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Hello, I am new to the forum and I am in need of help!! I have a 1989 Chevy 5.7 TBI (150k). it has always run perfectly, purred like a kitten. I just replaced the valve guide seals and since it has had a slow fluctuating idle(about 200rpm flux, can see it on the vacuum gage too), and if I step on it fast it hesitates and sometimes backfires (intake). test drive exhibits the same issues, dogs until rpms come up then it accelerated great. this is what I have done:

-rechecked the valve lash 3 times.
-new cap, rotor, plugs, coil.
-no vacuum leaks.
-checked timing.
-no codes.
-checked all connectors on sensors.

one item of interest, when I checked the timing after the valve seals replacement, the timing was really advanced. not sure why, i did nothing to disturb it.

Any help is greatly appreciated.
 

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When you timed it, did you unplug the electrical connection on the firewall? You have to unplug that, time it, turn it off, plug it back in then start it up and confirm that it is right.
 

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I'm having a similar issue with my 78 350. Bought it mid summer and would idle high, checked timing and it was at 16-18 btc. I set it backed down to 8 btc as it said in the manual. As it is getting colder out, I was having starting issues and also wanted to get rid of the quadrajet. Installed a Edelbrock performer 600cfm carb with the matching air gap intake. Installed new plugs, wires and msd cap and rotor (have HEI dist). Drove it home about 20 min all backroads and started to diesel when I shut it off. Talked to some people and was told to set initial timing to 4 btc. Did that (with vac advance off) and the truck ran like a champ. (was about 50 degrees out if it matters) Drove the truck to work, went to lunch around noon when temp dropped to below 30 and the truck backfired tru the carb when I went to take off from the parking spot after warming the truck up. Drove down the street, drove back, drove home after work and under any load or acceleration it would bog and backfire thru the carb. Checked my timing and it was roughly 5-6 atc. Dist bolt was tight, set timing again but this time to 6 btc and again runs great. How can my timing change on its own?
 
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