backfiring thru carb

Can anyone figure out what is wrong here. The engine is backfiring thru the carb when you rev it up. I have been working on giving it a tune up. Here's what I got so far: Delco Dist. cap and rotor looked amost new on the inside. I have a dwell/volt/tach meter. I adjusted the dwell to 30 deg. I checked the points using the meter it said it was in the Ok range. Set timing to 5 deg before using timing light. The mark kept jumping around a little. I tried to set the idle and mixture but it was near impossible since the rpm's keep going up and down slowly. This is a new ly reman Rochester 2bbl carb reman by Holley. It was for a Buick 3.8L, it was the closest thing I could find. I disconnected the automatic choke and put the manual cable on it. It is the 225 V6 W/ offenhauser 4bbl intake which I put a 2bbl adapter on. Yes I put a piece of hose in my ear and checked for vac leaks, I didn't notice anything but could have missed since the headers are so loud and the fan is blowing over the engine. The coil (Accel super coil) had the + and- switched for who knows how long, I switched it back. Plugs are almost new and the wires ? but they look almost new. Oh, also when it is running the fumes are so incredibly bad you think your eyes are going to burn out.

Anyone?

 

Bad fumes usually mean a very rich condition. If it is firing on all 6, then I would adjust the idle mixture screws. Too much advance timing will cause a backfire through the carb too. You're sure the wires on the distributor are in the correct places too right? wires out of order can cause both the fumes your talking about and backfiring. I would ensure the plug wires are in the correct place and then readjust the carb if need be. Don't let the timing light fool you either. Sometimes the outer ring on the balancer slip some and will give an incorrect reading with your light. just go by ear if your not sure.

 

Once it gets warmed up pretty well it seems to run alright. I don't know about the wires being in the wrong place, wouldn't that make it run really bad- barely at all? I'll have to check on that.

 

Maybe, but it depends on how much they're off and what your timing is set at. Check your idle mixture as well.

 

sounds like mixture to me.... But I am stupid!!!!!!

 

Mixture alone won't cause the backfire through the carb. I'd start with the timing. I've actually had a dist installed 180* off and the engine ran. It ran awful, but it ran. Timing will cure the backfire.

Once you are sure that the timing is right and the wires are right, then adjust the carb if it is still running rich. If you can't adjust it to your satisfaction, have a rebuilder check it out. there are a few things that could be wrong internally to cause the engine to run rich. Worth the few bucks to have a pro check IMO

 

That's what crossed my mind too. 'Cept mine wouldn't run, just an occasional BLAMMO out the carb during cranking.

 

I know nothing about the V6. However with the F134 I have had similar problems. And have fairly good results with just rotating the disturber cap by trial and error when it starts to back fire at a certain rpm. And after a few tries would get it smoothed out.

How are you spark plug wires? Got any of them laying directly on the motor?

 

A worn timing chain would cause the timing to jump around.

 

try adding adjustable fuel meter to line (need 2psi @ carb). how much advance do you have? should be about 5*

 

Stole the words.....


You might also want to pull the distributor to see if the bushings inside it have a lot of slop in them. The only way you'll get a backfire through the carb is to have a plug (or plugs) ignite the fuel mixture while the intake valve is open. So it's a timing issue, ignition and/or valve timing. Check that chain!

 

Really sounds like timing. After checking the distributor setting (timing) and confirming wires are going to the right place and it still does it you have to confirm engine timing. I had a volvo B18 that idled ok (not the greatest) and ran up to 50% but over that under load it would backfire up the carb, found a flat exhaust cam lobe, it was opening some but not enough for proper operation. Improper cam operation is timing also. Hope it's wires.
Joe

 

I had the same thing happen to me... Everyone said basicly the same thing I have read here, but didnt mention one thing. To check the pushrods. I had a bent/broke pushrod. Replaced it, and it ran better than ever. Did you check yours?

 

I adjusted the mixture with the aid of a vacuum guage. It does seem to run better. If I hit the throtttle too hard when it was warming up and I didn't have the choke open far enough, could that cause it to backfire thru the carb? I mean I do have the timing set to 5 deg before like it is supposed to be.

Josh

 

throttle not throtttle!

 

Hey,
I pulled the dist cap again. I turned the crank by hand and it took quite a bit to get the rotor to move. However, I also tried letting the crank stationary and just turning the rotor, that moved almost as much as in the previous test. What should I do?
Josh

 

How much is "quite a bit"? if you have any slack at all you probably need to replace your timing chain.

 

It's hard to say what is wrong for sure, however, it is common for the odd fire timing chains to stretch. The uneven firing puts a lot of load on the chain. So, it's likely that your chain is stretched and a stretched chain could cause the timing problem that causes the backfiring. Your turn the crank and watch the distributer test results sounds like the chain. I figured out my chain stretch problem because of the noise it was making rubbing on the cover - I'm pretty slow sometimes.

 

Well when I turned the crank(back and forth a little) I had to turn it maybe 1/16 to 1/8 of a rotation. But like I said, the rotor end alone had that much "wiggle" in it. Is the distributor gear driven directly off of a gear on the camshaft. I can't tell from the diagrams. Do these typically wear out? If so, can they still be found? Someone mentioned a distributor bushing, Where is this? Can it be purchased/replaced separately? If I replace the distributor too, what is a good source for an HEI? Or shouldn't I bother with HEI? I know too many questions. I may just rebuild the whole#$$$## thing. My cylinder compression isn't that great either, but then of course the manuals don't give you a number reading to go by. Something like 110-105-115-110-115-105 just a guess but it was something like that. Is it worth rebuilding something this old? will I be able to find all the parts I need?

Josh

 

Well, 1/16 to 1/8 turn is not really that bad. There's always a little slop in there.

The dist is driven off the cam by a seperate gear that is not part of the cam itself. These can wear but is unusal for the normal life of an engine.

The dist bushing is on the dist shaft itself. These can wear and cause eratic timing. British cars were nortiouls for this, I know first hand. Check by trying to wobble the rotor sideways and see if there seems to be excess slop.

The best place for a new/rebuilt HEI is Cape Conversions ( www.driveajeep.com ).

HTH.