C3 - Backfire through carb

[Sebastian Aruta]

1972 survivor. Engine was not running right and carb. primer pump was leaky anyway, so i had it rebuilt by a local shop that does great work on these classics. Engine starts and runs smooth, but when i hit the throttle it back-fires through the carb. and almost stalls. I have to feather the throttle to get up to speed. Vaccuum lines and Timing are OK, the ignition wiring has been changed and checked over, new rotor, distributor cap, points, etc. People keep pointing me back to the ignition (crossed wires, whatever) but I know it's not that. This is driving me crazy. I don't believe it is a timing chain or cam lobe wear, because it has run right not too long before. Any ideas would be greatly appreciated.

[george romano]

Re: C3 - Backfire through carb

Sounds like the secondary air valve is opening too quickly. That would cause a backfire.

George

[Patrick H.]

Re: C3 - Backfire through carb

From http://duzy.8m.com/monte/ccquadrajet.htm : (slightly edited)

"Setting the Air Valve (AV) tension
The AV is the flap that covers the secondaries of your carb. It opens up when airflow through the secondaries demands it. Decreasing tension on it has 2 main benefits. It allows the AV to open faster, giving you full power sooner. It also allows the AV to open more easily with airflow, giving you the possibility to make more total power. It's held shut by two things:
First by the choke pull-off diaphragm (which is on the front pass. side of the carb.) When the engine is running , the choke pull off keeps the AV closed by pulling the linkage via the rod that connects them. When you go to/near WOT, the choke pull-off pulls out (due to the loss of manifold vacuum) and allows the AV to open. The pull-off generally releases slowly, slow enough that even with no AV spring tension you shouldn't get a bog. The rate at which it pulls out is not adjustable with the metal can pull-offs our carbs came with.

The AV is also held shut by spring tension, and that is adjustable. To adjust it, you will need a small allen key (either 3/32 or 5/32, I forget) and a small flat-head screwdriver. (**note: it is possible that instead of an allen key you will need a small torx bit, but they are rare.) Now, here you go:

Look at the pass. side of the carb (with the air cleaner off)
Observe the linkage on the side of the AV. Through the slot you can see a small flat screw. That's the one you need to adjust.
To adjust it you need to loosen the setscrew, which is on the underside of the lip of the airhorn. The setscrew is where the allen/torx bit comes into play. Loosen it only enough that you can turn the adjusting screw.
Turn the adjusting screw counter-clockwise until the AV flops open on its own. Now slowly turn it clockwise until the AV just shuts. That is 0 tension. If you were setting it to factory specs you would then proceed to turn it down the specified turns (usually 1/2-1 turn). Instead, tighten the setscrew and go drive. (**note: You may have to hold the AV rod to the pull-off out of the way while you adjust the tension. You can also remove the pull-off, but that is really excessive. If you exercise some common sense, it isn't hard to figure out how to hold everything.)
With your car fully warmed up, come to a stop and then nail it. Did it bog? If it didn't, consider yourself done. If it did, then there are a few ways to proceed. First ask yourself if you have a fresh tune-up. If not, what are you doing making performance mods when your basics aren't even done. Shame on you. Also, be sure your choke pull-off is working properly. If it is bad, it could cause you problems too. ...Finally, ...you can increase the AV tension until the bog goes away. Go in 1/8 turn increments at a time, and stop when the bog goes away. Do not increase the tension to over 1 turn down or you will permanently distort the spring."

[george romano]

Re: C3 - Backfire through carb

"The pull-off generally releases slowly, slow enough that even with no AV spring tension you shouldn't get a bog."

Thats not true. If the valve opens too quickly because its out of adjustment, you can bet the farm it will bog.

George

[Patrick H.]

Re: C3 - Backfire through carb

True. That's exactly what you were trying to say. I was mostly after some description for him of how to adjust it. Sometimes it's not enough to say "find the adjuster on the passenger's side of the air horn and tighten it up." Sorry I didn't proofread it enough. I already deleted some irrelevant stuff. I was hoping to find a web site with pics for him.

I found one now; look about half way down: http://vetteworks.tripod.com/Qjet.htm

Adjusted mine too far the wrong way once on the 71 and it sure did bog. A SMALL amount of adjustment the other way fixed the problem. If you go too far tight, I've also found it's a lot harder to "kick in the four barrels" and will also be relatively obvious.

