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sb chevy backfire through exhaustPosted by Tom McKean on September 07, 2003 at 00:38:04 from (66.248.97.21): I own a 1972 ¾ ton Chevrolet truck. I installed a rebuilt engine [350 cid long block] and ran it trouble free for approximately 12,000 miles when it began to backfire constantly through the exhaust. Highest passenger side compression readings were approximately 55 lbs. Readings on #1 and #7 were 172 lbs. Readings on #3 and #5 were 52 and 57 lbs. respectively. When I pulled the heads I discovered no hardened seats on the right head and the exhaust seats had been hammered flat. Additionally, the MT revealed the right head was cracked. The left side head and gasket was ok, however when I reinstalled this head the threads on one of the holes in the block between cylinders 3 and 5 pulled and had to be Helicoil repaired. The right head was replaced with a spare that was refurbished and assembled by a local machine shop. The left head was blue checked and reassembled with the existing springs and valves. When the engine was started again, it backfired constantly through the exhaust. There is no backfire through the carburetor. The fuel system is ok, carburetor rebuilt, new fuel pump, etc. The engine starts and runs easily. Carburetor cleaner sprayed around the intake and carburetor flanges do not seem to affect the backfire. Right side compression is 150 - 156; left side is 172 - 175. The exhaust system is in good condition. Ignition system is basically new; i.e. distributor replaced approximately 12,000 miles ago. New points, plugs, wires, etc. replaced with this installation. Dwell is steady @ 30° Valve timing was checked with a degree wheel and a dial indicator. The timing marks line up properly and opening and total lift are within specs. It’s obvious the timing chain and gears were installed new during engine rebuild and show no signs of wear or distortion. The only thing that seems totally out of whack is that I have backed off the adjustment on the hydraulic lifters until there should be valve noise. And there isn’t. The engine quieted for a short time after I backed off the adjustment, but sounded like the gunfight at the ok corral when I put it in gear and it then continued to backfire. I am beginning to suspect weak valve springs and/or bad lifters. Any help or suggestions would be appreciated. Thanks, Tom
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