Background:
225 engine with 30 over pistons. Engbldr stainless steel valves.
Rear mounted turbo setup for a blow thru carb application.
Carb is a holley 390cfm race-type, modified for blow thru application. No choke tower, smooth venturi type with mechanical secondaries. Four corner idle adjustable.
Carb is mounted on a modified Offy intake. Intake has had the 4-hole divider removed from the intake and the carb mounting plate.

Problem:
Carb was running wayyyyy rich at first. We jetted down to I think 50 size jets on the primaries, before we could get the A/F ratio into the 13s at idle.
On the dyno, we can get it tuned at idle and wide open throttle only. Runs wayyyy rich at part throttle. Tuning technician strongly advised me to ditch the modified intake in favor of an unmolested one. He reasoned the carb was not receiving proper feedback with the lack of the 4-hole dividers in the intake.

Solution:
I got a brand new off the shelf Offy intake, and we swapped them out.
We have had to go back and start re-tuning the carb from scratch. Primary jets are now near 60 sizes, not positive what they are exactly. And we are now able to tune at part throttle.

Has anyone else ever run into an issue similar to this with their intakes?

Thanks,
Brian

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Flyin' Brian

74 Duster, 225, rear-mounted blow through turbo at 12psi boost, street/strip car
"Sixy Beast"

I have not heard of this issue. Doc probably has played more with this stuff than about anyone.

I assume they tried a lower vac reading power valve to get leaner part-throttle cruise? Air bleeds are another issue. What does "waayyy rich" mean in numbers?

I have a really hard time getting myself to work on carbs now, even though I think I'm pretty good at it. Once you go to tunable EFI, it's hard to go back...

Lou

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Home of Slant6-powered fun machines since 1988

Lou is right. Just ask Doc as he is having issues with a modified intake on his car right now.

Just got back from Asheville and the car.

wayyyyy rich means my wideband O2 was pegged at 10.0

Right now we are waiting on some more air bleeds to arrive before progressing. Looks like I might end up with primaries in the 30s, and secondaries in the 80s. Not sure yet.

I think the power valve circuit was blocked for the blow thru application, I will have to check on that.

Brian

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Flyin' Brian

74 Duster, 225, rear-mounted blow through turbo at 12psi boost, street/strip car
"Sixy Beast"

I think blocking the power valve for blow thru sounds...wrong... at least if it is not a race only setup??
From what I have seen, they increase the size of the holes behind the valve?

My understanding of this is: the increased flow thru the pv allows you to jet it leaner, to make it run less rich, without boost?

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I have no idea what I am talking about.

The trick thing to do to a holley on the draw through application is blocking off the power valve opening at the baseplate of the carb, then tapping the passage and installing a tube so it can be fitted with a hose so the carb can use an external vacc. source for reference. That being said if the PV is not rated for the reference vacc. then it will close and the A/F will lean out. You are correct though, if the PV engages and it is not delivering enough fuel, it will also lean out, and someone will need to pullout their pin drill and open up the PVCR passages in the metering block.

A quick and good read on the subject is Page 66 in Dave Emanuel's Holley book on Turbocharging.


-D.Idiot

I didn't know Dave Emanuel did a turbo book. I have his Carter carb book. He should do a sequel to it!

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'72 Duster 198 stock cam, 3:23's Hookers on jack stands for 8 years in the driveway
'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
93 D350 160HP Cummins Auto Dually Clubcab needs a injector pump
2005 Golden Couch Buick

I think the secondary power valve is blocked, and the primary is open. Now I am thinking the primary power valve channel restrictors may have been drilled a touch too large.

Brian

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Flyin' Brian

74 Duster, 225, rear-mounted blow through turbo at 12psi boost, street/strip car
"Sixy Beast"