Exhaust backfires- ignition throuble?

I'm getting an occasional pop out of my exhaust sometimes, if I've been deep in the throttle for a short burst, or at highway speeds for a few minutes and let off the gas. I'm never hearing backfires when the power is on, always when I get off the throttle completely.

I have noticed the last two times I tuned the engine (Last week, and mid August) that the initial advance has been up around 16* or so- I have been very careful to set it to 13* as per the distributor manufacturer's instructions, based on my engine specs. In August, i thought maybe I bumped the housing while tightening it and that was why i was off. Right now I think that the backfires and my advance being too high are related to ignition trouble. My first guess is my advance weights are sticking or slow to return. I also note that when the engine if first warmed up, it will rev like a small block, it winds up and falls off quickly. When it's hot and running for half an hour or so, it winds up quick as ever but the rpms fall off comparitively slowly. All this leads me to suspect my ignition curve in general and my advance weights in particular.

Any input on this backfire issue?

 

Possible running rich and burning raw fuel that has passed into the mufflers?o No:

 

sounds almost like a backpressure situation...not "backfiring"....o No:

 

Hmmm...if anything, I should be a tad lean, I should think, with the stock Edelbrock AFB 800 cfm carb (which I am now set against due to fuel slosh problems) and the new 2.5" exhaust. I have yet to re-jet the carb since having the new exhuast installed (read- lazy). I have richened it up a bit at idle, but this isn't an idle only situation. I tuned the carb to max vacuum at idle, then turned the needles back in about 1/8 turn. Right now it pulls just under 20" Hg with the needle quivering perhaps 1/4" either way. Perhaps I have a carb problem in general. Of course, that could be timing too, rather than the carb, allowing gas into the exhaust. The engine purrs, though, even during these pops.

maybe regional differences in terminology are against us here, Adam...I do not mean that the backfires are explosions as the norm, I mean I can hear it burble as the norm, but once or twice I have heard a definite "PAP " sound come from the exhaust. Certainly not like a machine gun :grin:, more like a muzzle loading musket loaded by a tranquilized tree sloth.

feel free to quote that:laugh:

Never having had a real performance oriented exhaust before August, I'm not 100% sure of what to expect. The pipes are damn near straight pipes after my headers; I can see through the mufflers if you angle them just right which is OK by me as long as they're legal- which they technically are. The neighbors may disagree but the DOT says "fine".

I am going to re-tune the engine tomorrow and see where the timing mark lies...I set it Sunday at about 13* initial. I am very curious to see where it is now. The balancer is 4 years old, maybe 20K miles on it and the engine. If it's moved again... When I installed this distributor, i checked the timing and took a small sharp chisel, and marked on the distributor housing and timing cover where 13* should be oriented, then put blue paint on the mark. It's dead on that mark, just like it's been every other time I tuned it- but this mystery advance thing makes me wonder about some type of cause/effect relationship. Certainly I will not rule out fuel management. Thanks for the suggestions, more input is always welcome. Car still goes like a 4000 pound scalded cat though:TU: But I prefer to be on top of any potential problems. I actually might like an excuse to tear the engine apart, but it's too late in the season to build my new heads. Next spring.

 

Possible timing chain stretch.. 3* is a whole lot less than one tooth off. How old is the chain and what brand?

Just something else to consider..

 

im probably way off, but isnt your advance when your letting off of the throttle also controlled by the vac canister. it may be too much or too little advance when backing off of full throttle, but cant the vac advance also be to blame, as well as the advance weights not returning?

 

Typically the vacuum signal for the vacuum advance is typically taken from a port that is just above the throttle plates, so that when the throttle is closed there is no vacuum advance.

When I've had this burbling problem in the past (forgive me if I don't quote the thing about the three-toed sloth musket whatever), it's been too rich- unburnt gas is combusting in the tailpipe or something.

Maybe also your timing is too retarded, so that combustion isn't 100% complete by the time you open the exhaust valve, so some of the combustion is occuring after the valve is open. A solution for this might be slightly lighter mech advance springs, to give it slightly more advance as the RPMs fall.

