I have a 502zz,the motor runs lean on number 4 and 6 cylinders.The carb is a holley 850 vac sec.It is jetted with #90 pri & #94sec.All cylinders but #4&6 are running rich.I did not find any vacuum leak.Can anyone steer me in the right direction. Thank You Lorne.

What kind of intake? Using a spacer plate? Open or 4 hole?

Has this always run this way, or is this something that has just started?

It always ran like that.The manifold is the dual plane that came with the motor,it looks like a edelbrock performer.It has no carb spacer.Twice the #4and#
6 cylinders were scuffed from running lean.

I can't help but pin it on the intake gasket area.

If 1,4 were lean I'd say your primaries (on the right side) are too lean and it was showing up WOT. It 6,7 were lean, I'd say your secondaries (on the right side) are too lean and it was showing up WOT. But 4,6 too lean? And 1,7 look fine?

I say intake gasket seal. Just cause it doesn't exhibit evidence of a leak up above, doesn't mean it isn't leaking into the lifter valley from below.

Are you running power valves? That sounds like a lot of jet if you are running PV's.

I had the same problem with a marine 850 vac. secondary. Tried various jets and cross jetting, spacers of both types and never did get it right. 6 and partially 4 were always lean. In order to get them to show any color I ended up at 92 and 96 but then the other cylinders were fuel washing. Next step was to go with a 800 and Dart single plane. Got better with cross jetting but not perfect.

Final fix was to put a blower on with Nickerson prepped carb. Perfect color all around at all rpm ranges. You might consider calling Nickerson and see what he says.

Good luck but be carefull about fuel washing, when I was runing jets in those size ranges it would water your eyes if following close.

The 850 may be too large, causing poor distrabution at your cruise speed. Try installing a 4-hole carburetor spacer between the carb and intake. This will increase the air velocity, and may help with your situation.

Yes I am running power valves 6.5 & 4.5 sec The boat weighs 10000 pounds and only has on motor.

Yes, the a 4-hole carburetor spacer will increase the air velocity going through the engine. This will make the engine think the carburetor is slightly smaller (or make the carburetor think the engine is bigger).