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Author Topic:   Having Carb probs using air Scoop
bluestreek
Gearhead

Posts: 1724
From: Athens,GA
Registered: Jul 2001

posted 04-05-2003 09:13 PM     Click Here to See the Profile for bluestreek   Click Here to Email bluestreek     Edit/Delete Message   Reply w/Quote
I just finished my forced air induction on my scoop and I'm having some problems with the HP carb. A 12 x 5" tall K&N air filter is fully enclosed and sealed in a round airbox and the scoop is pushing a lot of air in. I noticed a nice boost under light and hard excelleration and cruises fine but when I back off on the throttle after a hard run, the carb is loading up and dumping fuel for a few seconds. Then it smooths back out. From inspection, it looks like the fuel is dumping from the primary bleeds and booster area. But why wouldn't it do it all the time and why only the primaries? . It doesn't do it when I just run the open breather stuck in the scoop. I can't figure this one out?
Help!!

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1966 Mustang Coupe: Custom glass hood and BIG scoop sits atop a 289 stroked to 331 c.i., Steel crank and girdle, 5.4 H-beams, Forged slugs, TFS alum. heads, ported Stealth 8020 intake, Xtreme 274 Solid Roller, Holley 750 HP, long tubes, 4speed, 9" 3.50 posi.
2nd pass: 12.11 @ 114 mph
DanH

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V8 Thumper
Gearhead

Posts: 4377
From: Arizona
Registered: Dec 2001

posted 04-06-2003 10:46 AM     Click Here to See the Profile for V8 Thumper   Click Here to Email V8 Thumper     Edit/Delete Message   Reply w/Quote
That's the damndest thing Dan
I had a needle/seat stick a while back, and it DUMPED fuel down the primaries . So bad the thing wouldn't run below 3500 rpm. All I did was tear the carb down, give it a good cleaning and re-assemble. Haven't had any float/fuel level problems since. I have however since then gone to a regulated delivery system

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1965 GT coupe, 333ci aluminum headed/solid cammed stroker, four speed, 3.70:1 9"

All Blue Oval, no blue bottle
http://mustangsandmore.50megs.com/V8Thumper.html

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bluestreek
Gearhead

Posts: 1724
From: Athens,GA
Registered: Jul 2001

posted 04-06-2003 12:46 PM     Click Here to See the Profile for bluestreek   Click Here to Email bluestreek     Edit/Delete Message   Reply w/Quote
I posted this problem on a few other tech sites and got some insights on my problem. It seems that the HP carbs are very sensitive to flow requirements because of enhanced metering system and downleg boosters. Holleys are designed to run under normal atmospheric pressure and the added boost from the "Ram Air effect" is screwing up the metering.
What may be happening is the forced air is pushing pressure spikes in the bowl vents and causing it to continue to push fuel out momentarily when the throttle is closed. Does that sound right?

The GOOD news: The Scoop and sealed airbox is really working good and making more power!!

The BAD news: I may not be able to run the airbox on the street unless I can figure out a way to get the metering circuit to work under changing bowl pressures and throttle positions.

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1966 Mustang Coupe: Custom glass hood and BIG scoop sits atop a 289 stroked to 331 c.i., Steel crank and girdle, 5.4 H-beams, Forged slugs, TFS alum. heads, ported Stealth 8020 intake, Xtreme 274 Solid Roller, Holley 750 HP, long tubes, 4speed, 9" 3.50 posi.
2nd pass: 12.11 @ 114 mph
DanH

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DidgeyTrucker
Gearhead

Posts: 1190
From: Greenbrier, TN USA
Registered: Oct 99

posted 04-06-2003 02:08 PM     Click Here to See the Profile for DidgeyTrucker   Click Here to Email DidgeyTrucker     Edit/Delete Message   Reply w/Quote
I've seen this done on Holley carbs before:
Get some 3/8" fuel line (or whatever ever fits on the vent tubes) and connect them together. Plumb a "tee" into the middle and run a length of fuel line OUTSIDE your air box. This will put non-pressurized atmoshperic pressure in the fuel bowl.
It would cost a few dollars to try it and if it doesn't help your situation you haven't made any major modificationis that can't be undone.

At the end of a hard run, you are still pushing air through into carb under pressure and it is pressurizing the fuel bowl and forcing the fuel out.

Tracy

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bluestreek
Gearhead

Posts: 1724
From: Athens,GA
Registered: Jul 2001

posted 04-06-2003 03:00 PM     Click Here to See the Profile for bluestreek   Click Here to Email bluestreek     Edit/Delete Message   Reply w/Quote
Thanks
Sounds like a good idea. I'll try it!

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1966 Mustang Coupe: Custom glass hood and BIG scoop sits atop a 289 stroked to 331 c.i., Steel crank and girdle, 5.4 H-beams, Forged slugs, TFS alum. heads, ported Stealth 8020 intake, Xtreme 274 Solid Roller, Holley 750 HP, long tubes, 4speed, 9" 3.50 posi.
2nd pass: 12.11 @ 114 mph
DanH

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Fastymz
Moderator

Posts: 18703
From: Reno Nv USA MEM#1240
Registered: Apr 2001

posted 04-06-2003 03:29 PM     Click Here to See the Profile for Fastymz   Click Here to Email Fastymz     Edit/Delete Message   Reply w/Quote
When I 1st set mine up.I was having the choke close on me,when I got out of it.I think it was because the carb was getting so cold.I also had to jet up my carb.I was running a carter at the time.With my Holley I did not run into any problems.Now mine is not a sealed system either.About 3.5" of my 4" air filter sits in the scoop,and .5" below the scoop line.My carb will be real cold to the touch after a hard run with this set up.

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SCOOP
oddly obsessed with big scoops on little Mustangs

2.26 60'S
14.9 @ 90.86MPH

65 coupe,351w,C4,Big Boss 429 hood scoop,8"3.40 TracLoc,Cragar SS,Black Suede.

https://mustangsandmore.com/ubb/Fastymz.html

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bluestreek
Gearhead

Posts: 1724
From: Athens,GA
Registered: Jul 2001

posted 04-06-2003 08:23 PM     Click Here to See the Profile for bluestreek   Click Here to Email bluestreek     Edit/Delete Message   Reply w/Quote
I tried venting the bowls to outside the breather and things got really nasty. The carb would surge and stumble even at cruising. Those little vents are sensitive!! So after playing with all the settings and getting nowhere, I put everything back like it was supposed to be and decided to follow a hunch from Todds idea . I dropped the fuel pressure down to 5 1/2 psi. Now it's working great! WOO HOO !

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1966 Mustang Coupe: Custom glass hood and BIG scoop sits atop a 289 stroked to 331 c.i., Steel crank and girdle, 5.4 H-beams, Forged slugs, TFS alum. heads, ported Stealth 8020 intake, Xtreme 274 Solid Roller, Holley 750 HP, long tubes, 4speed, 9" 3.50 posi.
2nd pass: 12.11 @ 114 mph
DanH

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