Backfire through the intake.

[Huron]

My '71 Cutlass with a 350 is backfiring through the carb. It's been my opinion that anytime an engine backfires through the carburetor that it's an ignition timing, valve timing, or valvetrain issue. Has that you been your experience?

I've only owned this 71 Cutlass for a couple of months. It was running pretty poorly since I got it. Now it runs great at wot, but if you accelerate modestly it tends to be really tentative, and then once you back off the throttle at cruise speed it will often backfire through the carb once or twice. (The backfire existed since I bought the car.) The other issue is that sometimes the manifold vacuum is ~16" h.g. at idle, other times its down to ~12" at idle. The car generally seems to idle fine. It starts as soon as you hit the key, whether the engine is hot or cold.

I've since installed a rebuilt distributor, and new cap/rotor/plugs/wires, set the dwell and timing. I also first rebuilt the 600 cfm Edelbrock that was on it, then I installed a Oldsmobile Quadrajet that I rebuilt myself. I've also capped off all of the vacuum lines to ensure that none of them are leaking. I'll be checking the intake manifold gasket and the carb to base gasket today.

Forgot to mention: it's got the Edelbrock Performer cam installed, and it's got a 3.73:1 rear axle.

Anyone want to hazard a guess as to what the problem is before I start checking the valvetrain?

Thanks!

Mike

[Oldsguy]

Mike, it sounds to me like you are on the right track. I wonder about the inconsistent vacuum. With everything plugged, except the advance cannister on the distributor I assume, you shouldn't have a leak. With a TH400 you would have a vacuum operated kickdown, that might be it, or the accumulator for the brake booster assuming you have power brakes. Like you said, it might be gaskets, or could be the throttle plate bushings on the carb. If the bushings are worn, you would have a vacuum leak that would come and go. Would also effect the carburetor performance too. One other thought might be a worn timing chain, have you double checked you timing? It might be jumping teeth.

[Huron]

Hey Oldsguy!

Originally Posted by Oldsguy

Mike, it sounds to me like you are on the right track. I wonder about the inconsistent vacuum. With everything plugged, except the advance cannister on the distributor I assume, you shouldn't have a leak. With a TH400 you would have a vacuum operated kickdown, that might be it, or the accumulator for the brake booster assuming you have power brakes. Like you said, it might be gaskets, or could be the throttle plate bushings on the carb. If the bushings are worn, you would have a vacuum leak that would come and go. Would also effect the carburetor performance too. One other thought might be a worn timing chain, have you double checked you timing? It might be jumping teeth.

Thanks for the information.

Yesterday I found where someone had screwed a bolt into the intake where a pipe plug should have been used. It was loose. I applied the correct plug with some teflon tape. Then I discovered that the electric choke was wired into B+ for the ignition coil. I could make the quality of the idle deteriorate by plugging in the choke. I moved the choke power supply wire to another spot.

Bottom line is that the backfires through the carb went from about 20 Saturday night down to 2, so it's running a lot better.

I eliminated the brake booster and modulator when I plugged all those off. No difference in idle or manifold vacuum.

Good point about the throttle shaft bores. I had replaced the throttle shaft bushings in this carb, so the shafts are tight.

I'll look into valve train this weekend. I'll have to get some valve cover gasktets and an intake manifold gasket.


By the way, do you know what size that crank bolt is? I'd like to turn the engine over with a breaker bar to check my chain, but I don't think that I've got a socket that large. I'll need to buy one this week.

Best Regards,

Mike

[Oldsguy]

I don't remember off the top of my head. It is over an inch though I think. Definitely a big one.

[$tryker]

It's 1 1/8" for the bolt. Spraying carb cleaner in suspected areas while the vehicle is running, is a good source for finding vacuum leaks. If the idle goes up, that's the problem spot. Good luck.

[Huron]

Originally Posted by $tryker

It's 1 1/8" for the bolt. Spraying carb cleaner in suspected areas while the vehicle is running, is a good source for finding vacuum leaks. If the idle goes up, that's the problem spot. Good luck.

$tryker,

Thanks for the bolt size. I used copious amounts of carb cleaner over the weekend. That's how I found the bolt where the plug should have been. That's the only intake leak I could locate.

Huron

[millwrightrice]

Flat cam lobe if all your other trouble shooting doesn't pay off

[texasred]

May I start by congratulating you on the move back to a QJet.

"Rebuilt" distributor? By who? Distributor sideshaft play can cause unpredictable dwell results. Do you have the correct Vaccuum advance can? Weights/springs all hooked up correctly? (I saw an Olds distributor the other day that has springs but no weights....and he wondered why his car was such a dog!)

Another guess would be a worn timing chain.

C.J.

[$tryker]

The bolt should be 1 1/8".
Did you degree this cam? Also, when you check the timing, have you disconnected the vacuum advance? Keep us posted.