Hey fellow hambers. I've been a long time reader, but this is my first post. I tried searching this issue, and haven't found any help. I recently built a 350SBC for my Plymouth. Everything on it, in it, etc is new, rebuilt or machined. The engine runs great. Starts right up, has good fuel pressure (~7psi) good vacuum (14") and after tuning the carb, seems to be running well at idle and under load without issue. The problem is that when I really get on it like its a race, and i take my foot off the gas, the car backfires. Not normal rumbling etc, but a legit bang. I tried advancing the timing and still have the same issue. When I first timed the car, I timed it at 15* BTDC, but then gave it a bit of a boost so maybe its at 18* now. I tuned the carb using the vacuum method, rechecked my valves to make sure that they are adjusted properly (using the EOIC method) and otherwise the car runs well, but damn that backfire is loud. Here is the engine specs. Stock 72ish 350 SBC std bore, std stroke, block cleaned and honed. New pistons, rings bearings etc. Stock 1.94 heads (rebuilt) comp cams roller rockers (1.5 ratio) comp cams 268 cam kit (timing chain, hyd lifters, springs etc.) weiand intake (new) Holley 650 carb with elec. choke (new) points style distributor with petronics conversion and FTII coil (reman) stock style champion plugs with ~.032 gap 2300rpm stall conv. All help would be appreciated. Thanks I would assume that I need to adjust the timing, but which way and by how much?
You'll want to set your total timing, 34-38 is going to be what it wants. To find the exact sweet spot you'll need a dyno or track times but that will be close. You keep bumping your initial, you are also bumping the total and that can be dangerous especially if you don't know where the total is. When you close the throttle at 6000 Rpms, the mechanical weight timing is fully advanced, then the manifold vacuum rises and that pulls in all the vacuum advance too. Bumping your initial 2* is probably not helping your backfire. If its backfiring thru the carb, its a good bet that its sparking with an intake valve open. There's other possibilities too but an ignition source is getting into the intake tract. It could be extremely lean too. If its popping in the exhaust, its either a leak or extra rich when you let off the gas. Some carbs had dash pots to slow the throttle blades from slamming shut. They were devised to eliminate the problem you are describing.
I pulled a plug and it looks pretty good. I ran the car for a few hundred miles before I adjusted the carb and one side of the carb was running lean, but adjusted them out now (about 2.5 - 3 turns out on both sides now) How do i adjust total timing as opposed to initial timing? The dist has a vacuum advance.
Unplug your vacuum. Hook up timing light and tachometer. You need to figure at what rpm you want all your timing in by- let's say 2800 rpm but it may differ for your combo maybe your engine wants it all at 2500 or 2600 or 3100 Run engin up to 2800 and set your timing at 34. Run it up to 3500 and make sure it doesn't advance anymore. Back rpm down to idle and see where it's at. If its unacceptable or problematic you'll need a recurve. Dialing this in is a dance, you'll need to run a seat of the pants dyno and readjust. Keep notes and be methodical and you'll have it dialed in in a couple hours.
You never mentioned where it is backfiring from,exhaust or intake (would be helpful).If its through the exhaust look for electrical problems.
Read this thread and pay attention to the posts by HOOP98. All your questions will be answered. Are you aware your profile says you're an auto tech?
The factory service manual assumes you are tuning a stock engine with a stock distributor. They don't bother you with total because with a stock distributor the weights and springs were speced by them and the factory knows what will happen. Look at the factory specs thru the years andHp variants and you will find specs from 2 ATDC retartded to 22 BTDC advanced . This is to meet the standards they set with the weights and advance pot they chose. Your initial timing matters for starting and idle characteristics, but total timing is what matters for optimal performance and preventing catastrophic detonation. It's completely silly to not be on top of the total timing. When your foot is on the floor, the total timing is in total control as the vac falls out of the picture.
Hold on.......laid it right... over.... HERE! http://www.jalopyjournal.com/forum/threads/sbc-timing.804546/
I am a tech, but on newer cars where computers make all of the fuel trim and timing adjustments. I know that it sounds silly, but tuning cars with idle mixture screws and timing lights have become more of a dark art than common knowledge. Sorry oldheads. When I grew up adjusting timing and carbs with my pops, he did everything by ear and rarely used repair manuals or gauges/ lights, but when he did, it was all to factory spec, never for performance. Thanks so much for all of your guys' help.
