okay so i got a new holley 670cfm street avenger carb on my 302 and i am having backfiring problems. 2000rpm and beyond you canhear it stuttering and it backfires alot. it idles fine and it runs great until you hit 2000. this is not under a load and in neutral just pressing on the gas. but it also happens under load and seems to rob power. i do have a big cam if that matters. and i have the timing set at 12 initial and 32 total, within the specs of the cam. if i rotate the dist. and try again it doesnt help whether its counterclockwise or clockwise. im kind of stuck now and im hoping you guys can help me out. i heard something about the power valve on the carb? because i have a bigger cam then stock. but im just not sure. any help would be great and if you need more infor just let me know. thank you!
a power valve issue will not cause that issue under no load and you most likely have timing issues. Also 12 initial is not nearly enough for anything performance related and should also be supplemented with vacuum assisted timing from the disty's advance pot via full manifold vacuum port off the carb or manifold port. 32 degrees is also not enough total lead with typical bolt on parts. Advance the disty to 18 degrees initial timing. This will also bump total up to around 38 degrees. Plug the vac advance mechanism into full manifold source. Set idle speed and adjust mixture screws. Retest.
Definietely follow this mans advice. I can attest to the knowledge he dropped on me. I went from a rough running barely driveable car to burning the tires up with his advice.
Not really trying to go off topic here at all but what's wrong with his 12 degrees initial timing? That's what I have my car at right now and it runs great... And it's probably more modified that most.
12° is plenty initial. It's double what the factory setting is. The problem is that if your total advance is only 32°, that's not enough. You should run about 38° total. If you run 18° initial, you may not be able to get the idle RPM down to a desirable level. The backfiring could very well be a major vacuum leak. Are you running the smog pump? I doubt you are, but just in case, a bad diverter valve can cause backfiring when letting off the throttle. My money's on vacuum leak though. Something probably didn't get sealed up properly when you installed the new carb.
it sounds like it going lean on the secondary opening transition. 3 possibilities come to mind. the secondary idle mixture screws if it has them can be way out of adjustment. the secondary accelerator pump circuit is not working correctly or one or both of the boosters is plugged up.
I'd love to take credit for the breaking of the traction bar mounts but I'm pretty sure the previous owners 347 took care of that lol.
Wow lots of good info from you guys! Thank you all so much! I will try the idea with the timing I'll set it to 18 and plug it into full vacuum and see if that helps. If not I'll check for vacuum leaks and cross my fingers
And yeah groberts has helped my on a few threads while I've been here lol he knows his stuff for sure
The key words I picked out of there and based my response on were the bold sections above. A few random thoughts about things I have learned through trial and error over the years. I can take a large vacuum hose off of all my engines and they will not backfire one little bit. Sure.. they'll drop some manifold vacuum which may affect the vacuum advance mechanism to some degree or another and lose some throttle response in the deal.. but they still run without major hiccups like the one being mentioned here. All newer Holleys have PV blowout protection built in these days so it's highly doubtful that's the issue.. particularly at only 2,000 rpm under no load. Secondary metering/fueling issues aren't even in the troubleshooting diagram under no-load situations. Too much front transition slot coverage(front throttle blades too tightly closed) and more than needed on the rears can cause stumbles, hesitations, and even lean pops like those described here when the engine transitions too late from the idle circuit over to the main circuit. Too little(retarded from what the engine actually prefers.. NOT what someone else thinks or "runs theirs at without issue")ignition timing lead can cause fuel puddling/loading to the point that it can backfire through the carb as well. The exhaust is more typical with the telltale popping and backfires on heavy deceleration.. but it does happen on the intake as well. This is especially true when fueling is overly rich from what the engine prefers and retarded timing compounds the issue with this result. A vacuum gauge is without a doubt the best bang for the buck that can be had for ignition and carb tuning. They visually tell you what the motor likes.. or dislikes.. and trump a timing light five times over for tuning effectiveness. In fact.. if you don't mind breaking laws on vacant side streets?.. all you need is a vac gauge and a decent set of ears connected to the old ass-dyno to dial it all in.
I put a spare boost gauge in my maverick to read vac so I can see what kind of vac I'm getting when I adjust the carb or timing.
Okay so i did what you said groberts and it worked wonders. But eventually it started to backfire again under the same conditions. But that was after i adjusted the idle mixtures and speed screw. Sounds like your right yet again groberts and i need a vacuum guage
How do i find out what timing the engine prefers? I noticed the rpm changes with moving the dist. Is that a way for me to do it? Or do i need a vac guage for it?
