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  Jetting a Holley 750 DP ???

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Author Topic:   Jetting a Holley 750 DP ???
70 Pony
Gearhead

Posts: 1815
From: Glenmoore, PA, USA
Registered: Jan 2000

posted 06-01-2001 10:53 AM     Click Here to See the Profile for 70 Pony   Click Here to Email 70 Pony     Edit/Delete Message   Reply w/Quote
Have a Holley 750 DP on the 351 (30 over) Windsor TFS heads, EBRock Perf RPM intake.
Running a 14" by 4" K&N filter.....Just bought and put on K&N's 14" flow thru filter top and have a functional Boss 429 hood scoop which is right in the scoop.
Since install I have noticed when I am slightly on the accelerator (About an 1/8 of an inch) I have a slight stumble, also when going up hills. If I give it just a little more pedal...like a 1/4 inch it runs great.
I have the 750 jetted at 69 Primaries 78 secondaries.
Have tried all differsnt squirters from 28 to 50 and the 31 in the primaries seems to run best with 37's in the secondaries.
Now the question...I'm figuring I'm getting A LOT OF AIR and should Fatten up the jets.
Especially the primaries....because I may have a lean condition right off of idle..
Anybody agree with this and if so I am open to suggestions on jet sizes....whether to do just the primaries....or if something else may be the fix....
PS: When I stab it the car goes with no stumble at all....or even part throttle...it's just that little bit of pedal when cruising or taking off slowly..which is gone with just a fraction more pedal...
Thanks in advance.

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EDDIE D
Resto Mod '70 Sport Roof
Arrest Me Red

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Dave_C
Gearhead

Posts: 964
From: Gadsden, Al
Registered: Aug 99

posted 06-01-2001 12:26 PM     Click Here to See the Profile for Dave_C   Click Here to Email Dave_C     Edit/Delete Message   Reply w/Quote
I would jet it up to ~73 on the primaries. What Powervalves are you running? Might need to go to a higher number to get it to open a bit sooner. Stock pv's are 6.5. Maybe try a 7.5. Also check the air bleeds. Make sure they are not plugged.

My setup is simliar, other than a few more ci (383) and a solid roller cam.

I also have a KN filter (regular lid) and a Boss 429 scoop on my 69. Carb is a 750 dp, shaved choke horn for more flow.

It is jetted 73 front and rear with 4.5 pv's (due to large cam). Squirters are 41's front and rear.

Later,

David Cole

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kid vishus
Gearhead

Posts: 6060
From: middle of NC
Registered: Oct 2000

posted 06-01-2001 02:01 PM     Click Here to See the Profile for kid vishus   Click Here to Email kid vishus     Edit/Delete Message   Reply w/Quote
I would only go up 2 sizes on the jets in the primaries and see what that does. I agree with changing the powervalve first. It may be opening too late and causing the lean condition.

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richard bohm
Gearhead

Posts: 373
From: tucson,az-luray,va
Registered: May 2001

posted 06-03-2001 04:34 AM     Click Here to See the Profile for richard bohm   Click Here to Email richard bohm     Edit/Delete Message   Reply w/Quote
seems to me the 351w likes 71's in the primaries. as far as power valve goes you can try the 7.5 but i would change the jets first.

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70 Pony
Gearhead

Posts: 1815
From: Glenmoore, PA, USA
Registered: Jan 2000

posted 06-04-2001 08:27 AM     Click Here to See the Profile for 70 Pony   Click Here to Email 70 Pony     Edit/Delete Message   Reply w/Quote
Thanks all.....I'm running the 50 cc REO acc/pump at the primaries.....I changed the cam from purple to yellow and about 60% of the problem went away...Am using the 6.5 valve and was thinking of going higher with the jets,,,I'll try the 7.5 and 73's and let you know..
Thanks again.
PS cam...Comp Extreme Energy 500-512 lift
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EDDIE D
Resto Mod '70 Sport Roof
Arrest Me Red

[This message has been edited by 70 Pony (edited 06-04-2001).]

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mvierow
Gearhead

Posts: 209
From: Bay Area, Ca
Registered: Apr 2001

posted 06-06-2001 03:19 PM     Click Here to See the Profile for mvierow   Click Here to Email mvierow     Edit/Delete Message   Reply w/Quote
The accel pump only provides gas as the pedal is pushed. So if the lean condition is at part throttle and only at that specific spot then the accel pump would not be the culprit.

Changing jet's may solve the problem, but don't forget they effect the entire equation, from full vacuum to none. While this may solve a lean condition at just off idle rpm, it may also create an overly rich condition as well.

A different power valve seems like the way to go. Use a vacuum gauge to determine the pressure at the effected rpm, then use the proper valve to allow gas to flow at that pressure. For even more tune-ability, you could try a double vavle, one that opens partially at a designated pressure, and fully at another.

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steve'66
Gearhead

Posts: 8796
From: Sonoma,CA,USA
Registered: Mar 2000

posted 06-06-2001 03:37 PM     Click Here to See the Profile for steve'66   Click Here to Email steve'66     Edit/Delete Message   Reply w/Quote
Eddie,

I use a 4.5 power valve. 72s in primary, 78s in back and don't have a bog. I'm not sure it's a carb problem.
I'm thinking advance curve.... What's the curve set up like on your Mallory? You still using Vac advance?

SteveW

[This message has been edited by steve'66 (edited 06-06-2001).]

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70 Pony
Gearhead

Posts: 1815
From: Glenmoore, PA, USA
Registered: Jan 2000

posted 06-11-2001 10:11 AM     Click Here to See the Profile for 70 Pony   Click Here to Email 70 Pony     Edit/Delete Message   Reply w/Quote
Well believe it or not I had one power valve other than a bunch of 6.5's.....it was a 10.5. I put it in and it has seemed to have cured the problem.....
Steve....if you remember I am running a lot of advance....the car seems to actually like 20 initial....and when you were over was the last time I checked the total and I forgot but think it was around 38 at 3200 rpm..DOES THAT SOUND ABOUT RIGHT...
And yes I'm still using the vacuum secondary....
I'm still going to try 72's up front and try to lower the power valve down to 8.5 and see what happens....thanks...

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EDDIE D
Resto Mod '70 Sport Roof
Arrest Me Red

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