is enough dwell angle to allow coil to build back it's field.

As point rubbing block wears, the points ride closer to the cam, holding them open too long. Then as speed increases, you get to a point where the points are not closed long enough to build up a full field in the coil. A weak spark results.
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Old 09-30-2020, 10:34 PM   #3
Chuck Sea/Tac
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Default Re: Carburetor backfire

It sounds lean to me. Have you opened up the GAV when it starts popping to see if it runs a little better? Be sure to let us know what the solution is.
Have you played with the timing recently?
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Old 10-01-2020, 12:45 AM   #4
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Default Re: Carburetor backfire

I would guess lean mixture as Chuck Sea/Tac said or timing is off for that engine rpm and load.
I recently had a Porsche ( I know way different beast hear me out though) that had some surging and backfiring problems at cruising speed . I ended up finding the intake valves way too tight and a series of small vacuum leaks. Adjusting the valves cleared up a lot of issue and fixing the vacuum leaks cleared up the rest.
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Old 10-01-2020, 03:06 AM   #5
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Default Re: Carburetor backfire

yep often a symptom of a weak mixture/fuel starvation, ... so easy things first ... check the flow of fuel from tank to carb and the various stariners filters etc.. then move on to other stuff.
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Old 10-01-2020, 06:22 AM   #6
Daba
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Default Re: Carburetor backfire

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I’ll give the GAV a shot today and check back. Can I use the new timing wrench I got for my other model A on a model B engine ?
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Old 10-01-2020, 06:46 AM   #7
John
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Default Re: Carburetor backfire

It sounds like it is time to get that 'B' carburetor cleaned out.
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Old 10-01-2020, 06:52 AM   #8
Patrick L.
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Default Re: Carburetor backfire

Good info, but, loosen the fuel cap and take it for a drive. The vents are known to plug.
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Old 10-01-2020, 08:36 AM   #9
rocket1
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Default Re: Carburetor backfire

Intake valve hanging up try MMO in the gas tank.
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Old 10-01-2020, 11:06 AM   #10
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Default Re: Carburetor backfire

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Originally Posted by Patrick L. View Post
Good info, but, loosen the fuel cap and take it for a drive. The vents are known to plug.
Yes, Thanks Patrick.. I'd forgotten about that one ..... That's another one that has caught me out!
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Old 10-01-2020, 11:58 AM   #11
Patrick L.
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Originally Posted by johnbuckley View Post
Yes, Thanks Patrick.. I'd forgotten about that one ..... That's another one that has caught me out!




John, with our age and what we have rolling around between our ears its easy to forget where we put all that info in that grey matter.
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Old 10-01-2020, 03:37 PM   #12
Daba
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Default Re: Carburetor backfire

I tried the gav no difference then the timing much worse. I used the New timing wrench I got for the model a apparently the timing is slightly different. It moved about a quarter of a turn more than it was to start I think I’m going to go back to that point and try again.
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Old 10-01-2020, 06:08 PM   #13
Daba
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Default Re: Carburetor backfire

I opened up the points and BINGO it did the trick. Thanks again for all your suggestions.
Daba
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Old 10-01-2020, 09:25 PM   #14
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Default Re: Carburetor backfire

Glad to see that fixed it for you.
George
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Old 10-02-2020, 03:28 PM   #15
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Default Carburetor backfire

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Glad to see that fixed it for you.
George

Love the graphic. Yup, Iíve definitely had that feeling after finally getting the car to start when it was broken in the past.


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