Mercruiser 7.4 top end issues

Scott Danforth

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you would have to have casting numbers of the block and heads to identify what you had / got

Gen V was cast, Gen VI had forged crank and rods, however they were the small bolt rods
 

alldodge

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The 502 heads are rectangular ports, so if the current heads are not, then you have less HP, so less available to get up
 
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The 502 heads are rectangular ports, so if the current heads are not, then you have less HP, so less available to get up

Oval ports, the gaskets i bought were rectangular so i had to get another set, so it’s the less HP model but it got on plane before with no problem, and also did 65MPH at i think 5,000 or so RPMs. This one takes 20 seconds to get on plane when it chooses it wants to and tops out 4,000
 

alldodge

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Oval ports, the gaskets i bought were rectangular so i had to get another set, so it’s the less HP model but it got on plane before with no problem, and also did 65MPH at i think 5,000 or so RPMs. This one takes 20 seconds to get on plane when it chooses it wants to and tops out 4,000

The above one is a bit different time line then the one below when thread was started

so long story short i pulled my engine two seasons ago due to a cracked block. Got a new used engine with forged internals and put her back together, drained and refueled. Won’t go up on plane, hits 3,000-3,500 RPMS and is boggy, if i throw a few guys up in the bow or go out solo it will get on plane and max out at 4,000.
Taking a system designed for rectangular port heads and putting it on oval port means its getting more fuel then it needs, so should be running rich. Running rich also means less power.

Knowing that it use to do just fine with new setup helps but for me makes it also puzzling. I'm going to have to think some more
 
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The above one is a bit different time line then the one below when thread was started

so long story short i pulled my engine two seasons ago due to a cracked block. Got a new used engine with forged internals and put her back together, drained and refueled. Won’t go up on plane, hits 3,000-3,500 RPMS and is boggy, if i throw a few guys up in the bow or go out solo it will get on plane and max out at 4,000.
Taking a system designed for rectangular port heads and putting it on oval port means its getting more fuel then it needs, so should be running rich. Running rich also means less power.

Knowing that it use to do just fine with new setup helps but for me makes it also puzzling. I'm going to have to think some more

The only thing that has been changed is the block. The heads, cam and intake/fuel system are all from the original motor so nothing should be different unless the compression, bore or stroke are different from the new block and i don’t think they are.
 

alldodge

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The heads, cam and intake/fuel system are all from the original motor

Recon I'm just to old to understand. A Gen 5 and 6 blocks for 454 (7.4) MPI all use rectangular port 502 heads, and yours has oval port. If yours was a L29 then it would make since to me, but being an MPI things don't add up. Guess I'm just not up to this, I'll bow out and let someone smarter then me continue and I'll quit side tracking your process
 
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Recon I'm just to old to understand. A Gen 5 and 6 blocks for 454 (7.4) MPI all use rectangular port 502 heads, and yours has oval port. If yours was a L29 then it would make since to me, but being an MPI things don't add up. Guess I'm just not up to this, I'll bow out and let someone smarter then me continue and I'll quit side tracking your process

I’m fairly certain these are oval ports, this is how the original engine was setup. Still haven’t had time to pull valve covers and check for issues but hoping tonight or tomorrow I’ll be able to look into it. Any other suggestions?
 

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