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Discussion Starter #1 (Edited)
My Chevy 350 jet boat engine is running lean at the top end. It has a Holley 600 vacuum secondary with 66 primary jets and a metering plate with hole equal to 55 jets. It has a 2.5 power valve and a stiff plain spring on the secondaries. I have never seen a carb with smaller secondary jetting. Is this a normal set-up on a jet boat? I want to drill out the plate to richen it up and eliminate a slight top end bog. The engine is a Marinepower 325 HP, my third one over just 8 years, same original carb. The first two blew head gaskets and ate up the blocks and pistons. I want this vortec to last.
 

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lean problem

i would talk to a local professional, a lot of variables here .sounds like a move to a bigger carb is in order.possible 750 at least a 650.good luck
 

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Bouncing off rocks
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600cfm is plenty for that mild of motor. id lose the vacuum secondaries in favor of a double pumper. i have a newer 600 dper that i ran on a similar small block last year. as i recall, i had 68 primary, 73 secondarys, and a 6.5 pv
 

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I think I'd also change the power valve to a 6.5 if you are going to stick with the same carb. Is this the carb that came with the boat, the only time I've seen them setup like that is on OMC stuff. I think you'd be happier with a 650DP. Do you have logs or headers?
 

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Tough to read plugs with todays fuel and additives they use. You can put in a can of octane booster and it will show red on the porclain, light is lean darker is richer and so on.
 

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E-7 Sheepdog (ret)
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Must be one HELL of a cam for a 2.5" PV, switch it back to a 6.5, and put a lighter spring in teh seccondaries (kits exist), just don't go excessively light.
 

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Discussion Starter #9 (Edited)
It has the high performance water cooled 4" dual shiny risers. If I went to a larger carb it would need even richer jetting to compensate for the lower air velocity. I've seen that Holley page, mine's not on it #75009-2. This is the original carb but third engine. But it is a 600 cfm similar to a model 1850. It is a mild roller cam, with a very smooth idle, lots of idle vacuum. I did try a 6.5" power valve but it doesn't have a noticable effect because it is a jet boat. It revs easily and gets on plane in about 2 boat lengths. The problem I notice with drivability is at top end W.O.T. Not during transition.
 

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Discussion Starter #10
I took the secondary metering plate to my tech school and had the machine shop check it. The holes were larger than what Holley said. They were .066 and .067. We enlarged them to .070 and I can't wait to try it out.
 
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