popping through exhaust

[B5charger]

What causes the popping sound through the exhaust when I take my foot off the throttle?  Under load there is no problem, only when I take my foot off the throttle at speeds of 30mph +.  Is it a timing problem?

[resq302]

Could be timing a little off or distributor bushing is wearing causing excessive play.

[B5charger]

If the timing is off, should I advance it more or retard it?

[resq302]

Hopefully Ron could chime in here but I would say it needs to be retarded just a tad bit.  Almost sounds like it is firing late when the exhaust valve is opening causing the backfire through the exhaust.

[B5charger]

well I just tried advancing the timing and started to get pinging.  Then I retarted it and the pinging is gone but the popping is still there with the throtlle off at cruising speed.  There's no popping at low speed (in town).  Could it be a leak in the exhaust?

[Musicman]

Your fuel mix may be a little on the rich side...

[nascarxx29]

Vacuum leak will cause you to lean out.Also check heat riser operation

[oldschool]

exhaust leak...

[maxwellwedge]

Bad sealing valves, rings. Do a compression and a vacuum check as well.

[B5charger]

An exhaust leak I can check (covering the exhaust and listening for a leak elswhere)? But could it be because of too lean or too rich? how can I adjust it either way?  It's a Carter 2BBL.

[FLG]

Too rich or an exhaust leak will cause the issue, or an exhaust leak by itself could as well.

[B5charger]

Well I turned out (counter clockwise) on the air/fuel mixture screws at the front of the carb about 3/4 turns.  They are now about 2 1/4 turns open and it seems to have done the trick.  The only question I have now is whether I made the mixture more rich or more lean?

[Brightyellow69rtse]

out is rich

[FLG]

You sure your backfiring from the exhaust not the carburetor?

[B5charger]

I'm not sure if it was backfiring.  It was more of a popping sound; not a shotgun bang.  The closest sound I can think of comes from bikes as they let off the throttle.  You hear the same pop,pop,pop etc.  Whatever it was, it's gone now.  So I take it that I richened the mixture a little?

[Musicman]

Sounds like you were popping out the carb then 

Either way, your good to go now right

[firefighter3931]

A popping sound out the exhaust is a lean mixture. 


Ron

[440]

I thought it's because it gets loaded up with fuel and continues to burn in the exhaust....

[Musicman]

I thought it's because it gets loaded up with fuel and continues to burn in the exhaust....

I would have to agree... popping exhaust is usually a sign of a rich mix, timing issues, or worn valves.   

[FLG]

A popping sound out the exhaust is a lean mixture. 


Ron

Generally any overly rich mixture will flood the exhaust with unburt fuel and cause a backfire.

I usually never disagree with Ron...

Id imagine it would be possible if the idle circuit is very lean it never burns in the chamber and just goes in the exhaust until it collects enough to backfire?? 

[B5charger]

Well if Ron says the popping is due to a lean mixture and turning the mixture screws out a little more enriches it.  It did stop the popping, but it does seem to make more sense the other way around.  Can you help explain Ron?

[firefighter3931]

Can you help explain Ron?

Sure 

The original quote stated :

"What causes the popping sound through the exhaust when I take my foot off the throttle?  Under load there is no problem, only when I take my foot off the throttle at speeds of 30mph +.  Is it a timing problem? "

The problem exists with the throttle blades closed under full load....so it's not a main jet issue. The idle jets & circuit come into play in this scenario so when the idle mixture was fattened up (richer) the "lean pop" was eliminated. Other conditions that will cause this occurance is leaky exhaust where outside air is sucked into the mainfolds creating a lean condition and popping.

Both are lean conditions created through different mechanisms but the results are the same.


Hope that helps....



Ron

[Musicman]

There's also no air entering the manifold when the throttle plate closes either... When you close the throttle plate a high vacuum is created in the manifold which pulls a rich fuel mix in through the idle circuit. The rich fuel mix does not burn completely in the engine because there is not enough air, but when the exhaust valve opens there is a supply of fresh air and it ignites causing an exhaust backfire (afterfire).

[FLG]

Yes but an overly lean condition can cause the mixture to not ignight and pool up in the exhaust as well 

[Musicman]

Yep that's true, just as Ron said... but usually a Lean mix situation will result in both backfire (intake) and after-fire (exhaust), where as a momentary Rich mix will result in after-fire. Whether or not the vehicle was popping through the carb, or through the exhaust, or both, doesn't really matter at this point... the problem was fixed. This thread has provided some good information for future readers who may have a similar issues however, so all in all 

[440]

Would a higher octane also reduce the popping as it would burn more efficiently ?

[B5charger]

I don't think it's an issue of ignition as much as the amount of fuel.  I think Ron's explanation painted a pretty good picture of what's going on.