Right now I think all 3 of my Q-jet equipped cars are adjusted about perfect. Time to go drive one with the top down. It's sunny here.

Patrick

[george romano]

[Patrick H.]

Re: C3 - Backfire through carb

Only 55 here, but the sun....

Went and tried to get 110 octane for the Cutlass since it pings on 93. They won't let me pump it right into the tank - have to use a container. Bummer. I'll have to try some other day.

Patrick

[Dick W.]

Re: C3 - Backfire through carb

I would bet that this fuel is being sold as Non Highway Use fuel. Exempt from road taxes. It is illegal to dispense this fuel into a highway vehicle.

[Patrick H.]

Re: C3 - Backfire through carb

Actually they had a sticker that said that, but it said OK to use in marine, aviation, etc, etc and automobiles designed to operate on leaded fuel.
I tried to convince the clerk that a 1970 vehicle was thus designed, but no luck.

So would a 1970 vehicle fit under "non highway use" or does it fit under "designed to operate on leaded fuel"?

[Joe C.]

Re: C3 - Backfire through carb

You are probably not getting enough "pump shot". Try temporarily setting the idle speed to about 1300 rpm, and try nailing the throttle at this idle speed. It probably won't backfire. If this is so, then there are a few things you should try.

1. Slightly richen your idle mixture screws.
2. Slightly raise your idle speed.
3. Lengthen the duration of your "pump shot".

Joe

[Greg L.]

Re: C3 - Backfire through carb

I had a very similar problem as you have on my 69 L36. After doing all the usual carb and ignition checks I decided to check the compression and found that all the cylinders were within a few pounds of each other as well...now what? Well dear ol dad said to pull the valve covers and look for a worn cam lobe or broken spring on the EXHAUST side. Sure enough it was a broken exhaust spring. I guess the mixture can get in to ignite but can't all get out untill the intake valve opens and there's your backfire. I also had this problem two other times on the same car within about a two year period and each time it was a broken exhaust valve spring. I replaced all the springs at that time and its been good ever since.

[Sebastian Aruta]

Re: C3 - Backfire through carb

I should have mentioned that the shop that rebuilt my carb. does only that. They acid bathed it, rebuilt completely, adjusted and pressure tested it. That's all they do, so I am confused that I'd have to fiddle anymore with the secondaries spring adjustment.

I did fiddle with it today, and found that the secondaries remained locked by the vacuum controlled plunger, even after a 10 minute drive. Don't seem to want to release??? When I disconnected and plugged that vaccuum line, it performed slightly better, but I noticed gas in in that vaccuum line - is that normal?

Thanks again for advice.

[Dick W.]

Re: C3 - Backfire through carb

Your Corvette comes under the Highway Use catagory. It was also originally designed to burn leaded fuel. The difference between highway and non highway use is the federal road use tax. Unka gotta git his buck.

[Joe C.]

Re: C3 - Backfire through carb

There is nothing wrong with your secondaries, stay outta there. You won't be able to see them open unless you put the engine under load. Now, if you dont have access to a dyno, then you could try removing the hood, have someone tie you to the engine, and drive the car on the highway at about 90 MPH, NAIL the throttle, while you look to see the secondaries open.
Keep it simple, first do the easy stuff.............do what I told you before. If your carb was just rebuilt, then it has to be tweaked to your engine to fine tune it.

If you had a worn cam/broken spring, the engine will run like crap all the time.

FWIW
Joe

[Greg L.]

Re: C3 - Backfire through carb

Actually Joe all three times that I had a broken exhaust spring the engine ran just beautifull. It idled nice and smooth,ran nice and smooth at 60mph and accelerated nice and smooth unless you really stepped into it...then it would backfire and miss like crazy. Even at idle with no load,quickly opening the throttle would cause the same backfire. Another thing that I did notice at this time was a "fuel spray" coming out of the top of the carb. All three times I had this problem it was a broken exhaust valve spring. I remember that the last time that it happened all I did was yank the valvecovers,replace the broken spring and it was as good as new. My engine is bone stock and in my case the engine did not run like crap all the time.