Holley 750 vac sec tuning question....
Thread Starter
Le Mans Master
Joined: Apr 2000
Posts: 7,562
Likes: 9
From: B'Ville NY
Cruise-In III Veteran
St. Jude Donor '05
Holley 750 vac sec tuning question....
I was recently at the strip and pulled a plug after a pass & noticed that my plug was dry and completely white so I feel it's running lean at wide open throttle. Under normal driving on the street the plugs look perfect with a touch of tan to them. My throttle response is good right now the way it is. If I take the throttle cam & move it from the #1 hole to the #2 it stumbles really bad off idle. Being that it's a vac sec carb with a metering plate and not jets on the secondaries, I'm not sure if I should install larger jets into the primary becasue it runs good under normal drving conditions :confused:
Any tips appreciated :) TIA :cheers:
Any tips appreciated :) TIA :cheers:
Race Director
Joined: Jan 2000
Posts: 13,018
Likes: 2,262
From: Corsicana, Tx
2020 C2 of the Year - Modified Winner
2020 Corvette of the Year (performance mods)
C2 of Year Winner (performance mods) 2019
2017 C2 of Year Finalist
Re: Holley 750 vac sec tuning question.... (SmokedTires)
If it's running good on the front side, I'd leave it alone. If you change primary jets it's going to affect that nice cruise mode you have.
Moving to #2 acc. pump hole brings the pump shot in quicker, which obviously you don't need if it bogs. You did readjust the accelerator pump when you tried it didn't you? If not, that would cause a big stumble.
You need to remove secondary plate ad check number on it. Then crossreference it in catalog or Holley book to see what orifice size it has for main metering. Then re-drill it to a size a couple of jet sizes larger. Very common to do it and it works fine unless you just want to buy a bunch of metering plates or convert carb to screw in jets.
Are you sure it's not running low on fuel pressure at the track? That could cause plugs to show lean too.
JIM
Moving to #2 acc. pump hole brings the pump shot in quicker, which obviously you don't need if it bogs. You did readjust the accelerator pump when you tried it didn't you? If not, that would cause a big stumble.
You need to remove secondary plate ad check number on it. Then crossreference it in catalog or Holley book to see what orifice size it has for main metering. Then re-drill it to a size a couple of jet sizes larger. Very common to do it and it works fine unless you just want to buy a bunch of metering plates or convert carb to screw in jets.
Are you sure it's not running low on fuel pressure at the track? That could cause plugs to show lean too.
JIM
Thread Starter
Le Mans Master
Joined: Apr 2000
Posts: 7,562
Likes: 9
From: B'Ville NY
Cruise-In III Veteran
St. Jude Donor '05
Re: Holley 750 vac sec tuning question.... (427Hotrod)
Thanks for the info. I will pull the secondary bowl and take a look at the metering plate. I have access to a number of different drill sizes (number, letter, and fractional sizes) so I should be able to drill it out some.
The fuel pressure question is very interesting. I have a gauge in the dual feed line connecting to the carb, and it reads good (6.5-7 psi) when at idle or rev'd, but I don't know if it changes when I go down the strip ?
Thanks :cheers:
The fuel pressure question is very interesting. I have a gauge in the dual feed line connecting to the carb, and it reads good (6.5-7 psi) when at idle or rev'd, but I don't know if it changes when I go down the strip ?
Thanks :cheers:
Race Director
Joined: Jan 2000
Posts: 13,018
Likes: 2,262
From: Corsicana, Tx
2020 C2 of the Year - Modified Winner
2020 Corvette of the Year (performance mods)
C2 of Year Winner (performance mods) 2019
2017 C2 of Year Finalist
Re: Holley 750 vac sec tuning question.... (SmokedTires)
You're likely OK, but of course it's one of those things you can never rule out 'till you check. Under acceleration, it's a lot harder to keep the bowls full. You might get a longer line to extend it out from under hood temporarily (hold in place with wiper) and make a run to make sure it stays up there.
JIM
JIM
Le Mans Master
Joined: Mar 1999
Posts: 6,744
Likes: 1
From: delmont pa
Cruise-In I Veteran
Cruise-In II Veteran
Re: Holley 750 vac sec tuning question.... (SmokedTires)
any time you check plugs have a new plug to compare it to because you do not need a plug to be tan to have the correct mixture. off white or light grey is preferred :chevy
