backfire on carbureted '72

SnakeDoc · 10-25-2004, 10:51 AM

I've got a '72 911T with Webers.

Had really gunked up carbs, that's been fixed by a local good shop.

It continues to backfire on rapid deceleration, after coming off a heavy acceleration period. These backfires are like gunshots, not the pleasant burble of engine breaking, at all.

I'm a newbie. Is this normal? I think not. If not, though, what aside from dirty carbs could explain? It's supposed to have been tuned up before I got it, and is accerating noticably better since the carb rebuild.

Timing is on my mind, and I need to buy a strobe light. Feeler guages for the points, I do have.

Thanks for any help.

Tom in Memphis

klaucke · 10-25-2004, 11:15 AM

I would say timing. Keep in mind that even if you set your timing at idle with the right points gap, it could be your advance that is screwed up. That and you could be running a little lean, although I think a little backfiring on decel is acceptable and "normal" depending on application (although I realize yours is more serious).

jpnovak · 10-25-2004, 11:25 AM

Could also(and more likely) be running way too rich and air leaks in the exhaust system. Usually between the head and Heat Exchangers.

Nitrometano · Nitrometano's Garage

Yeah anoter Porsche with nitro!!!
jeje

David W · 10-26-2004, 08:47 AM

I had the same problem with my 72e, MFI, boy the backfires were LOUD. It was a leaking muffler, when you back off, air (oxy) is sucked into the system, when you get back on it the engiens run so rich, so bang! So I woud ck the timing and then exhaust leaks.

SnakeDoc · 10-26-2004, 01:21 PM

That's the first thing Pat Williams suggested, muffler hole, when I complained about it before the carb cleanout. Guess I need to have a look for holes. Would lousy heat exchangers qualify for that effect, you think?

snowman · 08:59 PM

Back fires are from one of two places. one is out the exhaust the other is out the carbs, less likely. Which do you really have??

I would suspect you actually have backfires out the exhaust, much much more common. THis is caused by two things. One is an air leak into the exhaust. Without an air leak you cannot have back fires in the exhaust. You MUST have an approx 12:1 A/F ratio for an explosion. Without the air you CANNOT have an explosion. So this is the first place I would check. Leaking gaskets, holes in mufflers, pipes, heat exchangers. The second condition necessary for backfires in the exhaust is excess gas, ie unburnt fuel. Leaking floats, bad valves, whatever may be responsible. LEaking valves can be rulled out by a leakdown test. Carbs are the last thing to check.