Help me find my superjet's 500 missing RPM

[motobehrens]

I recently purchased a PET tach, so I could make sure my SJ is tuned properly before dumping more money into the motor. But after several smooth water peak RPM tests, I saw I was only hitting 6450 rpm. That's about 400-500 shy of where I should be. Dave/Impros said I should be about 7000 give or take due to setup or location. I verified it against another tach with the same results. I normally ride at lakes between 1500 and 1800 ft elevation. Local Phoenix temps have been 105-110. Peak MPH is about 47. It runs dead even with stock SXR's and other similar b-piped sj's. Ski runs very good and the plugs look good after extended high speed runs. Going from 3/4 turn out to 2 turns out dropped my rpm to 6350. I pick up about 2 mph when I ride at blythe, 200ft elevation, but haven't hooked up a tach while there.

Ski is set up as follows:
2001 SJ
B-Limited
msd enhancer
9/15 hooker
worx 201
stubby cone
stock 38 carbs/arrestor w/no choke plates and 1 screen w/ 75/130 jetting

I want to go down in main jetting, but that puts me back at stock total jetting.

Any one have any experienced input on this? pipe tuning, jetting, elevation, air temps??

thanks,

 

[Kirv]

i saw a thread on here about tuning carbs using the tach, wish i subscribed so i can find it quickly. sounds like you can tune that with the carbs (someone chime in if jetting /adjusters will make 500 rpm difference). also sounds like it can just be elevation since you're even with similar skis.

 

[Polish jet pilot]

WATCON.com has the info - and they say - do not use Tiny Tach!

 

[kevbo]

Your grate is keeping the top rpm down, not a bad thing. The stock grate will let peak rpm rise to the stock rev limit most of the time.

 

[Waternut]

I would first make sure the carb is tuned right. Since you seem to be able to maintain high speed runs, watch the rpms and see if they fluctuate more than a certain number. I think it's GroupK's site that has guidelines on how to tune with rpms.

 

[motobehrens]

Finally, I think I've found it.
I parked the dirt bike this winter and dedicated my time to tuning and characterizing changes on my sj. Since my original post, the motor has been rebuilt twice(thanks to issues with a new crank), oem carbs replaced with Jetworks 38's, went from stock f/a to K&N's, lightened flywheel, ADA head with 35cc domes(170lbs) modified oem waterbox installed, swapped back and forth between solas concord 13/16 and hooker 9/15. the Aftermarket F/A's put me at 6550, and I also picked up a 100 from drying out the pipe or ??

I closed my top screw and set the b- limited hot, sizzling first inch, with the bottom screw.

I went from 130 mains half turn out to 150 mains two and half turn out on the OEM carbs, trying every jet and screw turn in between. The ski ran very good to great during the jet size testing with all the mains tested, up to 6660 - 6720 rpm. It never burbled or bogged, just wouldn't get past that stupid RPM ceiling. I had Art modify a set of oem 38's and put those on, along with the filled crossover. Noticeable difference, and the top screw actually made a difference in the way the boat responded.

So then I was thinking maybe I was way overpropped, especially after talking to Harry at Groupk. So I pulled my Hooker 9/15 off and had Dave repitch it. Dave's a good guy. He set it to 8/14. I threw that on, and didn't really see any rpm gain, though it did seem to accelerate quicker. I put my concord 13/16 on, and didn't see any difference on the tach. I called Harry again, knowing that he had specs for the concord, and is familiar with having to tweak them quite a bit to acheive peak rpm. Harry's target for the 701 w/ b limited is 7200-7300. After discussing my setup with him, he suggested to hold off on sending him my prop, and throw in a stock or stock w/no rev limit CDI. He said that even though the ski runs great up to my 6700 rpm ceiling, it could be the msd. Even on the trailer, in the water, the ski wouldn't do above 6700, but it sounded healthy.
I pulled out the msd, and looked for anything visible. Rev limit set screw was loose. but it was all the way to the high side, 9000rpm.
I attached an oem cdi w/no rev limiter, and the thing sounded and revved great, just in the garage. I also had a new in the box msd enhancer that I tried, and it too ran and revved great sitting on the trailer in my garage. I quickly attached the one that I'd been running with, and noticed it definitely ran crappy at higher rpm compared to the other two. I was told that the stator output doesn't always match the input required on the MSD, which results in poor high rpm performance.

So this weekend I'll be testing that. It just sucks that the lakes that are good for tuning are over an hour away. My ski is at 48mph at 6700 rpm, so I can't wait to see how it responds now.

Sorry for the long post, I'm just relieved i'm making progress, and hopefully this helps someone else troubleshoot their stuff.

 

[kevbo]

I missed that you have an MSD. I could not get my PET tach to work with enhancers, the numbers never made sense. My Coffman Sizzler with longer than stock cone wont rev much past 6900 at 47-48mph on glass nomatter the prop.

 

[SUPERTUNE]

High load intake grate and a stock head @ high elevation = low RPM's

 

[rollin5150]

The light is at the end of the tunnel!! Lets ride!!

 

[motobehrens]

I tested yesterday and today.
I definitely had an issue with the previous MSD. This one actually allows rpm to climb when I hit light chop or the ski comes out of the water. Now that I'm past that roadblock, I monkeyed with the high screw. Art said he put in a bigger than normal main, since he wasn't sure of my porting/cleanup, and that proved true. The ski burbled and stumbled if I turned out the High screw. The ski hit 6780 and i knew I needed to rejet. I also saw an amphicar on the lake. That was cool.

I put in 125 mains and rode it today, and it definitely improved, but I still need to jet down. I was seeing 6820, but the lake was busier and difficult to find smooth water. I've picked up 1.5mph since replacing the MSD. I saw several 49.5 runs and a 49.7.

If I knew I had glass water to work with, I'd throw on the stock grate as a test to see how much it affects RPM, since I agree it would be an indicator I need to repitch. But long term, I need to tune this ski for the intake i'm going to run.

Next plan of attack is to rejet and test, then swap props and test again.

 

[motobehrens]

I've been using the Chuy screw to make it easier to tune the bottom pipe screw. no allen wrench needed and it's easy to break the screw loose if it's tightened up since last adjusted. The t handles are nice, but not as effective if your screws are a little tight in the thread from not being turned often enough.