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Old 07-26-2020, 03:16 PM   #1
rickcioppa
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Default 1930 Sport Coupe 11 MPG, backfire when turned off

I was told I should be getting 16-20MPG, is that true?
I'm getting 11, seems run fine.
I can close the GAV and it doesn't seem to make much difference, I run it about a 1/4 open, if I open it more she starts to run rough. After I come off a long stretch at 45MPH and stop to get gas she backfires, it only backfires under this condition. So I'm thinking I need to rebuild carb (Zenith-1).
Any other thoughts? or should I do something else first?
Thanks

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Old 07-26-2020, 05:37 PM   #2
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

Woof
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Old 07-26-2020, 06:09 PM   #3
WHN
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

Carburetor problem.

We have a 1929 Sport Coupe, car gets around 18 mph around town,

We run with the GAV open about 1/4 turn when car is warmed up. We start the car at full turn open. If I left it at full turn open when warmed up, car would run rough. You can tell it is to rich.

Enjoy.
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Old 07-26-2020, 08:37 PM   #4
rickcioppa
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

Thanks, what should it do when I turn the GAV completely off (clockwise)?
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Old 07-26-2020, 10:02 PM   #5
Jeff/Illinois
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

Our '29 Tudor and '30 Roadster both averaged 20 MPG, running a Tillotson carb
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Old 07-26-2020, 11:06 PM   #6
Chuck Sea/Tac
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

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Quote:
Originally Posted by rickcioppa View Post
Thanks, what should it do when I turn the GAV completely off (clockwise)?
If everything is correct, it will run lean. Hesitation when jumping on the throttle, possible carb pops, and at speed, surging. Also the engine runs hotter.
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Old 07-27-2020, 07:03 AM   #7
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

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Originally Posted by Chuck Sea/Tac View Post
If everything is correct, it will run lean. Hesitation when jumping on the throttle, possible carb pops, and at speed, surging. Also the engine runs hotter.
We also turn it completely off when no longer running car. Donít turn it down tight, just turn it until you feel it stopping. Keeps carb from dripping.

Enjoy.
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Old 07-27-2020, 07:08 AM   #8
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

Check the soot level - color of your spark plugs. Smell the exhaust. It should NOT smell like a diesel. Check color of exhaust. again, it should NOT be black. Both of these are symptons of running rich, which would account for the 11 mpg.
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Old 07-27-2020, 09:02 AM   #9
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

I adjust the GAV for best running at speed while under load myself. I don't mess with it after that. I turn the fuel off with the fuel valve.

If you close the GAV off, it shouldn't pull a load well at all. If it doesn't change the way it runs then its either stopped up with crud or there is and internal leak running raw fuel into the throat, or both.
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Old 07-27-2020, 09:39 AM   #10
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

The "bang" is likely un-burned fuel in the muffler that ignites when oxygen reaches it. You may have a small hole in the muffler or a loose connection at the muffler flange. A rich fuel mixture would contribute to the issue.
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Old 07-27-2020, 09:46 AM   #11
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

Quote:
Originally Posted by rickcioppa View Post
I was told I should be getting 16-20MPG, is that true?
I'm getting 11, seems run fine.
I can close the GAV and it doesn't seem to make much difference, I run it about a 1/4 open, if I open it more she starts to run rough. After I come off a long stretch at 45MPH and stop to get gas she backfires, it only backfires under this condition. So I'm thinking I need to rebuild carb (Zenith-1).
Any other thoughts? or should I do something else first?
Thanks



I had this exact issue. A split second after switching the ignition switch off, would get a loud backfire through the exhaust, loud like a shotgun. Had just gotten the truck back from a repair shop, including replacing the carb. Cause was a totally miss-adjusted carb idle mixture screw. Was loading up the exhaust manifold with unburned fuel, engine stop, heat in the manifold ignited the mixture.
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Old 07-27-2020, 01:52 PM   #12
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

Yep turning the switch off and coasting and then turning it on you get a bang! And then after that a new muffler! I was a dumb teenage way back then, please don't tell my grandkids.
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Old 07-30-2020, 11:11 PM   #13
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

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Originally Posted by rotorwrench View Post
I adjust the GAV for best running at speed while under load myself. I don't mess with it after that. I turn the fuel off with the fuel valve.

If you close the GAV off, it shouldn't pull a load well at all. If it doesn't change the way it runs then its either stopped up with crud or there is and internal leak running raw fuel into the throat, or both.
Over the last 30 years I have run 4 Model all on original Zeniths . Got 18 to 20 mpg on them all. All the GAV's were set at 1/4 turn open. Never touched them other than to pull them out to choke the car when starting.
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Old 07-31-2020, 09:09 AM   #14
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

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Yep turning the switch off and coasting and then turning it on you get a bang! And then after that a new muffler! I was a dumb teenage way back then, please don't tell my grandkids.

In my case I was not turning off off, coasting, and then back on. I would be driving, pull in somewhere, motor idling, and then turn off the ignition, split second later - boom. Was not turning the ignition back on.
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Old 07-31-2020, 10:04 AM   #15
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

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I had this exact issue. A split second after switching the ignition switch off, would get a loud backfire through the exhaust, loud like a shotgun.
Uncle Buck's Marquis would do that also but with the added bonus of dieseling for a few seconds first .
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Old 07-31-2020, 08:57 PM   #16
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

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Uncle Buck's Marquis would do that also but with the added bonus of dieseling for a few seconds first .

You got me good on that one - just now realized the reference was the movie with John Candy as Uncle Buck. Great movie.
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Old 08-01-2020, 07:49 AM   #17
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

Backfire is typically an indication of running lean. A little more GAV may help.

Don't get too crazy comparing your fuel economy to others. I track my mileage very carefully (I'm nutty that way). In about 11,000 miles I have averaged 12.4 MPG and my car runs beautifully. I run a Zenith and do not have any sort of overdrive or modified running gear. My spark plugs come out coffee brown.

I live in Connecticut where there are lots of curves, steep hills, and stop and go traffic. I can't accurately compare my mileage to someone someplace like Kansas. Also, be careful comparing your mileage to anyone outside your home country. Much of the world uses the Imperial Gallon, which is 20% larger than the US gallon.

Make sure it is running well, and your spark plugs show proper combustion. Then just enjoy driving it.

Good luck,

Ken
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Old 08-01-2020, 10:02 AM   #18
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

Use google maps to verify your speedometer is accurate. If the odometer off, your mileage would be off the same percentage. My A only gets 12-14 mpg, fresh motor standard head, 3.54 rear end, 16 inch tires, no Mitchell/etc, spark plugs read good, running a zenith.



If the plugs read good, and motor is OK, not much you can do with what you have.
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Old 08-01-2020, 10:12 AM   #19
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Default Re: 1930 Sport Coupe 11 MPG, backfire when turned off

Some folks refer to an after fire as a backfire. The exhaust should be mentioned if the backfire term is used or there can be a misunderstanding. The actual backfire through the throat of the intake manifold sounds more like a hickup so this leads to the confusion. The actual backfire is a sign of a lean condition under any form of load.

11-MPG is indicating a rich condition unless the vehicle is loaded extremely heavily. A after fire can also be caused by a sticky or worn valve. A faulty ignition switch can also contribute but a lot depends on what type of switch it is.
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