Best Guess for new jet size?

[dsilver101]

Sup everybody-

Well, I'm about ready to install a proform main body(750+) into my 650 double pumper. The jets I have now are 67pri-73sec. The jets that come with the kit are 73 & 81. I know that you always have to tweek your jets for tuning up, but I wonder could anyone guess what is a good place to start at? - Thanks

Here's the engine- 73 truck 454 - stock internals - 8.5 to 1 compr?
Heads - 336781.....73-85...oval...OPEN...454, 118cc chamber, 256/114cc ports
cam- dur 234,244 - lift .325, .345, - 7.2 rockers
Headers, msd ign. ,15 btdc, no vac adv,

[MagicRat]

Originally Posted by dsilver101

Sup everybody-

Well, I'm about ready to install a proform main body(750+) into my 650 double pumper. The jets I have now are 67pri-73sec. The jets that come with the kit are 73 & 81. I know that you always have to tweek your jets for tuning up, but I wonder could anyone guess what is a good place to start at? - Thanks

Fundamentally, every aftermarket Holley is jetted too rich out of the box for anything other than a race car. In my experience, jetting a Holley for a street application involves leaning out the jets a fair bit.

I might be wrong, but your existing carb might be WAY too rich as is, on the primary side. Whats the carb part number, (found on the air horn beside the choke flap)?

My Holley parts guide lists all 650 spread bore carbs as 56 to 61 primary jets and 72-75 secondaries, depending on the model and intended application.

It does not list any 750 spread bore carbs, but the Holley 800 spread bore uses 62 primary and 81 sec.

I would suggest using the jets that came with the kit. But chances are you might get better performance is you drop a jet size or two, especially on the primary side. I would try one jet size smaller at a time, although I have never needed to drop more tham 2 jet sizes from stock to get good performance.

I have a 650 double pumper on my '69 GTO. If you can, post the results you get with the kit.

[dsilver101]

The holley I have now is the 4777 , on the holley spec sheet it shows those jets as stock, they seemed to run pretty good, yet I never did do any dyno or track tuning. Just seems too expensive to me. It seems like it'd be cheaper to get an oxygen sensor and go that route. I looked at some similar double pumpers that were more like 750 -800 and it looked like there stock jets were simlilar to those that come with the kit. I think starting with those is a pretty good idea too. Yeah, I'll post how everything turns out- Thanks

[MagicRat]

For the 4777 carb, my book shows 67 pri and 76 sec. jets stock, so you are correct, the pri jets are stock, the sec jets are a bit leaner than stock, which is probably about right. Likely, Holley changed their specs at some point and they are original.

[dsilver101]

well, today I finished putting the car back together with the Proform main body, I used their jet set and it runs *way* better than before.
Throttle response seems a little better and the motor just seems healthier. An xtra 100+ cfm sure didnt hurt it either. I reccomend their kit.- Dave