HPBCANADA.COM

JETS, JETTING AND TESTING

D JETS
MOST TIMES the size stamped on the jet is a bit smaller than the actual jet.
Best to use a jet drill bit and a drill press if you want to be exact.

JETTING
Matt you are on the safe side with 56D jets.
Many times it is easier to play with the fuel mixture rather than changing the jet size.
Remember: think amount of gas in the mix, not the amount of oil.
A 20 to 1 mix has less gas [lean] so you run a larger jet.
32 to 1 has more gas in the mix so you run a smaller jet.
The colder the outside temp, the larger the jet.
Fuel with higher octane normally burns hotter.
Fuel with lead in it can improve performance a little bit. Maybe 100 rpm in a omc triple.

TESTING
Biggest mistake I see is people idle to the dock after the speed run and then check the plugs and pistons.
Don't idle the motor, shut it off immediately and do the inspection.
The closer you are to WHITE plug reading the closer you are to blowing up.
Many times you have no issue in testing but in a race things change and things like water flow may cause the engine to run a bit hotter. If you are right at a WHITE colour there is a very good chance that temperature increase will cause a problem.
NEVER EVER go by what some one else tell you what jet to run because they run that jet in their motor!

Great info. Im breaking in my new SST60 motor in the spring so good read!

I'm noticing gears are still available for the nitro lower... search for the 1985+ part number: 0397421

So will these work in older lower casings? (clearances, shimming, etc.. taken in to account?)

Yes Matt, however you will require the OMC shimming tool to set up the pinion gear on the vertical shaft. Any gear, bearing or case change requires re shim.

That's something I never understood about the OMC Nitro lower. If you are only measuring the driveshaft/pinion dimensions to shim only that sub-assembly, does it not mean that we are making the assumption that all other components are non-variable: The castings are perfectly machined within a couple thou, the gears too, and the bearings in between them.. If that is the case then all other parts besides the driveshaft are ALWAYS the same dimensions no matter what. You shouldn't have to re-shim if you are putting the same driveshaft/pinion assembly in to a different casing. Seems hard to believe that would be the case in real life (as to opposed to in theory).

Problem is all cases are not identical dimensionally. The only real accurate dimension in the case, is the distance from the centerline of the vertical shaft, to the forward gear bearing cup surface. I have measured that on several cases in the past. The vertical shaft bearing seat surfaces in the case do vary. Why, I don't know. That is why the shimming tool for the vertical shaft assy., but nothing elsewhere.

All the cases have to be identical in that regard if the only shimmed dimension is the driveshaft/pinion assembly..

So that pretty much confirms the way it is supposed to work. If you swap a pinion/driveshaft assembly in to another case, you don't need to re-shim. It is only the driveshaft/pinion assembly you are measuring and shimming. The rest is assumed to be to a spec (or just doesn't matter).

I've run in to some other issue now: when I bolt the lower to the mid, the lower doesn't fully line up with the mid. looks like the lower casing is offset forward by like 3/32in, and to the left by 1/16in. Gear shift works well, I can spin the whole motor by the flywheel and all seems ok, but the alignment of the casings (mid and lower) is off.

You should stick to Yamaha's and give me your defective parts lol or buy a $4 60grit wheel and smooth that lip out.
Touch it up with spray bomb

Spray bombs acquired. How many times do you have to re-assemble an OMC before parts fit together properly?