According to Dean Nickerson Performance- one of the "Supreme Carb" guys says a Holley 750 will actually flow more air & fuel than an 850...... ??? I used a 750 on a blown 454 full roller motor, and got plenty of fuel flow and air (over 650 hp)..... your problem is not the carb.....
a carb is protection from leaning a single cylinder... think about it... the same air/fuel mixture goes to ALL cyclinders, except in an EFI system. Look for a vaccuum leak!!! The carb ain't gonna fix it!!

Or water to that cylinder, A buddy of mine just holed a piston on a 401 due to a water leak.

I'm not buying it either. Merc ran 750 q-jet's and webers on their 502's. A 750 q-jet is plenty up through the 500-550hp level. You've got a vacumn leak would be my guess also.... Have your mechanic explain why '2 small a carb' would cause one cylinder to go lean and post it on the board for everyone to scrutinize. There's a lotta engine blowing experience on this board.!

Ok, here's the formula:

Displacement X Rpm / 3456 = cfm (assuming 100% volumetric efficency)

502 X 5200 / 3456 = 755 cfm

Now a marine engine can be upgraded a touch because the idle and part throttle characteristing are not so critical. That upsize is offset by the fact that your motor isn't 100% volumetrically efficent either.

Macdad260
If your carb was too small it would only act like a govener. You would only build power until the carb was unable to flow anymore air or fuel. My bet would be on Vacume leak or still too lean.
Tee

I agree with above also. A 750 will supply a 502 to 5200 r's.

I had an 800 on mine for the cusion of being able to go to 5500 r's.

Just as a point. Merc put an 800 holley on the carb'd HP500's.

DAVE

SuperV runs 800's off 5oo hp's on his motors he made 505hp at 5700 rpm's...never a lean condition...

Hi Mac
Long time no talk
Sorry to hear your woes.....but
One thing you mentioned was you do most of your running at 3-3500rpm. I really dont know much about webers..but that may be right before the secondaries kick in and your at your leanest point on the primaries. Running there alot could be a problem.
750 cfm is sufficent for a box stock 502...gonna guess 415-425hp?
CFM is the air rating...more cfm..more air, not fuel.
Having said all that.....My vote would be a nice shiny new 750cfm
4150hp Holley with vac secondaries. I have a 950 on mine and I think its great. I like the vac sec for the smooth transition and fuel econamy.
Dont get the 830..they actually flow closer to 900 so I'm told.
Good luck and enjoy
Dan

I agree with cord
750 cfm is enough for you.

100% efficiency in chevy/mercruiser = LOL!

Thanks for the help guys. It doesn't make sense to me either that a small carb, jetted correctly, would cause a lean condition. In theory, a 750 running WOT should act like an 850 running at 88% throttle, shouldn't it? I respect my engine guy's opinion but, just the same it, I think the problem was caused by the carb being just a tick too lean. I had never run it for that lenght of time at or near WOT (about 2 minutes). Prior to that it was just a short 30 or 40 second blast. I did stop and check some of the plugs a couple of times and each time I increased jet sizes it looked better. I thought I was OK. It looks like I was wrong. Oh well, it's only money, right? Also, when doing plug checks, the first couple of times I pulled all the plugs. The #5 plug seemed to have the least color so, I would just pull that one each time and one or two more for reference. From now on I will pull them all even if it is a *****.

I called Teague today to see what they recomended. They said an 800cfm 9022 would be the best for my application. They said Holley is having problems with the marine 830s and the 850 was more than I needed for my stock 502. That makes sense. The 800 is what came on the Carb HP500. The 800 is about $125 bucks cheaper than the 830 or 850 so that also gets my attention.


I am not going to buy anything for a while as I will not put the motor back in this year. I want to have it ready to go by next spring so I have a lot of time to do my homework. Any more thoughts or suggestions will be greatly appreciated. I can't think of anywhere to get better advice than OSO!