Backfiring

[oldnslow]

Have a 1941 DC-65 that has recently picked up the bad habit of occasionally backfiring but only between 1500 & 1700 rpm. Replaced wiring harness (had excessive mag drop and that has stopped) & checked for vacuum leaks. #3 cyl. runs about 100 degrees cooler then the rest, but compression seems to be about 74/80. Played around with the mixture a little but didn't change anything. I figured after 1000 rpm the mixture is out of the picture anyways. Have also swapped the plugs around to no avail. It starts good and idles good just an occasional pop when moving the throttle between 1500 to 1700. I'm stumped so any suggestions would be appreciated.

[mohawktipi]

Re: Backfiring

Check exhaust gaskets. May have a leak.

[Mark Julicher]

Re: Backfiring

And if the exhaust gaskets are OK, check the intake rubbers very closely.

[Pearson]

Re: Backfiring

Stuck valve on number three cylinder.

[Carrieve]

Re: Backfiring

Where is it backfiring from? From the exhaust or through the carb? If it is through the carb then the engine is probably running lean. Things like vacuum leak, distributiuon tubes, valve hanging up or maybe even a weak spring. If it is coming from the exhaust it is probably rich. This usually is rare because with a carb it would probably affect all cylinders. Thses are just old rule of thumb things that work on most "infernal" combustion engines.

[Edwin Otha]

Re: Backfiring

Check the intake gaskets at the elbows. They dry out and pull in.
EO

[magman]

Re: Backfiring

One way to check the induction system is with a leaf blower or pressure side of a Vacuum Cleaner. Plug into exhaust & seal with duct tape (what else?)
Squirt a soap & water solution at possible leaks. With winter near it's a good
idea to check the muffler while hooked up. You may find several problems with this technique.
ron

[flyguy]

Re: Backfiring

The internal combustion engine vocabulary needs a new word or two. When we say "backfire," no one knows if we mean the foof and fireball that comes out the intake, or the loud bang from ignited fuel/air in the exhaust.

As I understand it "backfire" originally meant "to burn back through the intake," but you hear it more often, for a long time now, referring to the kind of result that you get if you accidentally turn your ignition off for just an instant while the engine is running.

Here we have a chance to improve the English language.

DC

[3Dreaming]

Re: Backfiring

I thought Backfire was through the intake, and afterfire was through the exhaust. I think it is mostly people that don't know what they have that uses the term incorrectly.

[Dick Smith]

Re: Backfiring

More than you wanted to know...!

http://en.wikipedia.org/wiki/Back-fire