The Ford Barn

Fred K-OR · 08-04-2015, 09:32 PM

When I go down a little hill and have it in second gear and run on compression, it will backfire out of the carb and also a little out of the exhaust. What causes this? GAV not set correctly??? Valve problems?? Timing??? Spark is advanced. Would appreciate some ideas. It is not a big problem but just wanted to know.

marc silva · 08-04-2015, 11:30 PM

I'm no expert but in my experience and having the same issue at times, I opened up the GAV a little and it seemed to help in my case. My thought is it is running a little lean while off the gas.It also seems to help the "wants to stall while coming a stop" issue as well.

Brentwood Bob · 08-04-2015, 11:33 PM

Carbon buildup? Plugs clean?
Bob

Marshall V. Daut · 08-04-2015, 11:45 PM

Rotate the exhaust manifold clamp so that the gap closer to the carb's throat is not directed at the carb's air intake where the choke butterfly is located. If there is the slightest leak between the exhaust manifold outlet and the flared muffler pipe juncture, escaping hot exhaust gas is sucked into carb's throat during compression (such as when going down hill or suddenly backing on hard acceleration) and causes the popping you are experiencing. Loosen the clamp's two nuts and rotate the clamp's gap away from the carb's intake throat, more towards the hood side panel. See if that helps.
If not, then the fuel mixture is probably too lean. But my money is on hot exhaust gas being sucked into the carb, a very common Model A malady.
Marshall

P.S. · 08-05-2015, 09:32 AM

Manifold gasket leak causes this.

Benson · 08-05-2015, 10:06 AM

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A hole in muffler will cause back fire on deceleration as others have mentioned manifold leaks above. Any hole where air can be drawn into the muffler will do it.

The closer the leak is to the manifold (hotter conditions) will make ignition of vapor easier.

That is why the hole at the end of tail pipe is usually not a problem as the gases have generally cooled down by the time they get to the end of the pipe.

Mitch//pa · 08-05-2015, 10:08 AM

how do you know its poping out the carb?
maybe you just have a small exhaust leak

BILL WILLIAMSON · 08-05-2015, 04:46 PM

Coughing back out of the carb is usually when it's LEAN & you're ON the throttle, it has a "different" SMELL & sometimes lets a smoky vapor into the car.
Bill W.

08-04-2015, 09:32 PM	#1
Fred K-OR Senior Member Join Date: May 2010 Location: Stayton, Oregon Posts: 3,806	What causes back fire out of carb on compression When I go down a little hill and have it in second gear and run on compression, it will backfire out of the carb and also a little out of the exhaust. What causes this? GAV not set correctly??? Valve problems?? Timing??? Spark is advanced. Would appreciate some ideas. It is not a big problem but just wanted to know. __________________ Fred Kroon 1929 Std Coupe 1929 Huckster

08-04-2015, 11:30 PM	#2
marc silva Senior Member Join Date: Apr 2013 Location: santa rosa ca Posts: 257	Re: What causes back fire out of carb on compression I'm no expert but in my experience and having the same issue at times, I opened up the GAV a little and it seemed to help in my case. My thought is it is running a little lean while off the gas.It also seems to help the "wants to stall while coming a stop" issue as well.

08-04-2015, 11:33 PM	#3
Brentwood Bob Senior Member Join Date: May 2010 Location: brentwood, ca Posts: 4,241	Re: What causes back fire out of carb on compression Carbon buildup? Plugs clean? Bob

08-04-2015, 11:45 PM	#4
Marshall V. Daut Senior Member Join Date: Apr 2011 Location: Davenport, Iowa Posts: 2,110	Re: What causes back fire out of carb on compression Rotate the exhaust manifold clamp so that the gap closer to the carb's throat is not directed at the carb's air intake where the choke butterfly is located. If there is the slightest leak between the exhaust manifold outlet and the flared muffler pipe juncture, escaping hot exhaust gas is sucked into carb's throat during compression (such as when going down hill or suddenly backing on hard acceleration) and causes the popping you are experiencing. Loosen the clamp's two nuts and rotate the clamp's gap away from the carb's intake throat, more towards the hood side panel. See if that helps. If not, then the fuel mixture is probably too lean. But my money is on hot exhaust gas being sucked into the carb, a very common Model A malady. Marshall

08-05-2015, 09:32 AM	#5
P.S. Senior Member Join Date: Jul 2012 Location: California Posts: 1,696	Re: What causes back fire out of carb on compression Manifold gasket leak causes this.

08-05-2015, 10:06 AM	#6
Benson Senior Member Join Date: May 2010 Posts: 2,595	Re: What causes back fire out of carb on compression Sponsored Links (Register now to hide all advertisements) A hole in muffler will cause back fire on deceleration as others have mentioned manifold leaks above. Any hole where air can be drawn into the muffler will do it. The closer the leak is to the manifold (hotter conditions) will make ignition of vapor easier. That is why the hole at the end of tail pipe is usually not a problem as the gases have generally cooled down by the time they get to the end of the pipe.

08-05-2015, 10:08 AM	#7
Mitch//pa BANNED Join Date: May 2010 Location: Bucks County, PA Posts: 11,454	Re: What causes back fire out of carb on compression how do you know its poping out the carb? maybe you just have a small exhaust leak

08-05-2015, 04:46 PM	#8
BILL WILLIAMSON Senior Member Join Date: Dec 2010 Location: FRESNO, CA Posts: 12,560	Re: What causes back fire out of carb on compression Coughing back out of the carb is usually when it's LEAN & you're ON the throttle, it has a "different" SMELL & sometimes lets a smoky vapor into the car. Bill W. __________________ "THE ASSISTANT GURU OF STUFF"