Chevy backfires


Old 11-11-04, 02:40 PM
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Chevy backfires

I have a GM built engine installed in 1999 with approx. 55000 miles on it. I removed this from a 1985 Suburban and installed it in a 1966 Chevy pickup. After the swap, the engine bnackfires on acceleration. I have checked TDC on #1 cylinder and set the timing. I have installed a new distributor with cap and rotor. I have new plugs and wires. Yes they are in the right firing order. I have between 125 and 135 lbs compression on every cylinder. I have adjusted the valves to eliminate the question of one being too tight. Fuel pressure is within the 5 to 9 lbs spec. New carb installed. Idling has somewhat of a miss. I have found no vacuum leaks at all. Is this a cam problem??
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Old 11-11-04, 06:53 PM
Join Date: Nov 2002
Location: Canton Ohio
Posts: 1,397
Was your TDC placement on the compression stroke? The dist may be off 180 degrees if installed TDC on the wrong stroke.
Old 11-12-04, 04:09 AM
Join Date: Dec 2001
Posts: 2,538
if the engine is backfiring through the carb double check the ignition timing and make sure it is advancing when increasing rpm it is possible you have the wrong timing indicator for the balancer that is installed which would not allow you to set timing correctly with a timing light.
also take a vacum guage reading at idle and see if it has any bounce or fluctuatation and if vacum is low under 17 inches it is likely ignition timing is to slow. assuming the engine is stock.
rapid backfiring through the carb at higher rpm can be an indicator of a cam problem such as a flat cam lobe also a bouncing needle on the vacum guage would also be an indicator of something wrong in the valvetrain.
if point ignition is your dwell set correctly it should be set before setting timing.
Old 11-12-04, 02:40 PM
Join Date: Apr 2002
Location: Pittsburgh,Pa
Posts: 2,417
Which distributor are you using the hei or the points type.Bad condensors were notorious for that symptom.There was a time that unisets were so bad I would not install them I always put separate points and condensors in.Also if you are using the old points dist the shaft bushing may be so bad that the points are bouncing all over the place causing erratic dwell readings between cylinders.FYI the point dwell for a v-8 is 30 degrees.
Old 11-16-04, 07:14 AM
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I am using an HEI distributor. I checked the slack in the timing chain and found it was 7 degrees before moving the rotor so I installed a double roller chain and gears. I have the identical problem after this change. I pulled the intake so I could check lifters. Not even a slight dish in any of them. Cam is not worn so I have no flat spots. I really didnt expect it to be bad as the backfiring is not a steady popcorn popping. I did go ahead and install a new cam and lifters since I had it so far down. I may just stick some new heads on it and go from there.
Old 11-30-04, 08:04 AM
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I FIXED IT!!!!! after putting new heads, i deciced to take it a step further and installed mallory ignition. I also had to install an inline fuel regulator to drop the pressure from 9psi down to 5. one of the heads was bad as the valves were too far into the seat. they would not open far enough to handle the acceleration. why it started with the swap?? who knows .all i know is the truck runs like a---well you know. Thanks for everyones input.

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