1996 SportJet 120XR crank balance

[turbo.tony]

Hey guys, I know this is a stretch for this site, but where can I find bobweight formulas for crank balance on an inline 4 2stroke? I'm out of options and this is all that's left before assembly...thanx, Tony

 

[Paul Moir]

Re: 1996 SportJet 120XR crank balance

Check screamandfly.com. But half the folks there say it's no use even for racing 2-stroke engines anyway, and they like to spend money on that sort of thing. I know it's done as a percentage of the weight of your piston/rod/bearings/ring/pin assy, determined emperically to keep harmonics away from useful rpms. Something like 40-60% and usually 50%.<br />You know this is never, ever done normally right?

 

[turbo.tony]

Re: 1996 SportJet 120XR crank balance

Yep, heard that from EVERYONE (but I will add that ALL Merc Racing 2.5's and Marine Cranks performance builds are balanced, and it helps alot), plus, I'm not normal. I wouldn't have bothered either, except the new Wiseco's were 18 grams apart from each other (tech says if they're within 75 gms of each other they're fine. Ya, whatever), new OEM rods were 23gms apart, etc... Between finish porting and balance (which Force never did on ANYTHING [to keep cost down]), should increase longevity dramatically, not to mention 5800+ rpm's will rock on the little jet boat ;-) And you're also right on that being one of the 9 formula's they use, but that particular one doesn't apply to two stroke or inline 4 bangers. I figured it out tho, thanks to Gary Hildreth and Bob at Scat Cranks. Thanks guys...T

 

[Zamboni]

Re: 1996 SportJet 120XR crank balance

If your going to the trouble of balancing the crank with bobweights(google has some great results that apply on formulas) I would be weighing those Wiseco's and getting them within a few grams of each other also.<br /><br />You setting it up without the rev limiter or did you modify one?<br /><br />If you need the SJ120 merc service manual in pdf send me an email addy.

 

[turbo.tony]

Re: 1996 SportJet 120XR crank balance

Thanx Zamboni. Have a manual already... Pistons, rods, and all rotators are balanced to within .4 gms. Removed 81gms of material total. 5.4 left on the crank bottom side then it's done (when I get my missing oil pump gear back). I'm gonna try it with no rev limiter for now to see what it turns, but would definately rather have an option to set the max higher with a box. Haven't figured that out yet. Any ideas on a 5800rpm box would be very much appreciated. Thanx again guys!!...T

 

[funpilot]

Re: 1996 SportJet 120XR crank balance

Hey Tony,<br /><br />Unless you do something about the impeller, you probably won't see 5800 RPM except if your tach is 15% off. On hole shot, the engine will rev to the rev limiter as water is packed into the jet pump. After that, the amount of water moving through the jet pump tends to limit you to about 5000 RPM or so.<br /><br />My engine/pump installation on my Mirage is a bit buzzy though... so I know what you are going through. For me, this boat is only a toy.<br /><br />warmly,<br />fp

 

[turbo.tony]

Re: 1996 SportJet 120XR crank balance

I understand what you are saying funpilot. I was hoping for an increase just from the 30hp jump. Crank balance turned out to within .7gms. We're good to go guys!!!!!! Thanx again...T

 

[Capt Ken]

Re: 1996 SportJet 120XR crank balance

Just balancing and blueprinting the engine ain't gonna give you anymore power. Might be a bit smoother running but not enough to notice. Without doing something to the intake to allow more air and fuel and along with something with the exhaust to get the rid of the backpressure, you're not going to gain anything. Like Funpilot said, the impeller is going to limit your rpms. If you take a closer look at those Mercury racing engines, they don't look anything like a normal outboard. Ain't but two kinds of racing engines, blown up ones and those that are going to blow.

 

[turbo.tony]

Re: 1996 SportJet 120XR crank balance

Copy that Capt... .040" Wiseco ceramic teflons, full flow porting and tuner, .004" raised and flowed ports, 67cc milled head, searching for HO spark box, etc... If it blows, it blows. Tons O Fun either way!!!!!...T

 

[x1745]

Re: 1996 SportJet 120XR crank balance

If your going to the trouble of balancing the crank with bobweights(google has some great results that apply on formulas) I would be weighing those Wiseco's and getting them within a few grams of each other also.<br /><br />You setting it up without the rev limiter or did you modify one?<br /><br />If you need the SJ120 merc service manual in pdf send me an email addy.

Please email me Sj120 merc service manual x1745@hotmail.com, thanks