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4900 and nothing more

649 Views 4 Replies 3 Participants Last post by  Fyhr Factor
Hi folks. I seem to be a little perplexed about something and I hope you all can help me out.
I have a MerCruiser Gen IV 454 with a Volvo 280T outdrive. You can see the current build sheet here. http://www.supernova19.com/motor_build_ii_at_supernova19_com.htm If you look up a little on the page you will also see what I changed it from.
I say “current” because I have made many changes over the last year.
You see I have taken a junkyard motor and not wanting to do to much to it the first year I rebuilt it pretty much stock except I built it knowing I was going to do some upgrades later on. So when the motor was done last year it should have been around 330hp.
This spring I did the whole top end over again. Heads, cam, intake, you name it. Now it should be somewhere around 420hp.
Which brings me to my question. No matter what I do I can not get the motor to spin over 4900rpm’s. I mean I have done upgrades that should have added power but…. I know what you are going to say next. “It must be the prop.” I am comparing before and after the upgrades to the same props I have used before and have many notes on. And yes I have tried a couple props with the same result.
Leaving that aside for a minute I have to add this.
I have tried many, many props on this set up. Volvo aluminum 15X19L, 14X23L, 16X24L. Ellison racing prop that was a 14X22L and then reworked to a 14X24L with some cupping. A Johnson brass 15X19 cupped and a Stainless Steel 14X24L. They all have a WOT of 4900rpm’s with one exception.
The Volvo 15X19 did pop off the rev limiter once or twice. Again I know some of you are going to say, “What rev limiter?” Well even though the sticker outside of the little black box says “Thunderbolt IV” I have it on film coming out of wide waves at WOT. The motor never rev’ed over 5 grand when it should have jumped at least a couple hundred rpm’s.
What am I doing wrong?
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Did you upgrade the fuel system?

Put a pressure guage on at the carb and see if its falling off at 4900.

Check valve out of the tank,,, larger fuel line,,, weak fuel pump bigger fuel pump etc
I did have 6psi before I started all the upgrades but they are cheap so I will pick one up just to eliminate it.
But you bring up the fuel system. I did install a new fuel tank last year and did even think about the “check valve”. I think I will try a total fuel system make over this weekend also. :lick:
Rev Limiter or fuel starvation?

I was passing a barge in this video. Not very tall wake but deep (you don't see in the vid). You can hear in the vid the motor drop off as I pop over the tops and I always thought it was the limiter but now ............
This doesn't happen as much with the bigger prop I run now. It brought my rpm's down 100 or less when I installed it to around 4900.

but with the 14 X 24 prop I was using in the vid (below) WOT was closer to 5000rpm's and I had some issue then that I am sure is still around holding the motor back.

Video >>>> http://www.youtube.com/watch?v=YehWSTS9uPA
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I'd have to say something is seriously wrong if you can't get that motor past 4900 with a 19P prop. in a 19 ft. boat, I have a 19 ft. boat with a 350 mag that was pushing a 19P cleaver over 5100... So I'm thinking you have 1 or 2 possible reasons for your limited rpms..,
As was mentioned before, Rev limiter, maybe, but I'm not sure what the stock parameters are set for on that..I would think it would be set higher.,. restricted fuel lines, and/or fuel pump,, are very, very possible reasons for sure, but I can add to that list,,,, you can also possibly have restricted air flow ( try running without the FA AND with the engine cover Off.). another thing is, what kind of Edelbrock carb are you using, vacuum or mech. secondary's ??? are you sure the secondary's are working and opening ALL the way???.. If you have mech. or vac. secondary's, do a visual check their fully open and that fuel is dumping in the secondaries under load.. but I'm leaning torwards air starvation. not to be confused with a A/F problem..just another theory to concider..
But in any case, start with the simple basic stuff 1st, and verify you have enough fuel getting to the carb under load, and go from there..
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