Need my RPM's back

[glenwhitt]

I have 96 seadoo challenger with the single 787/800 carb engine. I have 20 hours on new motor with hotrods crank, wsm pistons, R&D 43cc head, carbon fiber rotary valve. I have also replace carbs with brand new oem ones and have replaced jet pump with new oem and added skat track swirl impeller. Also have Micro Touch rev limiter and set at 7400 rpm.

Problem is I used to be able to get the boat up to 7200 rpm with the old nu-jet impeller. With the skat track I have a much better hole shot but the motor will only get up to 6800 rpm and so I have lost 5mph on the top end (now 45mph on speedo and 47 on GPS). I would like to keep the hole shot and get my speed back. I figure the cure would be to get the rpm back up. I was told a new aftermarket pipe with water injection would do the trick. What do you think? Am I way off base here?

BTE: Rave valves are all new from seadoo and spotless with new valvles bases, o rings, cups, cap springs, etc....

 

[seadoosnipe]

Fuel....

This sounds more like a fuel issue than a pump issue. If you can only hit 6800 rpm and your not bogging on the limiter.........then it has to be fuel.

You said you went to the OEM carbs. You have a few mods on this motor. Are you jetted for that R&D head and "hotrods" crank? What kind of flame arrestor are you using.

Without waiting for an answer, you could try to crack open your high speed bypass screw 1/8 to 1/4 turn to see if the additional fuel makes for a noticeable difference...........

 

[Dennis]

R.p.m.

Is the throttle cable adjusted correctly so that the carbs open all the way?
DAWG

 

[Gunsrunner]

if the engine is tuned properly and should be hitting 7200rpm, you may have an impellor that is to high a pitch for the hp output of your motor, repitching the impellor will help.

 

[glenwhitt]

Sorry for the long delay in the reply. I think the carbs/fuel is ok. I rejetted last year and forgot the sizes now. I did just a bit over on the stock main jets. Idle is good, not hesitation, no load, and color is good on both plugs since I used a size up from the MAG side on the PTO side. I talked to some people at skat trak and they agree with me sending the prop back to them so they can pitch it. the boat has outstanding take off with little to no cavitation (All new propulsion system including new seadoo jet pump). Last year I ran a nujet (number on impeller is not in books so I have no idea what it is. I have calls and email into nujet but nothing back from them yet) and the boat was hitting the rev limiter at about 7200rpm. I have the skat trak impeller off now and will ship it back tomorrow. I have also put the nujet back on and will mount the pump back on tonight. I already sealed the bump and filled with new oil. My new shoe seal is waiting at my door step now so I can put this back together tonight. This will allow me 24 hour dry time for the thread sealers and then its off to the lake this weekend for some testing. Will be 85 Saturday and in the 90’s Sunday. I have 170 psi in each cylinder, will run high test pump gas with 50/50 110octane race gas, and have my rev limiter set to 7400. if I am able to get to 7400rpm I will should get over 50mph out of the boat. 50 or above is what I was hoping for. Last year with nujet impeller and running 7200 rmp I got it to 50 (on speedo, not gps).

If I am succesful with this then I will try the skat trak again after the repitch. I was also told the old brass jet pumps are way better than the new plastic ones. Is this true? How are they better? I have been using the plastic one since the introduction to the skat trak impeller. I had the brass one in when using the nujet last year.

BTW: I put back on stock air box as I could never get the mid range hesitation out of the thing with the proxis arresters. Read about this on groupk site and sure enough after I dropped the stock air box on all was smooth again, after rejetting and tuning, again!!

I will post next week on my findings. THANKS GUYS