"popping" on decel

[E. Murray]

Hey,
All seems to be well mechanically except that there's a popping noise through the exhaust when I let off the gas when under way. It is a repetitive "pop pop poppoppop pop" sound like a small backfire (but more of a dull thud). I just replaced most of the ignition stuff, so what does it sound like? New to the carb thing, so I tend to blame it for everything... might as well get all the info first this time .

 

[Jeff Jamison]

If you are running points,try changing the condensor
Jeff

 

[Jay Tabor]

it is caused by an exhaust leak, got a pin hole somewhere, oxygen sucks in and reburns the exhaust gasses

 

[E. Murray]

Exhaust leak where (at the header?)? I would think the gasses would be burnt enough to not be able to react with straight air. Am I running too rich maybe and expelling unburnt fuel? It sometimes diesels after it's been running...bad valve or something?

 

[Robert Mann]

Have air injection???
Disconnect it and it should stop. Just remove the supply hose for the right side and put it on the left side, then remove the belt from the pump.

 

[E. Murray]

No air injection... Why would it do it only on deceleration in gear (i.e. engine braking)? If I push the clutch it quits.

 

[Baradium]

Dieseling can be caused by a number of things, engine hot spots, timing too advanced, plug too hot a number, too low of a grade of gasoline, etc. Is the noise like the you get from the big tips on the exhaust? Maybe an exhaust leak like previously mentioned?

 

[E. Murray]

So it sounds like the two problems aren't necessarily connected. I think my timing is too far advanced so that may be the dieseling problem. The exhaust was new behind the manifold when I bought the vehicle last year; that's why I was asking about the mechanism in an exhaust leak that can cause the problem. Maybe I need to go over the exhaust more closely... I didn't mean to question Mr. Tabor's diagnosis; I'm just curious how it works. Thanks for all the help.

 

[Baradium]

Some exhaust systems just pop. Varies with muffler etc, but some of them always pop on decel.

 

[Jay Tabor]

static ignition timing has nothing to do with 'dieseling' when ignition is turned off. or any other time either.
what element is needed for combustion to take place?
Oxygen!!
when you decelerate the engine in gear, the engine is sucking in fuel, understand?
the engine's cyls dont burn all that fuel completely, because the throttle is closed. so- little/no air gets in, got it?
so- that means all that fuel gets pumped into the exhaust pipe unburned.
some were, somehow [ one of lifes mysterys] air [ OXYGEN] gets into the exhaust pipe, and mixes with the hot fuel laden exhaust gasses and goes -BOOM! got it?
when you coast to a stop, out of gear, the engine isnt sucking fuel, so you dont get gas reigniting in the exhaust pipe.
write this down and memorize it, there will be a test!

 

[75SII]

vacuum leaks?

Mine popped real bad on decel 'til I fixed a few vacuum leaks and it hasn't popped since. I had leaky diaphragms in the vacuum advance, throttle pull-off and egr valve.

 

[David Banner]

Execellent explaination John. Your explaination also explains WHY the airpump was added to lower emissions! A pox on anyone who removes their emissions equipment (with the possible exception of those switching to EFI) PRETENDING to make their vechicle run better.
Point in fact I recently reinstalled all the cut and plugged vaccum lines (done by the PO) on a recently acquired '75 S11 345--it runs noticably smoother at idle and I certainly don't notice any loss of power on the highway!

Sorry if I've hijacked the thread...

Moderators feel free (I'm sure you do ) to move this post...

 

[Jay Tabor]

David yes the A.I.R. pumps were/are great emissions reducers for free! injecting air into the exhaust pipes to afterburn fuel! heck yeah leave em on to fool the smog machines into thinking the engine isnt polluting much. my only gripe is the pumps and manifolds were in the way of changing plugs on IHV8 s.
except of course the defective'gulp' valves would cause the mufflers to explode!!!!!!!!!!!!

 

[pandorasboxg]

mine deisles when turned of i think (that is when it keeps turning over after you "turn it off"?) what does cuase this and how do i fix this? i have never seen a car do this before i started driving my scout but this is also the first car with a carb i have driven are these related the few people i have talked to havn't said much but " your timing must be off" but im with you john how does this happen with the ignition off? mine also pops sometimes on decel but its pretty quiet so i just ignore it i just don't like how it turns over sometimes when i turn it off

 

[pandorasboxg]

another symptom is if i coast down hill for a long time in gear but no throttle as soon as i get back on the pavment or what ever i have no power at all....the only way to accel is to pump the gas....i don't know if the two are realated

 

[Jay Tabor]

lower the idle speed that usually stops/eliminates the 'run on', or dieseling. if the carb is leaking fuel internally that also will make it 'run on'
why you have to pump the throttle several times after long decel is caused by accelerater pump not working. why? I dont know could only guess

 

[Paul "Misterfixit" Schulz]

at least yours is only popping - my ol c1200 international with the 281 - 6 cyl had a cracked exhaust manifold and on a down hill stretch with the foot off the gas it would easily blow a 5 foot flame out of the top of the 4 inch exhast stack, i could see it in my mirror as it was happening! the first time i saw it on me and my laundry man knew how scared i was.