-Bob Cunningham

p.s. Chris, I'm planning on going to the track tonight (Wednesday Oct 1st)

 

Grant- double roller from TA, 4 years old, about 20K miles on it.

I'm not ruling out any possibility at this point. I'll wrestle with it tomorrow or today if the rain holds off

 

A back fire in the exhaust is caused by unburned fuel and O2 going into the exhaust system then being ignited.

Why it happens on decel...as you take you foot off of the gas, the engine vac jumps up higher than normal since the engine is spinning faster than idle. As vac increases and trottle plates are closed, it sucks add'l fuel in through the carb through the idle circuits until the vac returns to normal. This is normal with all carbs.

What causes the burn in the exhaust...it usually is getting air somehow. Look for a header/exhaust manifold leak. I never saw one caused from timing, but, in the 20 or so years of doing drivability repairs I have been surprised many times.

Does your car have an AIR pump??? The diverter/bypass valve needs to dump the air and not send it to the exhaust on decel.
Yes a carb that is too rich can cause this also, but, it takes unburned fuel AND unburned O2 and heat or spark to cause a fire.

Hope this helps

 

Thinking about this...did this problem start about the same time you replaced the carb?
If so, did the other carb have an anti-stall dashpot on it?
I was replying to another tread on "secondaries not working" and was reminded some of these engines came stock with one on it. That would help control the vacuum surge on decel.

 

Jamie-

I last replaced carbs 3 years ago, the Edelbrock AFB clone is the one I've been using for years. It's an electric choke 800 cfm unit, no emission equipment has even been within ten yards of it.

This cropped up about the same time I went from a 2" exhaust with stock mufflers to a 2.5" mandrel bent exhaust with performance mufflers that should be illegal, in my opinion:grin:

The engine was from a '73 Riviera, but every bit of emission equipment was gotten rid of back in '91 when I made the engine swap.

4 years ago I built this engine, there is no AIR equipment mandated by law for the year car in my area, so that stuff is gone.

This issue only came about in the last few weeks to a month, about the time of the exhaust change. Before that, nothing has been changed with the exception of 8.8 mm plug wires and a new distributor, which was two years ago.

I am tuning the carb at idle by setting timing, idle speed, and a ballpark estimate on needle adjustment. Then I turn a mixture needle until I get max. vacuum, and reset idle speed. Then I do the same to the other needle. Then I back both further in about a qurter turn.

 

hey chris, where in boston are you. im originally from nj, but have an apt near fenway til i finish grad school. itd be cool to check out your car.

 

Joe- this one's easy. if you're in the Fenway, I'm about 2.5 miles from you. Take the Jamaica Way (also called, at various points along it's legth, the Arbor Way and the River Way) to Forest Hills. I'm on the side with the restaurants and bars, not the resident side, of Forest Hills T station. I can see the parking lot next to the T station from my front steps. Pick up the Jamaica way right near Fenway Park, by the cinema, and go past Jamaica Pond. You'd go through two rotaries and go past the Arboretum. Right past there is the Forest Hills exit. Taking Mass. Ave would be a bit bass ackwards, but you can do that, too:TU:
PM me with a time you'd like to stop by and I can tell you if it's a good time for me

 

Do you have any exhaust manifold leaks?
Is the carb dripping out the boosters at idle? (usually float level)
Or, since it's kinda hard to hear it through the internet (maybe hold the mouse closer to the engine....just kidden:grin: ), it really is a minor thing that has always been there and now with better flowing exhaust, you hear it.
The way I adjust carbs...after timing and base idle speed set, turn mixture screws in until engine slows, back out just a tad to bring back up, double check idle speed, double check mixture screws again. Mixture screws always last thing to check if going for perfection since throttle plate movement has a direct effect on the mixture. But, that is getting alittle AR over the whole thing.
Your way of doing it should get you right there though.

Just for kicks, don't go a full 1/4 turn back out and see what happens...may just be enough to make a differance.

Another thought...the 2.0 in exhaust may have had just enough backpressure to keep the vac down on decel. (now thats really guessing)