Put L98 heads and comp offroad cam in my '90 K5 Blazer 350 TBI and can only get it to idle with the black/tan wire connector unplugged. When it is plugged in it starts but backfires violently through the intake and exhaust and dies. Base timing is set between 0 and 4 deg. Any ideas?

I venture a guess that it is either the ignition module, ECM, or wiring in-between (also includes the connectors) that is bad. By plugging the EST BYPASS connector back together it places the timing under control of the ECM. Part of this entails the ignition module doing it properly.

First check the wiring and connectors, then if you have a known good distributor swap that in.

RBob.

I metered out the wires between module and ecm and they all checked out fine. I also swapped out the ecm and module with known good units and still the same results. It idles okay with the timing connector disconnected ( not the smoothest but it runs ). When I try to start it with the connector connected it sputters and shoots flames or just fuel out the intake and will backfire out the exhaust. Is it possible the pick up is shot? The way I see it, the fact it runs with the EST BYPASS connector disconnected I would assume that the pickup is still telling the coil when to fire.

I am unexperienced in this stuff, but I am gonna venture a guess. Now, my car, if I unplug the EST wire, it idles real rough and lopes alot (completely stock). This is because the 20* or whatever advance isnt there. With the wire still disconnected, I can get the car to idle smooth by turning the dizzy way advanced off the tab. But then if you plug the wire in like that, the car wont run because the computer timing + the base timing would = like 50 or 60 degrees advance.

If for whatever reason your dizzy is not set at a base timing of 0*, you could be running alot more timing than you realize, causing the backfire I guess.

I am taking it for granted that you properly adjusted the base timing. However, I read that if the harmonic balancer has slipped, the timing mark can be way off.

Good Luck!

I've got it running now. Turns out the pick-up was shot. Even though it metered out okay, ohms were within spec, and kept vehicle running with EST wire unplugged, when I replaced it with another it fired right up. The old one had lots of orange corrosion under the tin ring on the windings. After playing with the timing a bit ( cam wants lots of advance) it runs pretty good. Had to drill out min idle cover and adjust to keep it running. Idles about 1100 in neutral and 800 in gear. Any lower and it stalls in gear. Thanks for the help. Hope this thread may be of assistance to someone else sometime.

whats vacuum guage reading?

and how dos the needle act?

steady?

movments?