Down Hill Backfire

[barkleydave]

Anyone else have this going down hill. On a recent tour in Eastern Ky we were in the foothills. Very steep winding roads. Going down hill was worse than going up. I would have it in 2nd and the engine RPMs would be way up and then it would begin to backfire through the exhaust. Obviously I was over rev. the engine. Had to use more brake than I like to keep RPMs down. Sure glad I had just finished my brake job complete rebuild with Cast Iron Drums. No brake fade at all!

So was it just over revs or should I be concerned? Your thoughts.

Thanks

[BRENT in 10-uh-C]

Generally speaking, the reason for the backfire would be due to an exhaust manifold/pipe leak.

[C26Pinelake]

Were you the only one in the group that experienced that problem? I suspect as Brent stated that you have an exhaust manifold leak. Wayne

[George Miller]

Lean mixture. Most of the time vacuum leak.

[johnbuckley]

Many cars backfired when decelerating fiercely. At our local race track in the 1960s Lotus Cortinas and the 7 litre Ford Galaxies did it on the approach to every corner. They were set up absolutely correctly to win races but like all carbs the setting of the carb when your foot is lifted off is for low rpm, low vaccum. Have the throttle closed but high rpm ain't the setting any jet wants for smooth running

[MichaelSchwarzer]

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Opening the GAV a little bit more at high rpm without actuating the accelerator at steep downhill drives helps. Michael

[barkleydave]

I went at it and checked timing and cab adjustments. No change it still goes pop pop pop from high RPMs backing off going down a hill. Engine otherwise runs perfectly. I tour in hilly areas it has developed during the last tour. I adjusted the GAV and it helped I am not going to worry about it. It runs fast starts great and idles smooth.. enough for me on any old car! Thanks!

[steve s]

What's going on is that when the engine is racing with the throttle closed (like when going down hill) the pistons' power strokes are coming fast and furious, with the spark plugs trying to ignite the fuel/air mixture for every one of those strokes, BUT the only source of fuel is the little bit that can be sucked through the poor anemic little idle jet. Thus, the mix is too lean to ignite, and the unburned fuel is pushed on out into the exhaust system. where (for some reason) the conditions are more favorable for ignition/pop-pop-pop. Factors that mediate that favorability include (1) a source of fresh oxygen on the exhaust side (as Brent mentions) or on the intake side (as George mentions), (2) whether the carb is set up on the lean or rich side of the normal spec range, (3) how strong the spark is, and (4) all the other vagaries of what's going on in the combustion chamber. The quick fix is to open the GAV going down hills, but this gets tiresome fast; I've seen it necessary to open the GAV two full turns to shut down the popping. I've seen the muffler blown apart when the problem was ignored. Otherwise, minimize downshifting and the use of engine braking to only that absolutely necessary to save your regular brakes; this can be a tough, even risky, tradeoff call to make. If your problem is very serious, you will also notice the popping during that little time interval when your foot is off the throttle during normal gear shifting, especially if you've wound out the rpm of the lower gear.

[barkleydave]

Nice explanation

[Marshall V. Daut]

Check everything recommended so far, but also rotate the muffler clamp so that the gap between the two halves is not facing the throat of the carburetor. Upon serious deceleration (such going down steep mountains at speed), the back pressure increases and can force its way out between the exhaust manifold and the muffler flange. The carb sucks in the hot gas and causes the popping noise you describe. You may not hear an exhaust leak at this juncture under normal driving conditions, but it doesn't take much of a leak under heavy deceleration to force these gases out and into the carb's throat. Just rotate the clamp so that the gap between the two clamp halves faces away from the carb's throat (more towards the hood side panel) and see if that helps.
But do also go through the checklist of other suggestions offered so far.
Marshall