Slight backfire when taking off.
 

I have a slight backfire most times when starting off and after say slowing down and turning into a parking lot etc. into 2nd on the low RPM range, just a couple of pops and it is just on the right bank and is in the exhaust. Just finished rebuilding my 355 with a Summit top end kit with Trick Flow Heads. It's rated at 420HP with flat tappet cam hydraulic lifters I had a 670 Holley Street Avenger (backfired then too)carb I had before and that wasn't enough so went with a 750 Holley DP same results. MSD mechanical advance and we have change curve for the best performance and adjusted the advance and nothing makes a difference. Jetting down to 68 and 78 from original 71 and 80 and have went back to 72 and 78 primary and secondary jetting, nice light brown plug readings. Compression tested all cyl. 180-200. Car pulls great 4.11, 4 speed and easily spins the rears going into 3rd, good power just cannot solve the obvious build-up causing the backfire in the exhaust. Advance seems to make a slight difference on the back fire but not much. It holds great manifold pressure at idle 800rpm=12-13lbs so 6.5 power valve seems to be correct. I have a Torquer 2 Edlebrock open plume intake with ram heads and new 2 1/2 exhaust front to rear that were put on and ran before the rebuild (no back fire) Any ideas guys would be appreciated greatly..!! THANKS


SEE last post by me today 2-27-09

For starters, You said when start off. Does it make any difference on throttle opening and is it consistant?

It is very hard to think outside the box sometimes. There have been times that I have been stumped. The most embarrasing was when my wife was telling me to check something before I tore the motor down. It was on her 77 which she drove more than I did. You can guess the outcome.

If you are really, really sure about the compression numbers than I would move on. I think you said right bank. I would definetly run that again. Is the misfire enough to feel performance loss or is it so slight that it can't be felt, only heard? One of those annoying things that will bug you to death unless it is corrected.

After 38 married yr observation: They are right more than we would like to admit! I would be looking at the compression and then that "touchy advance" would be the next thing. Perhaps dropping an old HEI in see if any change. Is there even any misfire under hard acceleration?

Is it possible the pickup wires running from the 6AL to the dist are to close to another electrical source? If so they can pick up extraneous interference. I purchased the shielded mag cable just for this reason. Is the engine running lean?

Problem could be the Torker and it's 2500-6500 RPM band. Was this intake used on previous engine? Might want to consider a dual plane.

Are using a vac adv dist? If so, are both ends of vac hose a nice tight fit, no vac loss?

i have chased these pesky problems before and after exhausting all the usual possibilites, timing, plug wires, compression checks, vacuum gauge readings etc. the issue is usually a "lean" pop at off idle. it could be fuel contamination by a very slight coolant leak in one or two cylinders caused by a head gasket. this problem is hard to detect without actually tearing the mill down. if by richening the carb , the issue goes away or improves slightly, then this could be the problem. the leak could be too slight at first to detect but im assuming you have done all the standard checks on your engine. if it were a big leak then it would be easy to detect, water out the exhaust etc. but it could be slight enough that the coolant level would not seem low until after driving for quite awhile. if it is an intake leak (vacuum) im sure you know how to check it with propane.......i have been here before and know your frustration, i hope im wrong in my diagnosis and you find something simple, any water puking out the overflow on a cold start up? any moisture out the exhaust? like i say sometimes you can try to find this with a pressure check of the cooling system but if it is a small coolant leak at the gasket to block area it is hard to show up, especially on a fresh build mill. a company called p&g makes a tool called block check that turns fluid different colors when used, you might want to find a rad shop that has one.....good luck

popping or afterfiring through the exhaust on deceleration usually means you are running too lean.
backfiring up thru the carb on take-off also usually means you are too lean.

not sure about how you would only get the popping or afterfiring thru only one side of the exhaust though unless maybe an exhaust leak on that side but I had an exhaust leak on my '78 and you could hear it all the time as a ticking sound, it didn't produce afterfiring on decleleration.

Another thought for you and easy to check. Part throttle acceleration creates some high cylinder pressures with lean mixtures so the ignition system is really stressed. Assuming new plug wires I would take a quick peek inside your distributor cap. Look for a very fine line (carbon track) running from one of the terminals to the base. Use a bright light. Electricity is always looking for the easiest path to ground or another cylinder. Another item to try is the accelerator pump cam. There are several different profiles you can use. One may give you just the right shot of gas that you need if your problem is a lean misfire. Working the bugs out of a new combo can be frustrating. Good Luck

Complete new system from manifolds and it was installed and running on it before rebuild. 2 1/2" from Corvette Central with Magna Flows. No leak there, already checked and put the real nice stainless sleeve clamps that are like 6" long. Great system for the price and fit like a dream. I highly recommend that choice.

