Offenhauser intake /carb problems

[Magnuman]

Two years ago I had a 350 ci engine for my Donzi restoration built, and unfortunately the engine builder is no longer in existance. I have finally put the engine in the boat and have been trying to get it to work but to no avail.
It will rev fine when not under load (attached to hose) but when its in the water it bogs down. I had TPC build carbs for it. They were modified 650's. The jets were 69 on the primary and 76 on the secondaries. I tried to lower these but still get too much fuel. (I believe). The plugs are all black from a way too rich condition.
The specs on the engine that I have are the following.
Compression on Pistons – 10:1
CC Chamber on Head – 72cc
Intake Runner - 18cc
Lift on Cam – 515
Duration – 246 Degrees @ .050”
Roller Rockers – 1.5
I have been told to can the Offy manifold and go to a Edelbrock performer, but I love the look of this setup if it can work. Any suggestions?
http://www2.***boat.com/image_center/data/500/710Engine_1-med.JPG
Thanks.

[Moneypitt]

First, WAY too much CFM......Second, WAY too little vacuum......Third, Probably 6.5 power valves wide open due to the staggered manifold. IMO, 1 carb would be plenty for that relativly stock engine, alot less problems and lots more boating fun............MP

[Floored]

That would be tits in a 69 Z28 but not in your boat. borrow a dual plane single carb intake and bolt on one of your carbs and give it a shot.

[SmokinLowriderSS]

Suggestions for the Offy Manifold ...............
Idea #1:
Locate a pair of 300CFM 4-barrels to replace the 650's (you have waaaaay too mauch carb there)
Get some oh... about #2 power valves in those 300's (the 6.5's are likely open way too early, maybe even at idle)
Idea #2:
Put together something like Oh......... 500 Cu In's at 10:1 and put THAT under the offy manifold and keep the carbs then.
Regular suggestions...............
Go with the performer intake in your boat and 1 of the 650's. You'll be MUCH happier.

[DeputyDawg]

You are definitely over carbed, but if those carbs have 6.5 power valves in them that ain't gonna work. The general rule of thumb on power valve selection is take a vacuum reading if you have a gauge, and take however many pounds of vacuum you are pulling at idle and devide it by 2. Example.. if you are pulling 5 pounds of vacuum you would need a 2.5 power valve.

[Tahiti350]

One of the Hot Rod Mags did an intake comparison a few months back, and had one of these mainfolds. The only way they could get it to work at all was to stay way rich due to the pulse timing and the plenum volume. They considered putting a balance tube between the two plenum's but didn't want to ruin the manifold.
Their verdict: Looks cool, but doesn't run for sh!t..
If you want to run dual carbs look at the "New" Edelbrock C-26 Airgap. It is equivilent to the Performer Airgap, but uses their in-line dual Quad top. I have an older C-26 that is going on my 350 as soon as I can find another Holley Economizer 450 CFM carb to match the one I have now.

[DansBlown73Nordic]

What would it hurt if he plugged off the power valves? Maybe just try it...

[SmokinLowriderSS]

What would it hurt if he plugged off the power valves? Maybe just try it...
Nothing wrong with trying it. I just think he's so overcarbed for anything short of 500CID that it's a dead end. Twin 650's add up to 1300CFM. IF he spun that 350 to 6-grand, and IF he could get 100% volumetric efficiency (never), he'd still only pull 1215CFM.
7,000RPM and 80% VE adds up to 1134 CFM needed - actually posible figures.
The motor just will never be able to utilize those carbs. Not enough airflow from the motor to signal the carbs properly for feeding, let alone mixture fallout /puddling/distribution troubles from low airflow velocity in the intake at anything but WFO above 5 thou.

[DansBlown73Nordic]

I run a pair of 450 cfm,s on a 402. It runs great. However thats only 900 cfm,s on a 402 I guess that could be the difference.

[SmokinLowriderSS]

You'd have to add a 3rd 450 to match up to those 1300 CFM numbers, and then I think you'd have similar troubles. Would look pretty cool though a 3x4 setup hahaha. Tripple 4-barrels WOOHOOO!! :D

[Magnuman]

Thanks for all of the advice.
I am planning on going with the Edelbrock performer.

[tbanzer]

Just out of curiosity what rpms are you planning turnig. The cam seems to have alot of duration for a marine motor turning about 5k?

[SmokinLowriderSS]

I'm not very experienced on small-blocks camming so I don't know how much duration 246* is (I'm running a lot more on a BB) but that .515 lift seems like a lot too. I do hope your problem isn't lack of torque to spin the drive. You may have to cut the impeller to get closer to 6K for the mouse motor's muscles to flex.
Had a guy on the board not long ago with a (I think) 502 pulling 685HP & 718 ft-pounds Q, but couldn't spin a berk A past 4,100RPM. His motor was set up as a hot street car motor without enough lower-end torque to carry it to the HP curve. He was running .629" lift and over 300* Intake duration with fully ported Rectangle port heads and huge (2.30" & 2.00") valves.
My nearly stock 454 on the same A is spinning it at 4700 on 350HP and pulling him by 10MPH. :eek:

[blowngas]

disconnect the secondarys on each carb and run it

[SmokinLowriderSS]

disconnect the secondarys on each carb and run it
Now there's another option, turn the 2 x 650's into 2 x 325's. Heck, give it a shot.