back fires when upshifting - dual intake/exhaust

[Powerhouse]

Howdy all...

I recently installed dual 2bbl Prog Carter/Webers on a "moose" dual intake/exhaust manifold to my 218. It runs great and is much more powerfull gettin' up all these hills.

Only problem is it backfires most times I let out the clutch to shift. Usually from 1st to second. Very annoying and scares my elderly neighbors while there on walks when i'm Leaving for work in the morning. Argh.

I am guessing it's a timing problem. I hadn't re-timed the engine since the new installation.

Could it be something else? What should I look for?

thanks for the wisdom

[greg g]

Could be one of your carbs is not returning to a completely closed throttle butterfly and is letting air through even on deceleration. Check to see that both butterflies are closed all the way or at least equally. I have a carb on my motorcycle that hangs open now and then and causes the same deal.

[Powerhouse]

I think they are pretty well sync'd up. I had a good time getting the spring tension right to get them both closing all the way. I think it might be an exhaust leak...Might need tightening somewhere...again. I just tightened things not long ago. I hope the custom manifold isn't leaking at a bad weld or something.

Edited September 14, 2009 by Powerhouse

[greg g]

Check for an air leak between the base of the carbs and the block. Do you have good vacuum and a smooth idle.

[Powerhouse]

idle is pretty smooth...vacuum was ok...I'll have to check it all tonite...or tomorrow since today is my wedding anniversary and the garage is off limits most likely. hahaha

[Powerhouse]

OK...so I tightened almost all the nuts. One of the brass nuts was stripped on the threads...it would tighten or loosen...just spin around slowly. I had to back it off with a wrench and pry at the same time with a screwdriver to get it off. The brass threads were broke off in the stud threads. After cleaning the stud I put a hardened steel nut on instead with a few washers and a lock nut washer. It didn't back fire, and sounded quieter too. It only made a small backfire letting out of first by the time I got to my destination after going up hill. Probably a bit too rich or something...tonite I will vacuum test and check timing.

[dezeldoc]

I have seen his manifolds and make sure their is no fuel puddling in the manifold, he has some areas that would lend it to doing just that.

[moose]

OK...so I tightened almost all the nuts. One of the brass nuts was stripped on the threads...it would tighten or loosen...just spin around slowly. I had to back it off with a wrench and pry at the same time with a screwdriver to get it off. The brass threads were broke off in the stud threads. After cleaning the stud I put a hardened steel nut on instead with a few washers and a lock nut washer. It didn't back fire, and sounded quieter too. It only made a small backfire letting out of first by the time I got to my destination after going up hill. Probably a bit too rich or something...tonite I will vacuum test and check timing.

Keep tightening it down through a few heat cycles, that way it will seat the gasket down and stay down. Also check your timing, sounds like it needs to be advanced a little, or maybe your vacuum advance(if equipped) is not working. There may be some puddling issues too, but if it starts, drives and pulls hard, I wouldn't worry about that too much.

Do you have any pictures of the installation?

[Powerhouse]

I'll get some pics up soon...she runs great and pulls hard too. Vacuum advance is working(fairly new), timing will be checked ASAP, I had the manifold thermal coated and pressure tested...seemed fine. I think it's gotta be timing or something? I juts have to get time to time it...hopefully by that time the timing will be timed on time.

[Reg Evans]

All I gotta say is I hope it's not Crying Time if you find out the timing is correct.

[Normspeed]

Did you change the timing prior to to when the backfiring started? I would look at what was changed before the problem popped up. My guess is carb synch, or an air leak somewhere in the exhaust system. Or, if you changed the timing, maybe it's a bit retarded. Also, is your vacuum advance unit hooked up to the new intake properly? It should be attached to a carburetor, not to the manifold.

[martybose]

Did you change the timing prior to to when the backfiring started? I would look at what was changed before the problem popped up. My guess is carb synch, or an air leak somewhere in the exhaust system. Or, if you changed the timing, maybe it's a bit retarded. Also, is your vacuum advance unit hooked up to the new intake properly? It should be attached to a carburetor, not to the manifold.

Norm,

You do realize that if the vacuum advance is hooked up to ported vacuum then it will have no advance at closed throttle,making it more prone to backfire .......

Marty, who uses manifold vacuum on his dual carb setup

[Normspeed]

Oops, I forgot about that discussion Marty. Is that with a stock type distributor?

My guess with Powerhouse's car leans more towards an air leak or a carb problem. I've seen motors with extremely retarded timing pop and backfire but also the performance at that point made them really powerless.

[Powerhouse]

sorry for the delay...I haven't been able to check it yet. Too much going on as of late. I'll try to get to it tonite. All I know is something is definitely "retarded", be it the owner... or the timing, we will find out soon.

I have the vac. adv. hooked up to the 2nd carb only. I thought that was correct.

[greg g]

i also have vacuum only one one carb, but i am pulling from the front one.

[Powerhouse]

some Progress..sort of...

I noticed that if I drive slower no backfiring occurs. I already drive slow enough to be easy on the engine. Could this mean I am getting to much gas in the mix?

[Powerhouse]

the timing seems fine....runs smooth and steady. vac adv is working as well.