If a hot wire in your electrical system is hitting a exhaust manifold,shorting to ground it is almost like shutting off the ignition while crusing at 50 mph. Try it sometime it will backfire thru the exhaust and blow the muffler up like blimp
No worries. I HATE the new crap and am at the mercy of new car mechanics. The daily I have now is nothing but a bank of transistors and computerized bull shit.
A little more initial timing can improve starting, raise manifold vacuum, give you better idle quality, improve off-idle driveability and make the carb more tuneable at idle. But as 31Vicky mentioned keep in mind that you're also increasing your total timing as well. It's backfiring thru the exhaust, right? I like the theory that an overly rich mixture is leaving unburned fuel in the exhaust system that ignites after you've closed the throttle. Adjusting the idle mixture screws is really only going to affect the mixture at idle. You may want to experiment with some smaller main jets. Do you have solid lifters? You might have an exhaust valve adjusted just a tad too tight.
I adjusted the total timing at ~2500rpm, took it for a test run and it ran great without backfiring after hard accel. It was backfiring through the exhaust - I'm 99% sure. I haven't had it backfire when revving it at idle, which lead me to think that it was a timing issue rather than a fuel issue. Tomorrow, I'll readjust the idle screws to double check that they are accurate. It's possible that the poor timing was causing it to want more gas. The engine has hydraulic lifters. I readjusted them first.
Another real important thing here, The timing marks should be verified with a piston stop test. There's no sense in worrying about degrees with out doing that. The cam card should have a spec on it that lays out some ignition timing starting figures.
Sorry, what I meant about the mixture screws, is if it is most happy at 2.5-3. I would be looking for a vacuum leak somewhere. Just seems that its really really rich at idle.
I don't remember if I checked afterward. It was getting dark and I wanted to take it for a test run. I'll check tomorrow. I set TDC myself using a dial indicator on the piston before I installed the head. The harmonic balancer is a new, one piece balancer so it can't rotate on itself.
I learned that trick the hard way, in my '54 F100. I'd get it rev'd up pretty good, kill the ignition, pump the loud pedal a couple times, and turn it back to run. Blew the crap out of a set of turbos. Dad came home and found me installing some new glasspacks, eyeballed the ballooned turbos, and lectured me, trying not to laugh.
When I was kid I got my jollies doing that. At night with just header mufflers, the ball of fire under the car is spectacular. It also sounds like two 12lb Napoleons going off. I know better now With that concept in mind..... With the switch off the engine continues to turn. Unburned fuel with the proper mixture is dumped into the pipes. When the switch is turned back on the engine fires and ignites the awaiting mixture in the pipes which equals a big boom. This is not really a back fire per say. Yours does this after revving??? Why? It may be a combination of things... Here are some candidates. Fuel in the pipes after revving... Too rich? Exhaust restriction? Dead or intermittent cylinder...2wires crossed? one tooth off advancing timing too far? Sticking valve? May need to do compression test. Timing off? Short in ignition somewhere? Faulty plug or improper gap? Weak or intermittent spark? Combination of all, maybe more?
Hey!. New here. Came across this thread and joined. Nice rides dudes. Had this problem going on for a few months. Ended being the 5 and 8 were switched. Runs killer now. Thanks
Adjusting timing at idle probably goes back to the fact that factory specs are set at idle. If you read far enough into factory specs it will give you total advance and the rpm that achieves it. High performance guys are more interested in total advance. I used to set the sprint car engine at about 42-44 degrees, but I was running methanol. Since the magneto had no advance, I could set it at idle.
Raw fuel in the exhaust system is a common problem. You should probably check the carb for overly rich mixture. when you let of the gas at speed the vacuum is much greater and improper mixture will cause it to explode in the muffler. I had an old Buick straight 8 that was a hoot to shut off the ignition and switch it back on. It would split a 4 foot long muffler from end to end and scare the shit out of people. Re check your firing order as mentioned. I seem to remember one engine that would actually run pretty good with a certain pair of wires crossed. Don't remember what engine it was, though. Check all primary wires for an intermittent short to ground. I don't think messing around with timing is going to cure it. Just my opinion.................
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