'78 and 79' 85hp comparrison

[Thumpbass]

I just bought a 1978 85hp to replace my '79 100hp and was thinking about putting the 100hp carbs on the '78 85hp. I did notice though that it looks like the '78 85hp uses a 45c high speed jet where as the '79 85hp uses a 48c....the '79100 uses a 50c. My question is why is there a 3c difference between the '78 and '79 85hp's? Looks like the 115 and 140's use the same jets (54c&65c) on both '78 and '79. I don't have a manual for the '78 85 yet so I haven't been able to compare specs for the possible obvious answer.

at any rate, considering that there is a 5c difference between the '78 85hp and '79 100hp, would that be too big of a jump and create a rich condition if I tried to just swap carbs?
If so, what about replacing the 45c jets (from the '78 85hp) with 48c from a '79 85hp?

hope this wasn't hard to follow.

 

[emdsapmgr]

Re: '78 and 79' 85hp comparrison

OMC constantly changed carb design over the years. Each carb was jetted for that engine for that year. If you switch carbs, run with the jets that were originally in each carb. Your 100 carbs should bolt right up and run fine on your 78. Run it with the 50 jets. The crossflow blocks were essentially the same for many years. Much of the horsepower differences were determined by the throat size of the carb-cast into the front flange of the carb face. The throat size varied from around 1" up to 1 5/16". The larger the throat, the more air it will flow, and the larger the jets it requires= more horsepower.

 

[R.Johnson]

Re: '78 and 79' 85hp comparrison

That 78, 85 is 92.6 cubic inch's, the 79, 100 is 99.6, there's some of your horse power difference.

 

[Thumpbass]

Re: '78 and 79' 85hp comparrison

ah ha, there is one of the "possible obvious answers" I was talking about.

with the 7 cubic inch difference in mind, would using the 100hp carbs with the 5c difference in high speed jets be a bad idea on the smaller engine?

 

[Dhadley]

Re: '78 and 79' 85hp comparrison

Either set of carbs will work fine on either motor. Just leave the jets in the carbs as they were produced. I think you'll find the smaller venturi carbs (if one set is smaller) will accelerate a little quicker and the bigger carbs will use more slightly fuel . If both sets are the same venturi (1 1/4" for instance) I doubt you'll see much difference.