engine and drivetrain 38/38 Backfire On A Stock '76

[Anschauer]

Hooked up the nearly-new Weber 38/38 w/Manual Choke and it starts, idles, runs well, rev's as high as I need to go, but in spite of resetting timing (vac line off, motor warm 1400rpm) it backfires heavily on decel.

 

No smoke or heavy fuel smell as I'd expect from excess fuel

 

Carb is stockand  except for a hogged out manifold, so is the rest of the motor except the PO had converted over to an electrical fuel pump (trunk-mounted), black coil, no resistor.

 

The 32/36 which came out was also prone to an occasional backfire but not nearly as bad as the new one.

 

Any ideas helpful.

 

 

[roadhog0]

If you have an exhaust leak it can do this. My old exhaust would do it. Not sure if it's directly carb related

[jrhone]

Don't have an answer for you but my motor did that before the top end rebuild. I always thought it was normal. It wasn't backfire so much as lots if little mini pops that were constant which sounded like a loud burble on decel. I don't notice the same noise now after a new head valve job rockers and a 292 cam.

[jerry]

a little off-topic, but i'm dealing with dieseling on my 38/38 carb with 292cam and 9.5 bathtub pistons.  my idles are down to 140's.  it idles well when it warms up and is responsive, but i have gotten into the practice of switching off the ignition as i let out the clutch in 1st gear.  otherwise it'd diesel for a fair bit.  i do not have backfire issues.  is a backfire an opposite end of the problem spectrum from dieseling?

[BDB]

my exhuast gas recirc tube had a hole in it and it would backfire very loudly on cold mornings.  once engine temp got warm enough, no backfires.  when i replaced the exhaust manifold with new metal gasket, no more backfires.

[Honolulu]

Shotgunning...

What is your fuel pressure?

Any leaks in the exhaust?

Is there an idle shutoff valve, as for the Solex carb?

Have you adjusted timing to see if this makes a difference?

Dwell okay?

Air/fuel ratio checked?

[tim385]

Yeah Id check for exhaust leaks even little ones , get it up on a hoist while running easiest was.

Dieseling 38/38 check your timing .

[Anschauer]

Thanks everyone - yes I am afraid the old Ansa exhaust has leaks so that would be my 1st priority. I don't know my fuel pressure but I can do that next. Idle shutoff no not really. This is a synchronous carburetor with both barrels operating in tandem as opposed to the 32/36 progressive which idled only on the 32mm barrel.

 

Each barrel has an idle mixture screw located below the throttle plate and depending on the setting you it also control how much fuel delivery comes out of the upper jets for acceleration (go to redline or pierce manifolds to see the diagram) but all this is news to me and I am trying to run lean.

 

Yes I did the timing (1400 rpm no vacuum) and I'll recheck it after I take care of the exhaust and the fuel pressure and get a CO meter and set the a/f mixture.

 

Thanks again everyone!

 

Next is getting the '74 tii running again, get some floors and rockers and go down to Hunts point and get me some quickie welding. That car is a pistol when it runs.

 

a taxi paint job and 

Shotgunning...

What is your fuel pressure?

Any leaks in the exhaust?

Is there an idle shutoff valve, as for the Solex carb?

Have you adjusted timing to see if this makes a difference?

Dwell okay?

Air/fuel ratio checked?

[c.d.iesel]

it's an exhaust leak someplace,

 

nothing to do with fuel pressure or timing.

Edited June 11, 2013 by c.d.iesel

[Preyupy]

I agree C.D raw air is getting into the exhaust system somewhere causing the excess fuel in the exhaust to burn (Rich on Decel is normal for any carburetor)

My other question about dieseling is why do people think it is a timing issue? When you turn off the key the ignition system STOPS FIRING THE PLUGS, there is no timing! If it diesels it is because there is air and fuel getting into the cylinders and you have enough heat being produced by compression and residual heat in the combustion chamber (this includes the top of the piston, cylinder walls, valves, glowing carbon deposits etc) to ignite the mixture.

[kbmb02]

My other question about dieseling is why do people think it is a timing issue? When you turn off the key the ignition system STOPS FIRING THE PLUGS, there is no timing! If it diesels it is because there is air and fuel getting into the cylinders and you have enough heat being produced by compression and residual heat in the combustion chamber (this includes the top of the piston, cylinder walls, valves, glowing carbon deposits etc) to ignite the mixture.

It's my understanding ignition timing can contribute to dieseling/run-on, albeit indirectly. Here's how:

 

An engine with moderate to severely-retarded ignition timing will likely run hotter (combustion temperature), and that condition causes the higher residual heat as mentioned above by Preyupy. Add other factors and it runs on, and on, and on, and on.... -KB

[ndog]

^   really hot cylinders dont need spark

along with a worn out carb,,,, overrunage

Edited June 11, 2013 by ndog