Mighty Demon going lean under boost..

Here's the setup.Twin turbo Buick 355 in a 71 skylark.8lbs of boost.. Barry Grant 650 blow through with 6an feed... Aeromotive a1000 with 10AN feed and return. Mallory comp regulator with boost port. 1/2" fuel pickup. Custom mild turbo cam. Ported heads..Timing is locked at 25*.. Hogged out buick spreadbore intake with a 3/4" or so open adapter.

Here's the problem.. My cruising AFRs are at around 12s.. As soon as I get into boost it starts leaning out into the 14s and I even saw a 15 something.. I can make 8lbs of boost and never open the secondaries and it goes lean. It goes lean with the secondaries open as well..

I called BG tech support and they want me to put a 1" 4 hole adapter on the carb before I do anything else. They think the open area beneath the carb is causing it to lose signal.. I'll be trying this as soon as it comes in.

Do you guys think this could make a difference??

What do you think I should do?? I haven't been into the carb yet, so I don't have a clue what the jetting or PV #s are..
THanks for reading
Jay

 

Jay,
I am only running 4 PSI so far but the Quickfuel 650 does not go lean at WOT. In fact it seems to be a little fat at low 11's. I run 12.5's at cruise.

What kind of booster do you have?

 

I'm not sure I understand that question.

I stayed in it some today, and it's not as lean as I thought. It spikes up for a slit second and then levels out at about 13.3 or so. That can be fixed with jetting..

I'll put the spacer on since I already ordered it and see what happens. The car pullls great even in dry ass 87* heat

 

The boosters are the devices in the middle of the venturi that feed gas into the air stream. Annular boosters are bigger and take up a large portion of the venturi. They are designed to give you a better vac signal.

Glad to hear your making progress. It's a long road.

 

They are annular boosters..

 

There should be a sticker on the outside of the box the carb came in that will have the PV, squirter, and jet sizes. The Mighty Demon has removable air bleeds to fine tune air fuel, try the spacer suggestion first, if the intake is coming up short on volume/capacity in the plenum and runners, the 4 hole spacer would at least help pick up air flow velocity. If this doesn't work, the next thing would be the regulator not increasing fuel flow enough to compensate for the boost. It is possible the air bleed sizes may have to be decreased to allow the mixture to fatten up under the boost. I have essentially the same fuel system, and have seen others use the same basic setup to support 1,000 hp. :idea2:

 

Well. I went 3 sizes higher on the secndary jets and got it down to the low 12s.. 12.3 or so..

I'm at 75 primary and 86 secondary. I don't know the primary PV size because it looked like the gasket was gonna tear. The secondaries do not have a power valve..

What exactly does the power valve do????

My cruise is a little rich so I was thinking that I might go three more sizes up on the secondaries and one size down on the primaries???

I see the air bleeds that are removeable, but I have no idea what they do..