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  Found cure for rich idle on my 750 HP

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Author Topic:   Found cure for rich idle on my 750 HP
bluestreek
Gearhead

Posts: 1460
From: Athens,GA
Registered: Jul 2001

posted 01-30-2003 12:09 PM     Click Here to See the Profile for bluestreek   Click Here to Email bluestreek     Edit/Delete Message   Reply w/Quote
I have been tuning the 750 HP on the 331 and finally got some positive results. It seems that the HP carb body(Proform is similar) is tuned for big cam low vacuum setups. I was having a little problem with it being too rich and jerky at lower rpms on the 331, so I started playing around with the adjustable idle bleeds. These are the larger bleeds on the outside. Here's what I found out:

Changing the size of the primary idle bleeds can either lean out or richen the idle and transition circuits. It's just like tuning any other jets. You will reach an optimal size that allows the idle and transition to run clean and also smooth.

If you go LARGER in idle bleed size, you will LEAN out the idle/transition mixture. A point can be reached where the idle/transition is so lean that opening the throttle may create a slight bog. At this point, you can either back down in bleed size or go up in main jet size to see what work best.

If you go SMALLER in idle bleed size you can RICHEN up the idle/transiton. Going smaller may eliminate an unresponsive condition normally caused from low vacuum on a BIG cam. You might want to do this before trying larger main jets.

Keep in mind that you also have idle bleeds in the secondaries. These can be tuned the same way. You want to be very careful not to lean them out too far and create a bog here. There is usually no power valve there to compensate for the quick secondary opening, and you might want to stay a little on the rich side.


The main air bleeds (smaller ones in the middle)should probably be tuned on a Dyno, since they greatly affect the A/F mixture under hard pulls and WOT. They react the same way. Smaller is richer, larger is leaner.

Hopefully this will help some of you that run a performance carb on the street or strip. The air bleeds give you a little extra fine tuning tool when the jets just don't give you what you want.

Dan

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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, ported TFS alum. heads, ported Stealth 8020 intake, CompCams Xtreme Solid Roller, Holley 750 HP, long tubes, 4speed, 9" 3.50 posi, BFG Drag radials..

[This message has been edited by bluestreek (edited 01-30-2003).]

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capri man
Gearhead

Posts: 4320
From: doerun, ga.
Registered: Nov 2000

posted 01-30-2003 02:21 PM     Click Here to See the Profile for capri man   Click Here to Email capri man     Edit/Delete Message   Reply w/Quote
i couldnt have said it any better myself dan!!! lol seriously thanks for the info cause i got a proform and will probably need it one day.

------------------
mike r
racing is real
everything else is just a game.
81 capri-7.51 @89mph 1/8
1.54 60 ft.
http://prestage.com/site/site_display.asp?SiteID=141

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steve'66
Gearhead

Posts: 7226
From: Sonoma,CA,USA
Registered: Mar 2000

posted 01-30-2003 08:57 PM     Click Here to See the Profile for steve'66   Click Here to Email steve'66     Edit/Delete Message   Reply w/Quote
Thanks for the tips Dan!

SteveW

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kcode
Gearhead

Posts: 2760
From: alvaton,ky,usa Suburb of Bowling Green, M&M #79, MCA #29208
Registered: Jun 99

posted 01-30-2003 10:37 PM     Click Here to See the Profile for kcode   Click Here to Email kcode     Edit/Delete Message   Reply w/Quote
Dan,
I'm really surprised that you had that problem with a 750 HP. I had the same problem on my 347 at first trying to run with a BG Road Demon. It ran good, but it would spit out so much black smoke the neighbors thought I was fogging mosquitos. BG tech advised to drill out the air bleeds, then came back and said they wouldn't put that carb on a 347. I didn't want to drill out a fairly new carb, so I switched to a Quick Fuel Technology 750 DP with screw in air bleeds. The carb was custom built and test run on a BBC test motor. I can honestly say I haven't had to turn a single screw for the year and a half its been on the car, even with four corner idle air adjustments. I'd suggest QFT to anyone looking for a carb.

Mike

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bluestreek
Gearhead

Posts: 1460
From: Athens,GA
Registered: Jul 2001

posted 01-31-2003 12:32 AM     Click Here to See the Profile for bluestreek   Click Here to Email bluestreek     Edit/Delete Message   Reply w/Quote
Don't get me wrong, the Holley HP is the best carb I've ever put my hands on. There's never been any black smoke. It's almost like it was too responsive. When I would idle around in 1st gear (stick shift), it would start jerking like a top fueler. I tried everything to cure the surge. Lowering the power valve to 4.5 seemed to help, but it forced me to increase the main jets to keep the "neck snapping" takeoff. So I went back to 6.5 PV and previous jets and tried tuning the air bleeds. That's when I found that larger idle bleeds would increase vacuum at idle without changing anything else on the carb.
Now it cranks and idles cleaner, the "jerk" is gone, and the throttle response is still killer. Can you believe I'm only using #69 primary jets in a Holley 750 in the Winter time? I love my Holley HP!

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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, ported TFS alum. heads, ported Stealth 8020 intake, CompCams Xtreme Solid Roller, Holley 750 HP, long tubes, 4speed, 9" 3.50 posi, BFG Drag radials..

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