backfiring

[ruckman101]

Can an exhaust leak cause backfiring? Or a drop in idle? Had low idle issues before Maupin, tightened the loose cut off solenoid and thought I had the problem solved based on the drive up from Eureka, but it seems to be back.

Nothing loose on the carb. Dwell at 50, timing is right, idling fine at the moment. But does seem poppier than I remember it at the exhaust.


neal

[bajaman72]

I had a crack in my flange that was causing pops from the exhaust.

[sped372]

Exhaust leak will give you backfiring on deceleration (engine braking down a hill).

[ruckman101]

Sheesh, just took it for a test run decelerating down a steep hill and just fine. Idle stayed up, no worries. The only thing I did was pull out the Idle adjusting screw, the big one, and put a film of axle grease onto the little rubber gasket on it with the thought that since it spins so easily, maybe there was a bit of a vacuum leak there.


neal

[vdubyah73]

take a wet rag, ball it or fold it, try to plug the tail pipe while it's idling. if you can stop the engine you got a tiiiiiighhhhtttt exhaust. if you can only slow it down and you can't hold the rag tight, it may be leaking but you might not hear it over the rag leak. of course, if you can stop the tailpipe ya got a leak. you may not have an exhaust leak until you reach highway hot temps. around town hot and highway hot is a pretty big temp spread. it can also take awhile for the exhaust to get hot enough to start popping off unburnt fuel/air charges in h/b, pipes or muffler. if your muffler is cheap or seen better days it can pop seams or just start shredding where the rust is.
here's why it happens, and why we have decel valves. the idle fuel/air mixture can't be fully burnt under certain conditions of sudden or extended decel, highway hot exhaust keeps it slow burning on the way out. introduce fresh air, through an exhaust leak, that mix wants to explode. a decel valve, generally speaking, doesn't allow the throttle to snap shut. kinda just eases off instead. it reduces emissions by maintaining a burnable f/a mix for a portion of decel time.

if it only does it under full decel it's more likely just a leak in the exhaust. in my mind, there are some leaks i'll tolerate, for a while, waiting for warmer weather..... in january.

[vdubyah73]

if this is the rebuilt engine, might check torque on the rocker box head studs.

[ruckman101]

Not rebuilt, stock exhaust, and yes, it did occur to me that an exhaust leak could vary depending on temp, and that after a bit of in town errand running, it could be much different than a run down and up the hill. Keeping my eye on it.

Thanks everyone,
neal

[SlowLane]

I had a crack in my flange that was causing pops from the exhaust.

Exhaust ... popping ... crack.
Must...resist...fart...joke.

[Amskeptic]

I had a crack in my flange that was causing pops from the exhaust.

Exhaust ... popping ... crack.
Must...resist...fart...joke.

Must. Resist. Deleting.
Colin