Posted: 6/28/2008 7:21:09 PM EDT
| I have a SB chevy engine that backfires as i accelerate. it is carburated and i have timed it, done the mixture correctly, gapped the plugs, replaced the points. why is it still backfiring? how do i remedy this? |
Been there done that got the T-shirt and burned it. The small Chivverlay hasn't had points in over a third of a century, and none of the point-ignition ones came with roller cams. The original design went into production over half a century ago. The engine didn't oil as well as modern engines do, and let's face it- at this point in the engine's life, it's probably been subjected to both incompetent operators AND incompetent maintenance and/or repairs. Tearing the cam up is often the result, and then they start shooting ducks when the throttle is open. |
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Is it backfiring through the intake or exhaust? Constant backfire or just once and then goes? Possibilites are many. Lean air fuel mixture, hanging intake or exhaust valve, firing order incorrect, distributor a tooth or so off, etc. When did this start? Was it after doing a tune up or repair? |
Gm should have thrown in a spare cam shaft in the trunk with the early 305's . |
Firing order should be 1-8-4-3-6-5-7-2 Number 1 should be the first post past the window on the distributor cap that is past the window used for adjusting the points. Number 1 plug is the front of the drivers side on the engine. Drivers side is odd: 1-3-5-7 in this order front to back and the passenger side is even 2-4-6-8 in this order front to back. ETA: Oh, I believe the foring order goes counter-clockwise on that engine. |
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Well I inherited the car about 7 years ago. It was my grandfathers so I dont know what kind of stuff was done to it, but it is pretty much in original condition. It is a 1963 chevrolet with a 283 v8 with 327 heads. i never rebuilt it but i did replace the water/feul pump, belts, filters, gas tank, manifold gaskets, carb,rotor, plugs, wires-all the easy stuff. i even sent it out to get timed and tuned up b/c i didnt know if i screwed it up. anyways, they got it to start after it stopped starting a year ago. is it old gas or something or does it need to be fully rebuilt? the hoses were changed but maybe the radiator is shot and its getting too hot in the heads? i checked the plugs(new) and they dont look like it was running hot, more like shitty carbon buildup on the ends (not alot ) also, it is only under load that it backfires. not a heavy backfire, just a small one every few seconds randomly. at idle it runs fine |
Not that it may be the root of the problem, but this engine does have an archaic pcv system in it. if that is all gummed up could that be the cause? either way i guess i will take it in to have it looked over. i figured i could salvage it myself on the cheap but its probably best to have it done right |
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Put a vacuum gauge on the engine. If the needle hops around alot it's camshaft/valve related. Most vacuum gauges come with a chart to help a newb diagnois and engine. A vacuum gauge is one of the best diagnoistic tools for older cars, hell I use them every so often on a new 08. Since yours is a 283, camshafts weren't a big problem then. Could be fuel, the carburator, ignition, or mechanical. Taking it to a dealer is a complete rip off, most of these 'techs' now have never even seen points or carburators. Most schools don't even teach how point ignition or carburators work anymore. Call around, asks people who have older cars who does their work, being a Chevy you should find someone half way close by. |
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So it was sitting for a while? Could be a clogged fuel filter or some clogged passages in the carb. Could also be a vacuum leak or a weak fuel pump or any number of other things, although a vacuum leak bad enough to cause problems under a load will definitely show up at an idle as well (high idle speed, takes a long time to return to idle, etc.). In general, a backfire under load is usually caused by a lean mixture or retarded timing. Has anyone looked at the timing advance mechanism, both centrifugal and vacuum? What kind of carb? 2bbl or 4bbl? |
Start with the simple things. If it's old gas, change it first. Depending on the heads, that combo could be a mixture problem in itself. 327= 4"bore 283 = 3 7/8". There were only 87 different heads for the 327 and unless someone had some installed, exactly NONE of them had the hardened seats that are needed for unleaded gas. Unless there is a lot of sentimental value , I wouldn't chase this problem very much. |
+1..THe simplest and easiest tool to adjust carbs and check condition of an engine. www.earlycuda.org/tech/vacuum2.htm HEre is the site that I use. Ive got a cheapo gauge mounted in my engine compartment for adjust the carb. Its always been right on with me. If I dont have it reading steady at 15-17hg it runs like crap. as soon as I get it in the range and get idle adjusted it runs smooth. |
| very posible the gas is old, also just a thought,, how about sprkplug wires, are they old? also check for spots where the come in contact with any metal, you can mist them with a water bottle and see spark jump from the wire to metal. the kilovolts (KV) go up with engine speed (load) and will cause backfires. |
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plug wires are new, carb is rebuilt. the 327 heads are original on this engine, came as a "powerpack" option in 1963. when i originally had fixed everything and ran it, i drove it around for about 4 years without a problem. last spring it stopped starting, because i was rewiring some things. i just now have been starting it up and so i was afraid it could be old gas doing the backfiring. should i just run lots of carb cleaner through it or let the car idle alot and burn eveyrthing out? ive filled the tank with lots of 91 fuel and octane booster |
