Fwd facing Hood Scoop leaning out carburetors

[Leadfoot]

Just wondering if anyone out there has had problems with carburetor's leaning out at speed ( 250 ) with a fwd facing Pro stock style hood scoop ? I keep richening the jetting and a couple cylinders (5-7 ) stay running Lean. I have plenty of room above the carbs and running 7 1/2 psi fuel pressure.

[SPARKY]

I am not experienced with a Pro-Stock hood scoop but I would try it with an air cleaner of some type and see if it goes away--  I had a car that would pull hard at lower speeds and fell on it face as speeds increased---put an air cleaner on it and problem went away---yep---I changed the scoop.

-5-7 was the reason for a 4-7 swap on BBCs

[Leadfoot]

I do have a 4-7 swap cam. Has anybody tried putting a hose in between the vent tubes ??

[hotrod]

I suspect you are setting up turbulence or a swirl inside the base of the scoop at speed.
Back in the 1960's - 70's Chrysler published plans for their ram air hood scoop used on the cars like Sox  and Martin

http://www.gilchristbnw.com/GPC/DR/PS/S%26M7201.jpg

The scoop was called a Bowman scoop as I recall. It was important to have the right dimensions above the carburetors to avoid the air flow turbulence screwing up the air pressures in the float bowl air bleed tubes and the throats of the carburetor. As I recall the bottom of the "duck bill" on the scoop needed to be about 1-2 inches above the top of the carb so the air would settle down and dissipate turbulence.

I assume you have a block off plate on the bottom of the scoop that only exposes the carb bores and air bleed tubes to the pressurized air in the plenum of the scoop.
If not, you could be getting hot air coming up from in the engine compartment due to pressure changes in the engine compartment at speed. This could cause some bores to see cold ram air from above and others pulling in hot air from the engine area on other screwing up the mixture once your speed gets fast enough to cause major pressure differences under the hood and in the hood scoop.

If it is turbulence a couple layers of screen or as mentioned above an air filter box could break up any air turbulence and equalize air pressure over all the carb throats and float bowl air bleed tubes.

If I can find my old direct connection book that has those plans I will try to post the info they had on that scoop design I am sure the modern scoops need to address some of the same issues.

[kiwi belly tank]

 Don't re-engineer the wheel by condeming the firing order, find the real problem before you torch a hole. Connecting the bowl vents won't fix this.
5&7 are going to be the first to lean out if you have a fuel supply problem.
It's impossible to diagnose from here with no details on the engine but if jetting up didn't help, you might have a weak signal issue with a bad intake gasket or maybe a tight intake valve adjustment or a valve problem.
Leak it down to make sure those two holes are good & then pull the intake & eyeball the gasket.
Never assume anything, check all the basics first.
 Sid.

[Glen]

Tom Bryan did a lot of research on scoops, maybe he will jump in and answer some of your questions.

[Leadfoot]

Ok there is some good points made in the above threads. The heads are fresh new gaskets seems to be a on going problem. How much pressure could I be seeing at 250 mph ?

[hotrod]

http://www.wallaceracing.com/ram-air-calc.php

Theoretical about 1 psi, real hood scoops only recover about 60% -90% of the theoretical, so you are looking at maybe 3% boost in performance or about .3 - .4 psi in a real hood scoop, but keep in mind if you are making 2000 hp 3% is a free 60 hp. (minus airdrag from the scoop)

[WOODY@DDLLC]

Check out the TECH&FAQs tab on the home page for some scoop on scoops! 

[wobblywalrus]

What method are you using to select your jet sizes.  Are you doing it on a dyno?

[Leadfoot]

I am jetting to reading the spark plugs and best mph. And the " the Great White Dyno ". Going to try some diffrent things this year , I have allready changed high speed jets from 33 to 28 and mains from 93 to 96. I also took out the 45 -90 degree  fittings from fuel system.

[Crackerman]

Are you seeing these issues only  at speed or on a dynamometer as well?

[Leadfoot]

At speed.

[kiwi belly tank]

You don't have your foot in it for a minute & a half on the dyno so confirm fuel supply with a gauge during a run. Put a pyro in #7 so you know whats really happening. I don't see how a possible scoop issue is only going to affect two cyls.
  Sid.

[Bob Drury]

  Gary, aren't you running Nitrous?  If so, I am assuming your not running out and seeing a pressure drop at the top end, and as Kiwi mentioned, those would be the most likely to lean out.                         Bob