I am trying to see what kind of jetting i need for two carbs:


750 holley with vac secondary


Proform mechanical secondary


The 750 vac is non jetable secondarys so the primary is all i need.


It is on my 89 camaro with a 383:

Performar RPM air gap
1 inch spacer
292 angle plug heads HEAVily ported and polished
.030 over 350 with 3.75 stroke
9.7 to 1 dish pistons
headman long tube headers into custom Y pipe then straight out the back
Comp XE roller with 236/242 @.050 on 110LSA and 520/540 lift


Thanks for any suggestions.


Zane

Start with stock jetting, tune from there.

I have not "stock" jetting reall, the vac sec 750 has 70 primarys, i am going to bump to 73s today, the mech sec pro form was a main bodu i had to install so there is not jetting base line for it ither.

72 primary is stock for the 60's bbc vette motors, so that would be as good a place as any to start. I would venture a guess that the motor will run on the 70's that are in the carb now...once it is running, you can start checking plugs (or looking at an A/F meter) to see how good it is running on the primaries before the secondaries open.

Well, had 70s in it, went to 72...fely good, went to 74s and it is not bad. Ill check plugs tomorrow. The hesitation at idle to WOT is not as bad and olmost gone with the 74s, but it idles rich a bit.

Ok, time to knock back the idle mixture screws a bit to lean out the idle (You doing it via maifold vac gauge?)

Hesitation comming off idle could very well be the accelerator pump cam & shooter. What color cam are you using and what size shooters?

Vac sec has 32 squirter, the mech sec has dual 31s. I just go 2.5 turns out, i went .5 back in on the vac, it is ok, i might go to 73s in the vac, the 74 seems a bit too much.


The mech sec doesnt want to acclelerate under a high gear at low RPM, i am thinking it is normal for a mech race style carb.

Maybe start with 70 in the fronts and 80's in the rear. 3.5 Powrrvalve in the front and a powervalve plug(blocked) in the rear. The air bleed size also matters but jetting wise, 70/80 should run. I run 74/84 on my 396 that has more cam/compression/head... goodluck!

When you sell those and need a custom piece call Pro-Systems. They'll set you up perfectly.

my old engine liked 76/88, I believe stock for proform 750 was 74/84.

What cubes and cam/heads/compression was it?

each motor is different. Even identical motors will want different jetting. Start learning to read plugs, buy a wideband 02 setup, or get a dyno tune.

I know this, but two identical motors will have close jetting. I cannot aford wideband right now, still need rearend, trans and suspension. Judging by my plugs, it seems a little fat, just a bit. I need a different ignition module, my motor is falling off at 6000, it should be pulling a bit higher i believe

Make sure the floats are high enough. Mine once had issues with a low rear bowl. it wouldn't pull past 6000 without breaking up from being lean. Raised float, pulls hard to 7000 now.

Stock jetting in most 750 Vac Sec Holley carbs (3310 list#) is 72 on the primary side. The secondary metering plate (should be stamped with a '21') is non-adjustable but is equivalent to about a #76 secondary jet. Just starting points- plenty good to get you up and running "roughly right."

Alright, i am in the ball park, a little richer than that, ill try to make sure my floats are good too.