I will gladly give some one a prize that can nail this. I am at the end of my patience.

Back ground. Boat supposedly ran great last year. Seller was shady and its a long story please do not ask. Twin 1997 502 mags with vortech superchargers. 31ft Scarab with 4blade bravo 26p props. 65mph at 4k. Under propped.

Symptoms- Free revs in the water strong. In the water starts idles great and has good low throttle response. When trying to accelerate 1500-3000rpm motors feel like they load up on fuel. Smell rich and has black smoke. As soon as they hit about 3200rpm they will take of and run strong. Problem is not intermittent and both motors act the same way.

Motors- 140-160 compression on both motors.


Intercoolers pressure checked

Ignition- new plugs, wires, cap/rotor, Base timing set and checked timing advance with a light. Msd boxes bypassed with no change.

Fuel--Cool fuel system with inline feed pump, part of vortechs kit. Drained full tank to about 1/8 and filled with fresh fuel. Old fuel burned in car with no problem. It does smell old. Tank is spotless and no sigh of water. New fuel filters. Base fuel pressure sits at 50psi ( vortechs spec). Motors running the fuel pressure bounces 50-60psi. Pulled the regulator to check return screen and it was clean.

Injectors- cleaned and flowed with good spray pattern, also all 16 had ohmed out correct

ECM-Mefi 3- No stored codes ,tune up is unknown to me and was part of vortechs kit. Kit has been on the boat about 8years and claimed to run perfect last year then boat was winterized till I bought it.

It may be that the load is so great at those rpms with that much prop. Higher the load, the less the intake vacuum, the more the fuel psi goes up and the more the ECM itself adds fuel.

What gear ratio's are your drives ?

BTW: Since the seller was shady, I would not trust the ECM tune. I would get widebands on there and a person that can tune the ECM's afterwords.

BTW#2: I would think even stock 502's with Vortecs added , your boat would spin 26's closer to or at 5k. Hopefully some Scarab 31 owners will chime in on this.

Thanks for the reply.

Drives are 1.5

26p will run near 80@ 5400 or so is my guess. It has plenty of power once cleared out (over 3200rpm) and I only ran it up to 4k@65mph at half throttle knowing its not running 100%. Do not want to push it till its 100%. I know it really needs a 28 or 30 but the 26 should not make it load up.

As for the tune I need leave that as a last resort. I do have a dual bank wide band but will need to weld bungs into the exhaust.I would also need to purchase the software and cable to do the tuning.

Thanks for that info.

If you are convinced all is working well, then it's tune time.

Wide band is a must. Once you have it you'll never go without it.

It sounds like your tune is overly rich coming out of the hole based on the symptoms you described.

Quick update. Installed a wide band and ran it on a remote fuel tank. Same problem. Wow afr was 10.5 at idle and pegged my wide band rich at 9.0 under load.

Ordered up some flashing software and went to work. Had to wipe out the current tunes as they were locked.
They are only ruining about 2-3psi of boost (may get smaller pulley), 27* of timing and 10.50 afr. Time to sneak up on the tuneup and see what its got.

Fwiw, when I jetted from 11.2 down to 12.5 I gained about 500rpm and 5mph, so you can expect some huge gains from where you're at. Hopefully it's just bad mapping for you. I'm not familiar with your system but it seems like a fuel regulator isn't doing its job.

Thanks I am looking forward to leaning on the tuneup
. I have full fuel mapping control so its no problem to lean it out. Just need more time out on it.

27 degrees ? Wow. Get that sorted out, as well as the rich fuel mapping, you should be flying

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