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Too many ways to go with this. As suggested, the vacuum gage can help (a lot) to isolate your problem. Plugs , even new ones, can load up quickly and cause a miss . They may or may not burn clean. If you put good gas in it I wouldn't think you'd need to use carb cleaner or fuel cleaner unless the bowl went dry from sitting and put a little varnish in there. If it did and you had a power valve sticking because of it, you'd definitely have a lean pop going on that would get worse as the load on the engine went up. An intermittent miss doesn't fit that scenario though. If it ran ok before, (and assuming the timing is correct, the cam isn't flat, valve springs aren't weak or broken, the vacuum lines or intake manifold gaskets or throttle shaft aren't sucking air....see vacuum gage advice)I think I would probably hammer it a couple of times thru second gear and see if it cleared up. Naturally you get your foot out if it if it starts sounding stupid. |
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There are a couple of good points here that I'd like to condense into one suggestion. Kennedy was President when that engine was new. It doesn't have hardened exhaust valve seats, so it's only going to run until it hammers the valves so far back into the head that the hydraulic lifters bottom out and the engine won't create enough compression to run anymore. Old small-block Chevrolet V8 engines with bores under 4.00" tended to be pretty octane sensitive, even when you could get lead in your gas. Even more so with the less pedestrian versions (like the Power-Pack). You're going to have to pull out so much base ignition timing to keep it from spark-rattling, that it won't have any power, it'll run hot, the throttle response will be mushy, and it'll get crappy fuel mileage. You can get a brand spankin' new low-compression standard model Goodwrench 350 for cheap. It would be a fairly easy swap, and it would physically appear very similar to the 283 that it was replacing. It'll also last longer, run cooler, get better fuel mileage, have better driveability, and have more power (than a Power Pack 283 with the timing set to 4 degrees BTDC). I'd have a hard time sinking much time or money into an old 283 Power Pack, unless it was a hobby. |
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I have heard the suggestion to place a 350 crate in this vehicle before, and i have been reluctant to do so b/c i wanted to keep everything together on it but more and more it seems like the best idea. i guess ill buy a vacuum gauge and look at it, seeing as it sounds useful enough for my other restorations. thanks for the help guys. any new suggestions are welcome. now this means i will have to place gun buying on hold for awhile 
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Another suggestion would be to do a compression test on the engine. Pull all the plugs, pin the throttle all the way back and run a comprtession test and write down your readings on all cylinders. An engine that has sat that long could have a gummed up valve causing it to hang up and cause that backfire. You need to start with the basics before jumping into dumping a bunch of money into it. Check the firing order, make sure the distributor is set corectly, make sure you have approx 19 thousandths gap on those points with them on the high point of the distributors cam and then run a compression check. Also, check those plugs and make sure one did not get cracked when they were getting put in. |
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You've had lots of good advice here and it seems to me that it comes down to a couple of choices for you. There are lots of smart fellas here that can help you get it running correctly... BUT IMHO, If this car is a daily driver, I think you are wasting your time and money trying to make an old workhorse shine. A 350, or even better for that 2 spd... a 383-400 with a wiiiiiide powerband ( I can't help it  torque is your friend and will make life much simpler. ) can be had for a surprisingly cheap price if you are willing to do some or most of the work yourself.Some models had their own unique ways of bolting up starters, motormounts, alternators and such, but as long as you check the new castings and ask questions beforehand you should be good to go because it's the same basic engine. I fondly recall doing the old Points, Plugs and Condenser, along with the oil change thing EVERY stinking 3000 miles ! No Thanks ! OTOH, if this is going to be a one-weekend-a-month-and-Only-in-nice-weather Cruiser ...........I'd Still want a new motor that looked like an original Old motor in it. IIRC the powerpack 283 was a bit over 200 hp with probably 180ft/lb of torque. The new crate motors (with upgraded ignition) offers quite a bit more than that without beating your wallet up with maintenance costs. Heck, save all the old parts and if you ever want to sell it you can claim All Matching Numbers. |
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Well I dig the idea of a 350 crate and am friends with an old-school mechanic that works on older chevys. I have also procured an HEI system for the next engine I put in the car. I Do plan on keeping the original engine around in case i do sell the car. Until then, for the sake of pulling the car in and out of the garage, I will keep this engine going. I have found a few unopened cans of lead substitute in my garage. How long do these last? |
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Here is something you can try short of running the diagnostics that have been recommended such as compression test etc. If it is a gummed up or carboned up valve, get yourself a can of Gumout, not the spray but the can that you can pour in. Raise the idle to about 2000 rpms and slowly pour it down the throat of the carb racing the engine up and down as you pour it in slowly. If it is a gummed / carboned up valve this might free it up. You can do the same with some trans fluid or water but the Gumout will work the best. It is a cheap thing to try and cannot hurt. |