Idle drop while braking,backing out of driveway

[1994rcsbLS]

Hmm,
Good test for that would be block booster,run engine up to about 3000 in park and hold it there a few seconds and chop the throttle and see if it drops down below your set curb idle or stalls all together. If all goes good with that then set the parking brake HARD and using the foot brake do a stall test and see if it falters. If it passes that test it's pretty much a given the booster is shot.

doing this in morning. And checking vac as well. Also. Running a rear setup with no parking brake.

My best guess is the converter is too tight and possibly have a leaking booster and thus it leans out. What is your idle vacuum in gear? 6 in/hg? Are you using idle spark timing variation to help control the idle speed?

Will check in morning. Also it was tuned by a reputable shop in nc so I’m sure whatever needed to be done was done. Converter I don’t think has a play here as this is a newish issue.

 

[1994rcsbLS]

Hmm,
Good test for that would be block booster,run engine up to about 3000 in park and hold it there a few seconds and chop the throttle and see if it drops down below your set curb idle or stalls all together. If all goes good with that then set the parking brake HARD and using the foot brake do a stall test and see if it falters. If it passes that test it's pretty much a given the booster is shot.

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This is booster disconnected. At idle. With rpm increase to about 3k it increased

 

[L31MaxExpress]

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This is booster disconnected. At idle. With rpm increase to about 3k it increased

Is that in Park/Neutral? Probably 6" or less in Drive and even less in Reverse. With GM transmissions, Reverse is the toughest gear to make a fairly rowdy combination idle. The line pressure increases adding load to the pump.

 

[Schurkey]

That's really low vacuum. The booster hose is disconnected AND PLUGGED, right?

Was that cam degreed when installed, or just shoved-in "dot-to-dot"? What is the cranking compression pressure?

???

 

[Scooterwrench]

AND PLUGGED, right?


???

My thoughts exactly especially after he says it went to 3000 revs.
Pinch the hose or pull it and plug it.

 

[L31MaxExpress]

My thoughts exactly especially after he says it went to 3000 revs.
Pinch the hose or pull it and plug it.

The way he wrote his comment confused me at first too. As I read it, that was at idle and vacuum increased at 3K from that.

 

[Scooterwrench]

Hmm,cam timing off,lifters too tight,big honkin vacuum leak,spark timing way late?

 

[L31MaxExpress]

Hmm,cam timing off,lifters too tight,big honkin vacuum leak,spark timing way late?

Honestly with the cam specs given, I would expect about that much idle vacuum. My ball park guess was 6 in/hg given 16° overlap @ 0.050. My 383 only pulls ~13 in/hg @ 750 rpm with 6° @ 0.050 overlap and 218 @ 0.050 intake duration.

 

[L31MaxExpress]

My idle spark is set at 16° BTDC base and the Idle Spark Variation can move that 10° in either direction as needed for idle control. I reduced the idle timing to give the idle spark more control of the idle speed. I also have 30° SA programmed into the same area off-idle. It helps smooth the transition between idle and off-idle and back since the timing advance control is quicker than the IAC control.

 

[Scooterwrench]

Yeah,stepper motor is slow. I just went out and dug out the cam card for my Lunati Voodoo. 262/275 duration, .468/.489 lift 112 lobe sep. ,108 ICL I've never put a vacuum gauge on it but I'm sure it's got more than his. It's got a thump in the idle.