The Ford Barn

howie123 · 06-02-2011, 07:58 PM

I am just beginning to drive my 28 Speedster that is nearing completion. Occasionally when I shift up I hear a very faint backfire while letting up on the accelerator. It is more like a quiet belch than a backfire. When taking the car out of storage this spring I regapped the points which had worn in a bit. I checked the plugs which all had a light coat of black soot. I run with the spark at full advance and the GAV 1/4 turn open. Any ideas on what may be causing this would be appreciated. Thanks. Howie

Gary WA · 06-02-2011, 10:30 PM

check for manifold gasket leaks,carb to manifold tight, wiper no air leaks. letus know the outcome.

Jack '29 Sport Coupe · 06-03-2011, 09:20 AM

I would experiment a little more with the advance lever. Try backing off just
slightly and see if that makes any difference. Mine only requires full advance
at top speed. You also might have a timing issue. Unfortunately, the problem
you describe could be one of a dozen areas that could cause a slight backfire.

Jack

Chris in CT · 06-03-2011, 05:21 PM

Hi Howie, Ordinarily, when I hear of someone having a backfire problem with a Model A, the first thing I go to is gas mixture, because a too lean mixture will cause backfiring on deceleration. However, your plugs are sooty, indicating a too rich mixture! Try closing down that GAV valve and see how she runs. Its possible some unburned gas vapors in the muffler are getting ignited on the upshift...? As Jack has indicated above, too late timing could also be a culprit... Good Luck!

howie123 · 06-04-2011, 03:48 PM

Sponsored Links (Register now to hide all advertisements)

Thanks for all the input. I took a nice ride to a local car show and back. I set the spark at about 3/4 advance and set the GAV just slightly less than 1/4. I am also trying to shift at a slightly lower speed according to the Ford operating instructions. I had only one instance of backfire and I may have revved a little high between shifts. I plan to recheck the timing although I have to think it is pretty close given how good it runs otherwise. I do have a little leak from the throat of the carburetor when parked so I plan to investigate that further. I also plan to check the adjustments on the carburetor. On the subject of shifting speed, the recommended speeds seem quite low to me. Is lugging a problem for an old babbit engine? Again, thanks for the suggestions. Howie

06-02-2011, 07:58 PM	#1
howie123 Senior Member Join Date: Jul 2010 Location: Eau Claire, WI Posts: 113	Backfire question I am just beginning to drive my 28 Speedster that is nearing completion. Occasionally when I shift up I hear a very faint backfire while letting up on the accelerator. It is more like a quiet belch than a backfire. When taking the car out of storage this spring I regapped the points which had worn in a bit. I checked the plugs which all had a light coat of black soot. I run with the spark at full advance and the GAV 1/4 turn open. Any ideas on what may be causing this would be appreciated. Thanks. Howie

06-02-2011, 10:30 PM	#2
Gary WA Senior Member Join Date: May 2010 Location: Clinton,WA/Whidbey Island Posts: 4,104	Re: Backfire question check for manifold gasket leaks,carb to manifold tight, wiper no air leaks. letus know the outcome. __________________ www.whidbeymodelaclub.com

06-03-2011, 09:20 AM	#3
Jack '29 Sport Coupe Senior Member Join Date: May 2010 Location: Cincinnati, Ohio Posts: 798	Re: Backfire question I would experiment a little more with the advance lever. Try backing off just slightly and see if that makes any difference. Mine only requires full advance at top speed. You also might have a timing issue. Unfortunately, the problem you describe could be one of a dozen areas that could cause a slight backfire. Jack __________________ Cincinnati, Ohio

06-03-2011, 05:21 PM	#4
Chris in CT Senior Member Join Date: May 2010 Location: Connecticut Posts: 272	Re: Backfire question Hi Howie, Ordinarily, when I hear of someone having a backfire problem with a Model A, the first thing I go to is gas mixture, because a too lean mixture will cause backfiring on deceleration. However, your plugs are sooty, indicating a too rich mixture! Try closing down that GAV valve and see how she runs. Its possible some unburned gas vapors in the muffler are getting ignited on the upshift...? As Jack has indicated above, too late timing could also be a culprit... Good Luck!

06-04-2011, 03:48 PM	#5
howie123 Senior Member Join Date: Jul 2010 Location: Eau Claire, WI Posts: 113	Re: Backfire question Sponsored Links (Register now to hide all advertisements) Thanks for all the input. I took a nice ride to a local car show and back. I set the spark at about 3/4 advance and set the GAV just slightly less than 1/4. I am also trying to shift at a slightly lower speed according to the Ford operating instructions. I had only one instance of backfire and I may have revved a little high between shifts. I plan to recheck the timing although I have to think it is pretty close given how good it runs otherwise. I do have a little leak from the throat of the carburetor when parked so I plan to investigate that further. I also plan to check the adjustments on the carburetor. On the subject of shifting speed, the recommended speeds seem quite low to me. Is lugging a problem for an old babbit engine? Again, thanks for the suggestions. Howie