I am sure a few of you saw my other thread regarding my 383 stumble and I was wondering if maybe before I spent the time and effort to set up the car right if I should look into a different style/ size carb.

I am running a 650 double pumper now mainly cause I had it. But for a boat what is best? Mechanical secondaries or Vacuum? 650 or 750.... and so forth.

Keep in mind the boat is not a cruising boat. She is a little hot rod that is used for 1/2 hour blasts. Acceleration and speed are more important then fuel economy.

Chris

Call Demon's tech line. 706-864-8544. They were very helpful and nailed a TR setup for my 632. They will want lots of info about your current motor to get you the right carb.

Mechanical .....vacume secondarys can shut down too quick and at high speed in a fast boat could cause it stuff the bow punch a wave ......

On my 383 which is not used for crusing either I am running a 750 holley vac. secondary. Its in the GM parts book. I put it on and haven't touched it since works great... But it is not a marine carb..

650 Double Pump is the way to go.

I'm using a 750 QuadraJet on my 383 with the jetting done by yours truly. It works REALLY well and has no stumble.

I would suggest looking for a vac 750... is your carb stock out of the box?

Tom

I run 600 Holley Marine vacuum secondary carbs on each of three 383 engines. One 383 is in my X-18 Donzi and a pair are in my 27 Magnum. All are propped for 4800 to 5200 RPM. I did try a 750 cfm carb on the 383 in the X-18 an found no difference in top end.

I have a fresh Marine Holley 750 Vacume secondaries for sale if you decide one that. I can set it up just about anyway you want. Has the quick change secondary springs as well.

I run a Barry Grant 750 vac secondary on my 383 in my car. I don't know what is better for a boat but I know they will tell you that you need a 650 mechanical and 750 vacuum.

On the dyno my brand spanking new 383 tried both 750CFM and 650CFM Carter carbs.

Results:
750CFM flowed a max 511 CFM at 5400 RPM and made a max HP of 398 at 5100 RPM and a max torque of 438 at 3600 RPM. Jetting was on the money with BSFC in the .45 range and FA lb/hr in the 165 range

650CFM flowed a max 503 CFM at 5100 RPM and made a max HP of 387 at 5200 RPM and max torque of 432 at 3800 RPM. Jetting was rich with BSFC in the .51-.52 range and FA lb/hr fuel comsumption in the 180 range.

conclusions: leave the 650 on for breakin and then after 10-15 hours rejet it to the 750 specs, and pick up some HP and fuel economy. This is a case where bigger insn't better....

I'm looking for 400+HP once dialed and broken in

Should push my 2400lb bow rider very nicely with the addition of a new Alpha 1 SS drive I'm getting today.......