Thank you..!!

Maybe a shot in the dark, but have you checked the exhaust heat riser valve under the right exhaust manifold? Could it be sticking closed or not opening fully causing backpressure? Good luck. :thumbs:

Deleted it and put a spacer there. Electric choke on the carb.

Thanks..!!

This is a high quality concern that anybody I know would pay to have. "65 Roadster with a slight backfire. No sarcasm intended but I REALLY have to remind myself that, as patience isn't one of my stronger vertues, LOL. Thanks

you will get this figured out and i hate posting the worst case scenario but the fact you are having a problem in "off idle" acceleration and not deceleration and take for granted that all the usual checks have been performed, this sounds like fuel contamination to me. if you pull the head you would have a visual inspection of the problem that would manifest itself by discoloration on one or two of the combustion chambers. compression checks will not find this and the fact that you are setting timing with a vacuum gauge instead if a timing light gives me pause. i know some say it can be done but why?? if the engine is that sensitive to timing adjustments and by richening the mixture the problem seems better, then i still would look at fuel contamination by coolant....good luck

Hmmmm....

Backfires (slight..couple of pops) only on the right bank/exhaust

Does it with both carbs, so it's very unlikely to be the carb, IMO

Happens when just starting off, and when downshifting to 2nd to slow down at lower rpm's. You probably blip the throttle when downshifting, and with 4:11 rear gears the rpm's would climb up just as the throttle plates are fully closing. One situation is just as the accelerator pump is enriching the fuel mixture, and the other is just after revving the engine and letting off the throttle under compression braking load.

Open plenum manifold, which lets all the fuel charge go to whichever cylinder is drawing the charge. Single plane manifold, which makes me assume the engine is intended for higher rpm usage, and also assume that the cam has some significant degree of overlap, especially since it's a flat tappet, and produces 12-13" of vacuum at 800 rpm idle. A 670cfm carb wasn't enough, presuming it went "flat" at higher rpm ranges, which the 750 DP solved..you also richened up the primary jets one size, and leaned out the secondary jets two sizes.

Cranking compression is right up there, which would indicate a pretty stout cylinder pressure, especially in light of a cam intended for high rpm use. I'm kind of surprised by that number, I would think the intake valve would close late enough to bleed off some cylinder pressure at low rpm's, but then again, you didn't state your static CR, and also don't know if you degreed the cam or not, or intentionally or inadvertently may have retarded the valve timing to favor the top end.

If you've ever driven an earlier car with an A.I.R. system, and the diverter valve was defective, you get one hell of an exhaust backfire when you keep pumping air into the exhaust system as you let off the gas.

You didn't indicate that the car starves for fuel while running steady at higher rpm's, so that leads me to believe that the float level isn't the problem. The "popping' occurs under conditions where you have just richened the mixture, at rpm's where a cam with more than stock overlap will tend to let incoming charge follow the exhaust charge out the door, and fed by a manifold that promotes max fuel enrichment to every cylinder. Since it isn't happening on the left bank as well, I think you have a very slight leak in your cast iron manifold to aluminum head seal, or at the manifold to pipe gasket, since a cast iron manifold tends to hold heat very well, and the introduction of a little more air under transient exhaust flow velocities to a rich exhaust charge would seem to be a possible cause of your problem.

So, I would suggest that you recheck the torque of your exhaust manifold to head bolts ( I'm assuming you used exhaust gaskets, didn't you? ) , and the gasket flange to exhaust pipe. If those are tight, you might want to check the exhaust manifold to cylinder head flange for excessive warpage.

Anyway, that's my shot at your problem, perhaps it will help you track things down. If it doesn't, then I can honestly say that it wasn't for lack of typing. :)

David

:iagree: Right on David and a thorough analysis...I am under the impression that cam timing or manifold leak might be suspect.

Fred

If I read your posting correctly--You got the back fire AFTER the rebuild. If that's correct then it seems to me that you should recheck the valve lash. I know-how that you screw up adjusting hydraulics?:withstupid:
It happens!
:willy:
Good luck.

is it missing while it's popping?? you are now running 80 mainjets? what size did they replace??...if by going richer the problem seems better then you have a vacuum or slight coolant leak. if you drive it more and it gets worse over drive time, then this is a good diagnosis. you can go too rich and wash out new rings here.......