Carb Jet Sizing Questions

Humbucker · #1 (**permalink**) 04-04-2006, 08:27 AM

Hey folks! I just did some upgrades and was wondering where you think I should set my jet sizes.

What I had: Ford Racing 392 crate engine with Performer RPM intake making about 430hp. I have a Jet Modified stage 2 - 750DP 4150 carb with 72's in front and rear. It ran great with perfect throttle response, but perhaps a tad on the lean size according to the plugs.

What I have now: Same everything with AFR 185 heads and a Victor Jr intake. About a 80HP increase according to the articles and all that crap.

Where do you more experienced guys think I should start with the jet sizes?

Thanks!

jwoodard · #2 (**permalink**) 04-04-2006, 08:37 AM

Jake,

There are many factors that determine jet sizes. Your best approach would be to tune it on a chassis dyno if there is one reasonably close to where you are. That way you can get it jetted correctly across your power band. If no dyno and you believe it is running lean now, go up a couple sizes in the primaries and a couple beyond that in the secondaries and tweek it from there.
I'm sure there are others with recommendations, but that's my 2 cents worth.

Humbucker · #3 (**permalink**) 04-04-2006, 08:43 AM

What would you know about my setup? The holes in your jets are probably the size of my thumb!

392cobra · #4 (**permalink**) 04-04-2006, 08:44 AM

What Jim said.

You said you have 72's front and rear.

Do you have a power valve on the rear ?

Humbucker · #5 (**permalink**) 04-04-2006, 08:54 AM

Quote:

Originally Posted by 392cobra

You said you have 72's front and rear.
Do you have a power valve on the rear ?

I honestly don't know. I would think it would be whatever comes with a Holley 4150 750cfm. I'm not too sure if JET Performance changes anything there or not.

I do have 72's on front and rear. It ran a bit lean before the upgrades, and it seems to now fall flat on it's face at about 3500 rpm. I made one run down the street and came back. It seems it was pretty obvious the jetting was low (at least I'm hoping it's that easy).

392cobra · #6 (**permalink**) 04-04-2006, 09:06 AM

I certainly don't know what Jet did in a stage 2.They may have added a power valve in the rear.

If not,you are under jetted in the rears by about 8 jet sizes.

Zoom This · #7 (**permalink**) 04-04-2006, 10:44 AM

I had my 850 Holley done by Jet a few years ago and they removed and plugged the power valve in the secondaries. If you ran lean with old heads and intake I'd install a set of 76 jets and get it to a chassis dyno to dial it all in at part and full throttle conditions. With the 76 jets, if your A/F ratio is within reason at part throttle cruise (13.5 - 14.5) stick with those. At full throttle, if you need to richen up the A/F, increase the secondary side only as needed to get the A/F to 12.5 - 13.0 at full tilt max rpm.