Question for Holley experts

[Paul_G]

The question mainly regards the secondary side idle circuit. This a 950 double pumper with four corner idle. Engine is 505ci, manual trans, 3.91 gear, cam is 234/240 duration 542/539lift.

The engine runs smooth, no stumbles, pulls like a freight train. Idle AFR is low 13's, cruise is mid to high 13's, WOT drops to low 12's. It took a lot of work on the carb to get it here. Now, I am trying to smoothen the off idle, easy going street driving, clutch release area. The engine tends to go a little lean right there and gets jerky, AFR's spike to the high 14's +. Giving a light thump to the throttle as the clutch engages smooths this out, but is still jerky.

I have a lot of time tuning on the primary side to get my AFR's where they are now and still keep WOT safe and happy. The primary side idle circuit IFR is .041" and the IAB .062", secondary side IFR is a .038" and the IAB is a .069". I have not messed with the secondary idle circuits at all. Would enlarging them to the sizes in the front idle circuit make much difference?

 

[Aron Gleason]

Im no expert, but both primary and secondary idle feed restrictors should be the same size. As to your air bleeds you just have to experiment. Ive read the bigger the iab it will make the exhaust stinky cause the idle mixture gets richer from the ifrs. So to lean the idle circuit you need a smaller iab to compensate.

 

[WEDGEDYNAMICS]

You can adjust it with power valves if so equipped. The lower the vacuum number the faster they richen the mixture. 4.5 hg vs 5.5 hg. vacuum reads at the point of your issue. From my experience air bleeds are more for leaning out the mixture at higher RPMS when richer mixture from a jet change helps down in lower range, I looks like you are very close drive ability with an AFR is so help full. What is the centerline on the cam ? If it,s a 108 versus a 112 it can be all over the map until the cam "clears up". Lower HG vacuum power valves have worked for me , just a suggestion..

 

[6872n73]

David Freiburger just went over this on his channel:

 

[Paul_G]

You can adjust it with power valves if so equipped. The lower the vacuum number the faster they richen the mixture. 4.5 hg vs 5.5 hg. vacuum reads at the point of your issue. From my experience air bleeds are more for leaning out the mixture at higher RPMS when richer mixture from a jet change helps down in lower range, I looks like you are very close drive ability with an AFR is so help full. What is the centerline on the cam ? If it,s a 108 versus a 112 it can be all over the map until the cam "clears up". Lower HG vacuum power valves have worked for me , just a suggestion..

Actually with the PV number, it is all about manifold vacuum. A 10.5 PV will open when manifold vacuum drops to 10.5", and like wise a 6.5 PV wont open till manifold vacuum drops below 6.5". Fun stuff. High manifold vacuum = idle and cruise where the throttle is barely open, to low manifold vacuum where the engine is going all out, throttle plates open a lot. An AFR gauge is a necesity when doing this kind of tuning. Not optional.

It takes a while to get cruise and WOT sorted. The dance with the jetting and PV is a long one, trial and error. The PV opening point is by the number stamped on it, and PVCR size which is changed by drilling the PVCR larger when more fuel is needed, or threading in a tiny jet with a smaller precision hole drilled in it when less fuel is needed. In my case the primary jets are 80's, with a 10.5 PV, and a plug in the PVCR drilled out to .070". The secondary side is different. In my case it has 78 jets, with a 6.5 PV, and the PVCR's are drilled to .059".

Yes, air bleeds come in to play as more air is passing through the carb. Bigger air bleeds will create a leaner mixture at higher RPM, not changing mixture much at lower RPM.

 

[Paul_G]

All I did to it today was enlarge the secondary side IFR. Then readjust all 4 mixture screws for the winter time tune up. It seems to help smooth out the clutch engagement a bit.