Starflite 100S bottom end on Starflite 90S powerhead/exhaust housing

[Dave_A]

My boat came with a dead (idiot left it out disassembled in the weather) StarFlite 100S motor...

I picked up a StarFlite 90S for $250, running, with a damaged lower end (lower will not shift into forward - you get a 'click' when voltage is applied, but the gears do not engage... Reverse and Neutral work, but it's like the Forward gear is stuck...

My 'solution' to this is to run the 100S lower on the 90S top-half...

The driveshafts are compatible - same length, same splines... The bolt patterns match too...

Both units are pure-electric shift - 2 wires, current to one is forward, current to the other is reverse, power off is neutral.

There are 3 notable differences:

1) The 100S lower is HUGE compared to the 90S unit...

2) The 90S lower appears to have an extension added to the top, as if it could be run in a 'short shaft' motor with said extension removed. The 100S does not have such an extension. Due to this, the 100S water-circulation tubes are shorter (but identical diameter) to the 90S units...

2) The 100S prop is 2x in diameter to the 90S prop.

That said, here are my questions:

1) Does 10HP make that much difference in prop selection? Should I run the 90S prop or the HUGE 100S prop? Or are the gears in the 100S lower 'different', making the prop more a function of the lower than the powerhead?

2) How do I get those water pipes out so I can put the 100S ones in? Just pull on them?

3) Any other comments?

 

[F_R]

Re: Starflite 100S bottom end on Starflite 90S powerhead/exhaust housing

As you have already figured out, the lower units are totally different, inside and out. The 100 is much tougher. The props won't interchange. Short shaft 100s are rare as chicken fangs. So you say you have one? There are such things as 5" water tube extensions. See the parts books. It is a major job to replace the water tubes. Really major.

 

[Dave_A]

Re: Starflite 100S bottom end on Starflite 90S powerhead/exhaust housing

The 100 I have, with the lower end I have, is the same size/length as the 90... Both are 'long shaft' in their current configs....

The 90 just has an extra 'piece' in there to get it that way, and it's water tubes are longer... Almost as if the 90 gearbox could be used in a long or short motor (just swap water tubes & drive-shaft & remove the extra piece)

You say the water tubes are a PITA to replace?

At present they are too long for the 100 lower... Meaning I either have to fix my 90 lower, or cut the water tubes to fit the 100 lower...

Further, will the 'large' prop cause any problems with the 90hp unit?

 

[F_R]

Re: Starflite 100S bottom end on Starflite 90S powerhead/exhaust housing

I'm in sort of unknown territory here as I have never attempted the swap you are doing. But I have cut off many a V-4 water tube. I have a neat little tubing cutter that is only a couple of inches or so long and will get up in there to do the job. Try your Ace Hardware to see if they have one. I'll take a picture of it if it will help....tomorrow.

The 100 prop may be a bit much for your 90hp powerhead to swing. Depends on how much pitch is has, and your boat/load.

 

[thwack32]

Re: Starflite 100S bottom end on Starflite 90S powerhead/exhaust housing

You can cut the tubes easily to fit. The tube cutter is the best idea because it automaticly tapers the ends in to insert easy. The 100-S lower unit was OMC's attempt to retrofit their already existing IO units to fit that one outboard, made for only 2 years. Instead of a close to 1 to 1 ratio used in your 90-S, the 100-S and the IO units were like 3 to 1 ratio enableing them to swing as large as a 20" wheel! Your gains will be in the hole shot as these really dig in, and they can handle the power of an inboard V8 so they really are tough. If you have a big boat to push, this is your answer, however if you have a smaller hull, you may lose top end due to the shear weight of the thing! The 90-S lower unit used smaller props so they would spin and slip out of the hole allowing the motor to build rev's. The "small hub" props for these units give you the best Bite. Any way you go, it is an easy swap. I even did it on a 80 horse Johnson of the same era and it was a great, dependable motor!