Loss my rpm at 4000


B

Buker2006

New member
Newby here sorry I'd this has been discussed I'll try to be brief. Bought a formula end last year 223 with 5.7 magnum. Ran good had a bog rebuilt carb took it out worked amazing Rev to 5k . Figured prop was small I'd take it easy. Well not due to rpm but due to huge amounts of rust built up in engine heads and manifold I over heated cooked the engine. Winterized put away.
NOW spring 2021 arrives had a brand new engine built. Different block. I put on new manifolds new risers rebuild carb again new chock Stat new cap rotor plugs . Wires coil seem excellent (msd upgrades) all in starts and purrs like a kitten drop in lake to break it in. Keep my rpm between idle and 3500 ish. And now have 10 hours . Time for a little more juice. So remembering prop and rpm from previous season did some math figure I should have a 21 pitch. Borrowed one to try hit the lake and putt around a bit make sure all is good now wanting to give it some go go juice. I can only hit 4000 rpm max. Play for a bit no change just figured I put to much prop maybe a 19p be better.
So past Saturday hot need the lake need to boat. Through my 17p back on figure I'll be carefully and just enjoy the lake add up few more hours on the break in.. well surprised as I was again.. max rpm is 4000 . Stumped. I've now checked base time make sure it was right on the rebuild. And it's perfect 8 degrees.

So now I'm stumped the 17p prop should have had me over reving like season before and no gain. Only noticeable difference is time to plain .. much faster with the 17. But still no rpm. I should be 4600 to 4800 and 5k won't kill it But not a place to run all day..

Any other suggestions would be amazing help thanks ..
 
There are a lot of 5.7's out there, carb, TBI, MPI and they changed as years go along

Assuming the rebuild was to same model block (year, casting number), what is the original motor serial number?

Edit: You did say carb, but still need serial number or at least a year
 
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Wow some how deleted my comment anyway.
Mine is a 1988 5.7 magnum 350 second Gen.. I was told I was getting a second Gen block same vintage (year ) of boat. I even took my boat for re/re and they could make sure everything was perfect vs me just saying what I had. It's carburetor. Rochester 4bbl
 
Being a 88 it would have had a TB-4 ignition. You changed to MSD, which MSD components are installed (part numbers)?

With changing from 21 down thru 17 props and WOT RPM's not changing, its either the ignition components (MSD - My Spark Disappeared :rolleyes:) set with a limit of 4000, or the Tach is bad
 
Tach is brand new out of the box just did a gage conversion to get rid of the black square. And it's the same ignition coil as last year and I Rev out 5000 last year. Just a msd coil. And changing 17 to 21 didn't vary rpm even by 100.
Hence the whole confusing part
 
And yes thunderbolt ignition just was saying coil was msd. It came that way I didn't change . Sorry if my wording was incorrect
 
Have to agree this is a puzzler

Old motor reached 5000 and new-rebuilt can only do 4000, and changing props also does nothing to change

Even if the new motor was weak (something not done correctly), lowering the pitch should increase the rpm's

When the prop pitch was lowered (21 to 17), was their any speed increase?
 
It did go faster on the 21p but much slower to plain . I haven't yet checked the timing advance. (?) It's like it's under extreme load and just being dragged down
 
If its not advancing this means less power, but going to lower pitch is like gearing down, there should still be some kind of rpm change

You sure the boat has the same outdrive as before? It wasn't swapped
 
Using the bblades prop calculator, and not knowing what speed your going, I used 40 MPH for all, then changed speed to see possible differences

21 = 24% slip using 45 mph = 15%
19 = 16% slip using 42 mph = 12%
17 = 7% slip

Can you post a pic of the drive full down and the keel?

https://bblades.com/propeller-slip-calculator/
 
So put in lake last night timing is 7 degrees advanced is 38 degrees .
Did compression check hot and cold . Went up hot as it should.
But still low needs more break in time I do believe. I got a extra 300 rpm with some precision trimming . But it should still be reving more then this with less work
Everything so far is checking out as it should .
 
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