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Old 01-27-2008, 10:24 AM
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super rich mightey demon 750 blower carb

im running a 750 mightey bc on 350 sbc afr 195 heads comp nx268 camshaft and a weiand 142 running at 2.17 ratio with 8.33 c/r.
the carb wont cut fuel out of the idle circut no matter what i do it will run between 11.3 all the way into the 9's on a/f gauge with only about 5 lbs of vacuum. i figure the cam should make lots of vaccum with a 113 lobe sep. fuel pressure is at 6.5 lbs the floats are just barely at the bottom of the sight glass timing is 15 inital 30 all in by 2000 msd e curve them mains seem to cut in at about 2500rpm
The mixture screws don't really respond at all I've tried going down to an .027 ifr from .035 no change the car will barely run unless the idle screws are out less than half a turn out any help would be very appreciated
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Old 01-27-2008, 10:51 AM
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I spent quite a lot of time with the BG rep at the Delco Nats, I had the same issue with two 750 BC's on my SBC blower motor. His advice was not to mess with the IFR's, only as a last resort. Recommended opening up the primary idle air bleeds and go from there. I opened mine from .070 to .090, all four corners, and it's helped and I got away from the rich roll at idle. A spare set of IAB's was like $15.
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Old 01-27-2008, 10:54 AM
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I'll Try That Thanks
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Old 01-27-2008, 12:47 PM
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If you only have 5 lbs of vacuum, it is possible the power valve is open all the time. I know the stock one on a holley is 6.5 in of vacuum. If you only have 5 in of vacuum it will be open all the time.
As far as the vacuum issue, have you checked for leaks and are you sure your ignition and cam timing are correct.
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Old 01-27-2008, 12:49 PM
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Also where are you checking the vacuum. If you only have 5 in at teh base of the carb you definately have a leak somewhere. I have a 142 with a similar setup to yours and I have about 16 at teh carb base all the time.
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Old 01-27-2008, 01:49 PM
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i was checking vaccum on the 1/8th pipe thread hole on the #5 intake runner as for the power valve i put a 2.5 in to see if that was the problem and also dropped down to 72 primary jets with no change on the wide band or the vaccum gauge
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Old 01-27-2008, 02:11 PM
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I still think you have a vacuum leak somewhere. If there is a nipple on the carb for vacuum, check it there. Is the carb boost referenced if so what is the vacuum reading at the boost reference source
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Old 01-27-2008, 02:16 PM
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the carb is boost referenced i ran a line from the port on the intake to a distribution block and than out to vac advance carb boost reference trans modulator and vac boost gauge i also just read in the instructions not to run my brake booster off this port which is what i have done as there is no pcv or other vacuum ports on the carb except for the boost ref
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Old 01-27-2008, 05:03 PM
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ok i tried putting back the .035 ifrs and increased the iab's from .070 to .086
it dropped to about 12.5 air fuel and vacuum gained a little does and know if i should just keep opening the iabs up or should i go another route
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Old 01-28-2008, 10:16 AM
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after a little more messin around with fuel pressure etc i got it to idle in the mid 13's at about 10-12 lbs of vacum which still seems a bit low can i go bigger on the bleeds still or do i start making the ifrs smaller at this point
as i dont want to throw the fuel curve off its steady all the way from idle till the mains cut in but still seems a bit loaded
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Old 01-28-2008, 12:22 PM
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Quote:
Originally Posted by sheldonw
after a little more messin around with fuel pressure etc i got it to idle in the mid 13's at about 10-12 lbs of vacum which still seems a bit low can i go bigger on the bleeds still or do i start making the ifrs smaller at this point
as i dont want to throw the fuel curve off its steady all the way from idle till the mains cut in but still seems a bit loaded
I was getting ready to post when you called. Try the changes we discussed and feel free to call back or pm afterwards and let us know how it changed it.
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