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Discussion Starter · #1 ·
i have a 72 malibu, w/ a 69 motor. i posted edelbrock 1407 backfire a few days ago. well it ends up that the backfire is from teh tail pipe. i have a 350/300hp .040 over, w/factory HEI(stock wires, and coil plugs gapped @.055), performer intake, performer carb(1407 750 cfm), mild cam,hookers, 2 1/2 exhaust w/glasspacks . the furter i advance it the more it happens, as well as when i richen the mixture(1 1/2 turns out). as i check the timing with the gun, the timing mark will stay staionary for awhile, and then all of a sudden it will surge between 4-7 degrees advanced, and then backfire.it backfires even at idle(sounds like a fire cracker)but, if i retard it too far, it will back fire out the carb. people have told me everything from a vaccum leak to stuck valves, to a bad distrubutor. this all happens w/dist vaccum disconnected, and pluged, at idle. i need help from the best, or anyone who has had this experience before.

mike
 

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Boldly procrastrinating
66 El Camino 57 Chevy pickup 2004 Tahoe
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mike, I'm certainly not the best but I have seen a similar problem before. It turned out to be some weird failure mode of the HEI module. If the timing surge you're describing in not related to an rpm increase (mechanical advance coming in) then I'd feel pretty good recommending you get a new module, preferably a Delco one. I have also seen some weirdness related to failure of the two little wires that come out of the pickup coil and go to the back of the module, sometimes these wires break off due to being worked back and forth by the vacuum advance operation. They look OK but the copper stranded wires are all broken inside the insulation, you can pull on them like a rubber band. Normally this failure causes backfire and loss of power as the engine enters cruise mode and the vac advance is actuated. In this case I'll bet on the module. Is it a Delco or an aftermarket?

Tom
 

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Ditto on the module! My son had one go bad in a camaro. It would only do it when the engine (& dist) got warm. Blew bothof the mufflers out. They looked like a bomb went off inside.

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Steve Strasemeier (70SS 396, Fathom Blue/White Stripes)
 

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Discussion Starter · #4 ·
ok well i put in the new module, and that stopped the weird timing(where it would jump), there is not a vaccum leak. i have the timing(vac advance disconnected)set around 4-5 btdc, and it doesn't backfire(taild pipes) at idle, if i advance it much more i will get a backfire at idle. as i looked, i have a slight fuel leak at the pump on the line going to the carb. i also checked to see if all cylinders were firing, and to my extreme dismay, #'s 1,3,8 are all not firing(sprayed water on header pipes)and the exhaust smells gassy, which it obviously would if 3 cylinders weren't firing. why did my plugs foul out????? and how do i fix the backfire??? this is driving me completely insane, i need to get my car back on the road. thanks in advance,

mike
 

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Stock wires? Your next step is to check or just replace the wires. I donno if it's correct, but I used to simply check the wires for continuity with my VOM. It sounds like you have more than one problem at work. The variability of the backfire given the amount of engine timing points in a direction I don't think you want to go, so hopefully it's something simple and not a seating problem.

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Discussion Starter · #6 ·
ok, well i pulled the plugs on those 3 cylinders, and they all spark. but the headers weren't hot like the others, so do i have stuck valves?????? would that cause a backfire???? are the plugs firing, with no mixture in the cylinder? any help please!!!1

mike
 

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I responded before, but I'm not sure if you read it. Have you tried adjusting the valves? I have seen that cause backfiring also. Let me know if it helps. Good luck.
 

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Try new plugs. Once they foul, it will cause this. Ive bench tested plugs that were fouled and the spark will occur way up the side of the insulators instead of at the gap. Even scrubbed them down and wouldnt look as good as new plug. Plugs are cheap,easy thing to eliminate as a possible cause. As a rule, I always start my diagnostics into a problem with fresh plugs. (if poor running that is-doesnt do much for headlights that dont work!!)
 

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Definitely bounce this one off a real mechanic, but I was reading your symptoms and I could't help but think, due to the relationship of your timing and the backfire, that you might have some valves, or just one, that aren't seating properly or hung up. Checked the rockers/rods to make sure everything's copasetic?Compression test next?
 

