90 suburban, 5.7 tbi; hesitation with backfire

Hi, I'm an ASE tech and I used to consider myself a bit of a specialist
at solving drivability issues, but I'm stumped on this one. Hopin'
someone out there can help me.

The history: This 90 suburban, 5.7 TBI, has been setting for nearly 5
years and was towed to me in this condition. I have drained out the
stale gas and chemically cleaned the fuel supply from the tank to
through the injectors. The injectors were initially frozen but appear
to be in good shape now. I replace the inline fuel filter after
cleaning. Old oil was drained, refreshed and the crankcase chemically
cleaned and flushed again with new oil and filter. New plug, wires, cap
& rotor. Timing set to 0 degrees. After extensive driving the scanner
shows no codes and all data parameters are within tolerance (TPS, MAP,
EGR, etc). There is a puff of blue smoke on initial startup after
sitting overnight showing valve seal leakage. Vacuum circuits have been
inspected and repaired as needed. Compression for all cyls are 170-180
psi. Has smooth idle and no engine or valve noise. Upper distributor
assembly (magnetic pick-up) shows rust, which I have doused with wd40.

The symptom: The suburban will feel doggy on acceleration. If I
accelerate from a stop to about 25% throttle it will act weak or lean
and then, after a few seconds of lag, as if the system finally realized
that the throttle is indeed open, etc., she will come to life and take
off like a new truck. If I punch the throttle, from 0% to a sudden 75%,
she will backfire through the intake almost every time. This symptom
seems less obvious when the system is in open-loop and is in warm-up
mode.

Any ideas on this matter would be greatly appreciated. I've got
mechanics block and I need a nudge.

My thanks in advance,

Wes

"Hog Whisperer" wrote in message


Backfire has to be a timing issue. 5 and 7 crossed perhaps? or the timing
chain is in real bad shape, or the pick up you doused with wd40 has had it
would be my guess.

Whitelightning