My Boss has had a stumble in his 94 Blazer for a little bit now and asked me to look at it.

I had Datamaster running on it and was watching the timing, when I give it a good stab it pulls it down to almost 0 advance. I'm just trying to determine if this is normal. If I ease into it(to avoid the stumble) it follows the curve up liked I'd expect. Datamaster is registering no knock when the timing is pulling back and doesn't trip a "Timing Retard" indicator.

Its almost like the map is set to pull that much timing when you stab it. I've not tuned any TBI engines so this seems rather strange to me, and I don't have a chip burner yet to read the PROM and see whats in the tune.

There are other issues I believe are the true heart of the stumble(Tune up needed, O2 fault) but I don't want to ignore this timing thing.

 

Its normal...Stock TBI 350 runs 0 to -4 btdc under 1000 rpm at 80+ kpa.

As for the tip in stumble I would check 4 areas.

A.) Fuel Pressure make sure its atleast 11-12 psi under load. Factory minimum spec of 9 psi is inadequate imo.

B.) Base Timing should be 0 or TDC with the timing wire disconnected.

C.) Throttle body base gaskets like to leak to the rear of the throttle body causing the MAP to read wrong.

D.) Test the EGR, easiest way I have found is to unplug the vacuum line off the valve and temporarily plug it off. If the stumble goes away your egr backpressure valve is carboned up and the egr valve will need replacement.