most common cause of backfire?

Peter Coomaraswamy

Peter Coomaraswamy

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Driving much more often now that temps are below 100 degrees, have dual Webers on a 74 3.0 cs, 5 sp. conversion. When I let off the gas, say going down a big hill, I have a pretty constant backfire/grumble/popping... In need of forum opinion as to what would be the most likely cause/causes- I have pertronix installed and the only other symptom is that the choke(s) really don't seem to do much as it doesn't idle correctly until warm and then sits pretty steady at 10,000 RPM.

Thanks,
 
Arriving Dec. 5th .....let me drive the car for a week and I might have an answer.

Haha!
 
Backfire on over run is usually a lean condition. So as mentined before if your timing is correct and the Pertronix is functioning and gapped correctly, look for vacuum leaks at manifold to head, manifold to carbs, carb body gaskets, and all vacuum lines. If no luck then check your carb jetting specs to make sure they're correct for your engine, and verify correct float levels. Sometimes pulling your plugs to check coloring will also provide clues to which manifold/cylinders are lean. You can also back out the mixture screw a half turn or so (one carb at a time) to see if it cures/isolates the problem.
 
900K is ninety times as high as 10K, which is about 10 times as high as 1K, which is in the ballpark, but probably a little high.
 
As a partial explanation- most modern FI systems add fuel on trailing throttle for better emissions. The reasoning is that suddenly shutting off the fuel to the limit of the idle circuit is not enough fuel for the higher rpms of trailing throttle which causes backfires.
 
After many months fiddling around with timing and mixture on my 74 with dual webers to solve an identical condition I finally replaced my exhaust system. Problem completely solved. Close examination of the old system revealed two hard-to-see cracks in the down-pipes which were likely the culprits.
 
sfdon said:
As a partial explanation- most modern FI systems add fuel on trailing throttle for better emissions. The reasoning is that suddenly shutting off the fuel to the limit of the idle circuit is not enough fuel for the higher rpms of trailing throttle which causes backfires.
Click to expand...

Interesting. Somebody once told me that the E24 completely shutoff fuel when going downhill in gear, while still idling when going down in neutral...
 
Backfire

I recently experienced some backfiring upon deceleration, which came about after I paid an expert shop to tune up my Webers.

I re-adjusted carbs to a more richer (less lean) condition. Problem solved.

Coincidentally, I also bumped up my idle to about 10,000 rpm (from about 7k) but you're already up there in revs, so try the enriching the fuel mix.
 
High revs

Basic laziness.

I changed my fuel air mix and bumped up the idle at the same time.

I read somewhere that either one (too lean and low idle) could cause backfiring, so I changed both.

Haven't found the time to dial back the idle. (World Series, domestic duties, etc)
 
Lots of good stuff here thanks, I think I'll try a couple suggestions, first replacing the exhaust, I have kind of a single muffler; solid but temporary solution since the original was trashed. There is a shop downtown that does allot of custom stuff so I think I can get them to do a two muffler system and quiet her down a bit and that may fix it (I hope) next would be the mixture 1/2 turn on each-one-at-a-time richer to see if that works after she's warm. After that I may have to have it "professionally tuned" so I can start the headaches all over. BTW here is a picture chokes cold.I'll send the warm pics after I run her around a bit. And thanks all again!
 

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carbs when warm

This is what they look like when the car has been driven for an hour.

Also, any opinions on this little issue; this is the second time this has happened in about 400 miles-

The car starts fine but then for some reason"runs out of gas" I have to pour a couple ounces of gas in each float bowl, through the top (Webers) and then she starts and the problem goes away- she'll run all day no problem. I LOVE THESE CARS!!!
 

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That is good and correct, showing choke plates fully open on a warm motor. I will assume they open simultaneously.
Couldn't say why the car won't run when first started. My zeniths seem not to have any gas if left to sit a few days and won't start until I crank enough to pull fuel from the tank. It's standard procedure for zeniths to do this. Drive every day, no problem.
 
Nice picture. Cleaner than my kitchen.
The hose clearly reads "Fuel Injection Hose", and it is being used in a carburetor setup.
Therefore you have a hose mismatch.
 
Agree with arde, the fuel line internal diameter is to small to fill the carb bowls fast enough to replace what the carbs are using. As for the back firing, that is the timing, have you had a new chain fitted or before you owned it? How old is the dizzy?
 
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