regalman4925

I have a new carb on my boat and i need help jetting it. the previous carb was a mechanical secondary 750 holley with 78/88 jets, that’s what came with the carb, it came off of motors that were similar. I personally thought that the carb was holding the motor back. I never messed with it last year because i had too many things i was finishing on the boat. i just put a like new proform 750 mech secondaries race series carb on and it has tiny jets in it. my motor is a gen 6 454 that started out as a 330hp, it has everything listed below. If someone with carb knowledge can give me a idea of a starting point and what to look for as far as going up and down with jet sizes. Last year i ran 5000 rpm 99% of the time, hit 5300 once spinning a 23 pitch 3 blade that was stock equipment, a stock 330hp should have been able to do the same from what i have read, i really think there is some tuning needed to get the rpms higher so i can switch to a 25 pitch mirage prop, maybe i am wrong?

330hp gen 6 454
Crane 731 cam
Gil hp500 exhaust
Edelbrock air gap dual plane
Thunderbolt 4 distributor
Crane hi-6m ignition
K&N flame arrestor

Many other upgrades but i don’t think they will matter with jetting

regalman4925

Bump

pantera232

How much approx hp now??

Griff

The 78/88's sound like a good starting point as long there is no powervalve in the secondaries of the new carb.

regalman4925

i will have to check but i believe there was a power valve in the old carb also? could this explain why it was running so rich? i may try it with the 78-88 this weekend and see how seh does. fuel system just got finished the other night so she is about ready to run

Griff

If you were 78 and 88's with a secondary PV, then it would definately be rich and sluggish when the secondaries opened.

regalman4925

this may explain the lack of top end i have been trying to get.... there was in fact a powervalve in it. so should i lose the powervalve or jet down on the secodaries?

Griff

Put a powervalve blocker in the secondaries and leave the jets at 78/88's. They sell them at auto parts stores. The PV is basically the same as adding 8-10 jet sizes. Whenever you get into the secondaries, the secondary PV is open anyway due to the lack of vacuum. With the PV blocker, you can control the fuel mix with jetting alone and don't have to worry about the PV going bad.

regalman4925

ok i got myself confused because i glanced at it real fast, the old carb did not even have provisions for the power valve on it. i have the new pro form installed with a plug in the secondaries, jet extensions (they were already installed from factory) and 78/88 jets. we will see how she does.