Reply To: Backfire/misfire thru carb

[Don]

Well, he is correct, to a point…The splines themselves didn’t actually change, but the way they were sealed from water was changed…. The older models, like the 78 powerhead you are using had an oring just underneath the splines that fit up into the crank slightly to help keep water out. The later models he mentions use a two seal set up and no oring…. The top seal in the lower crankcase head seals the crankcase off from the outside. The lower seal fits over the driveshaft to seal the water out… So, on the outboard models, the driveshafts were different even though the splines themselves were the same. Older driveshafts had a groove below the splines where the oring fit in, newer driveshafts had no groove, because the lower seal in the housing was there to seal off the splines….Sometimes, there is a slight difference in driveshaft height that could actually bind up conventional outboards when mismatched parts are used So, your set up certainly will work, but may not be ideal in terms of sealing the crank/input shaft splines….Hope you greased them liberally during assembly.
And yes, the turbo jet cylinder heads had the vent nipples on top of the heads to help bleed air out of the block, the turbo jet cooling inlet is subject to air intrusion more so than the regular outboard. Can’t remember if you used the turbo jet heads with the elbows, or put the elbows in the original heads of the 140hp powerhead…
As far as longevity goes, that old 140hp powerhead had more compression than the later models, like your turbo jet, this is why newer model V4s were rated at 115hp instead of 140, they had been “detuned” in an effort to help them survive on newer lower octane/unleaded fuels. But, this is something I have discussed with you in the past….A relatively easy way to combat this issue would be to use the original turbo jet cylinder heads on the older 140hp powerhead…
It does appear that the turbo jet crankcase is a different part number when comparted to the conventional outboard….Just what the difference is , I don’t know. Could be very minor, or perhaps the porting is a bit different for the different applications…
Nonetheless, like you say, the older powerhead is in place and does work…. Hope you can sell it off soon….Again, not one of OMC’s best efforts…