Exhaust Housing Size & Carb Orifices

[johnyrude200]

Hi All,

I’m in the middle of switching out carbs in an early 90’s 40hp OMC to up it to a 50hp motor. Well this has been interesting to say the least. Looking up the orifice jets shows changes to these sizes between 92 & 93 (the years I happen to have in front of me). They all change depending on if you have an all-aluminum carb, or the slightly newer ones which have the plastic tops. I noticed the plastic ones use larger orifices too.

One thing I noticed is that depending on if you have a short or long shaft motor, the orifice sizes are different for the carbs. I noticed the sizes are LARGER for shortshaft motors. Why is this?

Thanks

[fleetwin]

Well, these engines rely a great deal on exhaust tuning when it comes to hp ratings. So, it might just be that the exhaust tuning in a short shaft engine is slightly different requiring a slight jet change. The engineers really went too far in this regard, I seem to recall that some of the 50hp blocks had a slight spray of water introduced into the exhaust chamber that helped develop power. I can’t remember if WOT timing settings were different for the two hp models also, perhaps not… The sync and link procedures are ridiculous, a 10 step procedure that is a pain and "over engineered". The same engineers that developed the sync procedures for later model 6-8hps must have worked on the 40-50 models as well.
The only benefit of these over the earlier models are the split lower motor cover, nice trim unit, and more robust gearcase (except for the CPS shift rod cover)….

[johnyrude200]

I’ve only dived into a couple of these so far (looking to work more on these and then the 70hp motors later in the year when things slow down) but I know just what you’re referring to. Looking forward to getting more familiar with the different block changes to the shifter/throttle bosses. I’ve been accumulating a bunch of these 40-50hp motors in various conditions and will have to tear into several of them for parts piles in the next couple of months. Good to learn what’s compatible with what through the various revisions/generations.

I had to tear apart a 150hp mid 90’s motor as part of a marina clear out last week. No service manual, and I have to give thanks to you and the rest of the gang. First time I had fooled with one of those motors and just by looking at it I could see just how similar it is in terms of configuration (i.e. ignition, fuel set up, wires, set up, hardware, etc) to the 25/35’s and now the 40-48-50’s I am working on.

If it wasn’t for the help and knowledge you folks have shared, I wouldn’t be doing this right now. Learning something new everyday, but not so many ‘big gulps’ like a couple of years ago…

Thank you

[beerman57]

Are all the carb venturi sizes the same? Carb opening on the intake manifolds the same size?

[johnyrude200]

the venturi’s are different; the manifolds appear to be the same via parts diagrams. Holding a 40hp next to a 50hp carb, you can see the venturi’s have a larger diameter visually in the 50hp carbs.

I don’t have any of the newer plastic topped 50hp carbs in front of me, so I’d have to get back to you on that one if we’re comparing 50hp all-metal vs 50hp metal/plastic carbs.

Seems in late ’92 they switched to the plastic 50hp carbs with larger orifice plugs.

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