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hi,
if my car were running like yours I would do a vacuum test.this will let you know if there is a problem with any valves.guage should read from 18 to 20 with steady needle.
of needle on guage is shaking there is a leak or a bad valve.the bad valve will also show up on a compression test.these tests will cost zero if you have the guages.if these tests show normal(we hope)then try swapping out the distributer with a known good unit from another chevy.if none of these things fix the back fire then you will have to invest in some new plugs and wires.
one more thing to check is the voltage going in to the distrubiter,(not less than 10 volts)I had a chevette once that drove another mechanic crazy for a month,it would backfire didnt have any power on the road and then all of a sudden it would run fine.
turned out the stator on distributer was loose and slipping around causing timing to vary at times. hope you get that malibu on the road soon tony
 
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If this is a new motor, and this is all happening out of the box, I would be looking at the valve trane like the other post suggest. If you have fuel, spark, and it's timed somewhere near correct, then all that remains is to have the valves closed to make the big boom and make the final part of combustion which is heat, the part you are missing. You can save some time by doing a compression check on the cylinders involved.
This will tell you if the valves are closing.
 

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Discussion Starter · #12 ·
ok well i bought a compression guage, and did a test Vs. known good cylinder. the good cylinder read 165psi. #1 @ 160psi, #3 @150psi, and #8 @165psi. does there seem to be a compression leak? so what next? new distributor? plugs? wires? would a .055 plug gap w/hei cause the backfire? how about a bad ignition coil? thanks again

mike
 

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Did you try a good valve adjustment? My Dad was having the same trouble, we did a good valve adjustment and it was like a new engine!
 

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I think the old rule of thumb for compression testing is a 10% deviation ( for checking general engine condition), but in your case I would have xpected a few really low readings. Check them all. Don't assume it's # 1 3 8. Hopefully you won't find any outliers ( way off numbers).
 

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Discussion Starter · #15 ·
how or what do i do to the valves in order to adjust them, do i need to remove the heads??? any machine work required? thanks

mike
 

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Have you checked all of the "easy" things before you try the valves? Have you thoroughly inspected the cap and rotor for signs of arcing or tracing? I find it hards to believe that valves are a problem considering your problem occurs at idle and your compression is good. Have you checked the float level in the carb? Is the idle mixture set correctly? I think you need to keep looking.
 

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I would follow Greg’s suggestion as a CHEAP next step.
Change the plugs.
As I was in the final stages of my restoration, I was having the same type of backfire through the carb problem. As the engine didn’t have many miles on it and wasn’t running real good when I bought it, I suspected the worst, bad carb, valves, etc etc
I asked a friend who loves to build big block to go over the engine for me as soon as I got it on the road.

Well before I could get it to him I checked the compression, OK, and replaced the plugs and reset the timing from ~10 BTC to 6BDC. The problem is GONE and I still haven’t had the time to get the vales adjusted and carb gone over. Try a cheap next step, change the plugs.

Keep us posted,

Bill
 

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7duecemalibu, Yes .055 is way too much gap,the recommended gap is .010 over the stock gap. You do have 12 volts going to the HEI dont you.
 

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Discussion Starter · #19 ·
ok well, i have been busy the last few days, and finally i checked all cylinders for compression. here they are:1-160psi,3-150psi,5-160,7-165. 2-165,4-155,6-155,8-165. i just bought new plugs ac 45t's, i will gap them to .45 tomorrow, and i will update you from there, thanks to everyone so far!

mike
 

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Hey 7deuce!! Something you might want to check is the voltage going to the HEI. I believe, if I'm not mistaken that the '72 is the last of the point distributor cars. The points used a reduced voltage via a balast resistor or a resistor wire in the harness. The HEI requires a full 12 volts to operate PROPERLY. It will however "operate" & give symtoms like you've experienced. I went thru all the same things on my '67 & found that the voltage was reduced to 9 volts!! Just use a volt gage & measure what's feeding the distributor (from the ignition). It's worth a try. If you find less than 12 volts, rewire to a heavy wire 12 volt ignition source. Let us know what the result is.
Good Luck----ODIE.
 
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