Backfiring Problem

[parsok]

I just picked up a '80 and it has a back firing problem that manifests itself when taking my foot off the gas. The faster I take my foot off the louder the backfire - thought I had blown the muffler off once

Apparently the previous owner's son had 'rebuilt' the engine but could not tell me exactly what had been done. I believe it was rebuilt to the normal spec without any modes. I intend to replace the carb with a 32/36 DFEV and replace the intake and exhaust manifolds to start with then change out the head for a 1800 or 2000FI.

I'm hoping it related to the existing carb (leakage, loose connection) or related to the manifolds, that way it would be eliminated on the change out.

I'm wondering if any one has insight into the potential cause?

Thanks in advance.

[TX82FIAT]

Parsok, The backfire could be from many things including carb, timing, valve duration/overlap, compression, exhaust pressure, fuel, mixture....... the list of what could cause a backfire, detonation ping or knock is pretty long. Start by identifying if the 80 is indeed stock given the rebuild and if the timing is set correctly. If the engine was rebuilt and the timing belt is a few degrees of or ignition off a little that could cause issues. Have a friend stand by as you rev up the car and let off the gas to see if you hear/feel/see something in a particular location prior to the backfire. sorry i don't have more details.

[124JOE]

pull all your plugs one at a time to see if ones bad "oil,burned,ruined"
look at all the wires as well,both ends.just for kicks
do a compresion test
get a timinglight
learn how to set the 2 screws on your carb

set at 10*buy the way 28*max on timing"thats reving it up"


i think you have a bad plug
but these things we all need to know
well those with carbs

[parsok]

Question - the car idles fine and accelerates smoothly, besides the fact that it is an '80 stock. If the timing were off would it not also affect these two measures of performance?

[RoyBatty]

By "back firing", do you mean the engine is firing out the exhaust?
If so, as described earlier, you need to find out why the fuel/air mixture is still burning once the exhaust valve opens.
Something in the fuel metering system that is not leaning the mixture on release of the throttle?
Ignition timing incorrect in that power off condition?
Valve timing or sticking valve?
Is your distributor a vacuum advance model? If so is the vacuum portion working properly?
Good luck

[parsok]

i've decided that I'm going to replace the intake, carb and exhaust(maybe header) first and then see what i get.

Thanks to all for your advice

[bradartigue]

i've decided that I'm going to replace the intake, carb and exhaust(maybe header) first and then see what i get.

Thanks to all for your advice

That's a lot of work, you could simply eyeball that the following are true:

Engine at TDC (crank aligned to small pointer)
Camshafts aligned with holes
Rotor aligned roughly with #4

If yes then

Timing light at crank shows 10 degress BTDC at 850 RPM

If yes then

Check plug color, white=lean, black = rich, brown = correct

Then

Tune carburetor mixture screw

and so on

All of the above is 30 minutes of work at most.

[parsok]

I was doing some reading on Gulp Valves on this forum and think perhaps this could be the problem. I checked online and found this description:

A gulp valve sense a change of vacuum in the intake manifold. The gulp valve is intended to stop the air flow over the exhaust valves when a rich mixture is added to the system, i.e. whenever you let up on the gas pedal. If the gulp valve does not operate correctly, then this rich mixture would hit the exhaust manifold with the forced air and explode in the exhaust system.

Any opinions?

Dave

[majicwrench]

Other than the first sentence, that description of gulp valve operation is not accurate. The vavle certainly is not intended to "stop the air flow over the exhaust vavles".
If you have backfiring, you have fuel and air in your exhaust, burning. Exhaust leaks will do this. So will non-functional items, like the gulp valve, and idle up vavle on carbs.

[124ADDHE]

My 1800 on solex 40's backfire on the old fiat, much morso going from 1.75" exhaust to 2.25". You always hear harleys and old racecars backfire on decelleration; I always thought it was due to the rich running @ higher rpm with the throttle plates closed, i know some carbs have special provisions for preventing the backfire.....