'747 top end fuel leanout

["NickG" <nikog@mediaone.net>]

Vehicle in question:
'93 GMC pickup, 5-speed
1989 '747 TBI ECM installed
350cu in, ported iron heads, mild cam
ported alum intake, Holley TBI unit, headers, dual exhaust

The part throttle driveability is very good, with a smooth idle and good tip
in acceleration. WOT acceleration is also good, but only up to 4600rpm or
so. Using a Diacom, we've seen the O2 put out 950mv up to around 4600rpm,
and then it drops to 500mv or so (and the power dies out). Obviously, it's
running out of fuel.

So I installed a Fluke 88 today to see what the injector duty cycle was
(maybe they're going static?). The DC peaks at 57% right around 4600rpm. I
figured that there's plenty of DC time left, so I increased the injector
constant (BPW) from 108 to 130 as a test. I go for another drive, and the
max DC didn't change one bit.

Any ideas? Is there some set limit in the control program that limits the DC
to the injectors? How do I get more fuel in the upper RPMs?

Thanks,
Nick


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