Popping/backfiring on deceleration - need a rebuild?

[Fentonite]

My '73 710M has been popping / backfiring pretty loudly on deceleration. I recently installed the SSI, which helped some stumble on acceleration, but didn't really change the deceleration issue. It seems to run fine under power, but I now tend to not downshift anymore, to avoid the loud popping. I had noticed some gas leakage around the carbs, and thought maybe that was the issue. My mechanic looked at it briefly and said that oil is being drawn into the carbs, and that I need an engine (top-end) rebuild. Does this make sense? No valve clatter, runs strong, just pops on deceleration. Thoughts?

[Jimm391730]

The common answer is that air is getting drawn into the exhaust, typically through the flex joints, adding oxygen to the (typically) rich deceleration mixture, and detonating in the exhaust and muffler. replace the stock flex joints with the SAV flex exhaust (http://www.pinzgauer.com/showdetails.ph ... no=SAV6894) and most of the popping/backfiring will go away.

The richness of the exhaust is inherent in a well running Pinz engine; I doubt very much that any engine work (other than the exhaust) is needed.

[Fentonite]

Thanks Jim. That seems much more reasonable and likely to me. I did have the exhaust replaced not too long ago, but maybe it wasn't done right. I'll have it re-done. I appreciate the response.

[stiffler4444]

I've had this going on for a while myself and never bothered to address it. Thanks for bringing this one up, I'll be looking at a flex line overhaul as well!

[Fentonite]

Thoughts on the "oil being drawn into the carbs"? Glad its not what's causing the popping / detonation, but does it need attention?

[Jimm391730]

Thoughts on the "oil being drawn into the carbs"?

That's a very simple statement. What does he mean by that?

The crankcase breather feeds into the intake air; oils and fumes can condense on the carbs. These carbs tend to get ugly dirty, or at least uglier than other carbs on other vehicles that I've had. If this is what he is seeing, then I'd suggest that it is somewhat normal for a 40+ year old vehicle. But if he is seeing something else... who knows.

Vacuum can pull oil into the intake manifold, but not further than that. So I can't understand what "oil being drawn into the carbs" means, beyond breather fumes - how would oil be drawn into the carbs?! So the simple answer would be that if it runs and starts well, runs strong, I wouldn't worry about it. Certainly keep an eye on the general engine condition, oil consumption, check for carb screws loosening (and other places), etc. and enjoy your truck.

[Fentonite]

Thoughts on the "oil being drawn into the carbs"?

That's a very simple statement. What does he mean by that?

The crankcase breather feeds into the intake air; oils and fumes can condense on the carbs. These carbs tend to get ugly dirty, or at least uglier than other carbs on other vehicles that I've had. If this is what he is seeing, then I'd suggest that it is somewhat normal for a 40+ year old vehicle. But if he is seeing something else... who knows.

Vacuum can pull oil into the intake manifold, but not further than that. So I can't understand what "oil being drawn into the carbs" means, beyond breather fumes - how would oil be drawn into the carbs?! So the simple answer would be that if it runs and starts well, runs strong, I wouldn't worry about it. Certainly keep an eye on the general engine condition, oil consumption, check for carb screws loosening (and other places), etc. and enjoy your truck.

Are you a therapist?, because you made me feel better! Thanks.

[Jimm391730]

I'll send you my bill! $$$

[boeing7873]

I've had this popping on desceleration for a long time... also I've had the stumble/stalling on initial acceleration (low rpm). I wonder if they are related. Can it be a valves issue?

[audiocontr]

could ethanol impact of new fuels cause this?

[edj710k]

I also have been getting backfiring going down long steep hills. But it only seems to occur at higher altitudes. I live at 8400ft and the backfiring doesn't seem to happen below like 7000ft. And I hardly need choke to start. So I presume my pilot jets are too big.

[boeing7873]

I believe we have our idle screws way to out. They should be leaner, the problem is you will start to get other symptoms, like lack of power in acceleration from low RPMs if you lean out.
Are your acceleration pump nozzles all throwing a good clean shot of fuel as soon as you start accelerating?
JC

[rmel]

At altitude you will naturally run more rich due to lower Ox and vapor pressure.
So your already running rich at 8,000'. Try using regular gas if your not already
(lower octane). You may want to try increasing your advance a degree or two if
you wheel mostly at altitude -- the burn in the chamber is S L O W E R up where
the air is rare

[Fentonite]

I had the SAV flexible exhaust coupling installed. Problem solved almost completely!

[rmel]

What's really going on is one of two issues, predominately a rich condition
leading to unburnt fuel in the exhaust system. If Ox gets in the exhaust then
the telltale signs of post ignition -- popping. Elimination of the Ox in the exhaust
system with tighter exhaust plumbing may eliminate the symptom but the
cause is still there -- unburnt fuel going down the exhaust pipes. Tuning can
eliminate or at least minimize that.