[Gmecm] Two '747 problems

[David Haggard]

Thanks for the responses.

Turns out the oscillating idle on cold startup was caused by too much ignition advance. I discovered that the cold compensation spark table was adding 16 degrees with the engine cold. Add that to the base 16 degrees from the idle area of the main spark table, and it was getting 32 degrees on cold start! (Base mechanical timing and spark bias both set to zero.)

I zeroed out the cold compensation spark table and the problem nearly disappeared. Now it oscillates about 500 rpm around the target idle speed about 5 times, then settles down. That final bit should be fixable with IAC vs. coolant temp adjustments.

As for the flat spot between 1600 and 1800 rpm, there seems to be a bit too much spark advance there, too. I'm cutting back the timing a degree or two in that range. If that doesn't fix it, then I have to conclude that the manifold is too big for the motor, and in that rpm range under light load the mixture is dropping out a bit. The ultimate fix may be a manifold with smaller runners, which would mean getting the Edelbrock TBI manifold and having its throttle openings enlarged to fit the 454 TBI.

Thanks again!

David Haggard
   '88 Formula Firebird "SR-71 Black 'Bird"

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