posted 01-13-2006 07:16 PM ET (US)        

I am posting this request again to see if someone might now be able to help. I would like to make the change before the Spring launch.

posted 01-14-2006 01:08 PM ET (US)            

I also toyed with the idea of going to a 17 pitch prop. It'll give me a better hole shot but the motor will run up to higher RPMs. There is no rev limiter on the motor. Over 5250rpm your horsepower won't increase much. It just levels off so it is posible to still get 40 mph and be propped down a bit. Maybe I'll start looking for a 17 blackmax on ebay. That would be really cheap. I'll keep you posted.

posted 01-14-2006 11:48 PM ET (US)            

Merc 2-stroke 90hp full throttle rpm = 5000 to 5500

posted 01-15-2006 11:14 AM ET (US)            

Yeah, the Connelly Big Daddy didn't fit in Santa's sack this Christmas.

posted 01-15-2006 12:57 PM ET (US)            

Yet the price differences among the models is usually significant enough for people, particularly those with mechancal skills, to look into a home-brew upgrade (as someone said, you can be sure a dealer won't do it).

Someone made an excellent point - they can't make it easy enough to do or everyone would be doing it. I suspect there is something in addition to carbs you are not seeing in the parts comparison, possibly connected with the ECM or who knows what. Expert opinions in other forums where this has come up say it is a combination of carbs, reed blocks, gearing, timing (ECM) and enough other little things that it becomes not worth the cost.

I know you say you are looking at identical parts breakdowns, but I have a sneaking suspicion that there is something else hidden in this story that you will discover down the road.

I could be wrong, but I've just seen too many techs that I respect weigh on on this over the years (they've been building motors in groupings like this for a long long time)

If I am wrong, then I want to immediately convert my Merc 90 carbed 4-stroke to a 115.....

posted 01-16-2006 08:44 PM ET (US)            

After going through the manual page by page and part number by part number, the ONLY mechanical difference documented is the carburetor and the main fuel jet. I would sure like to hear from someone who knows what is involved with a conversion of this particular engine. I like the idea of having a 75 HP hood covering a 90 HP power plant.

Sal, since this is a carb'd engine, I don't believe that it has a computer, am I wrong about this?

Swist, the merc 115 carb'd 2-stroke has another parts manual so I have no factual information comparing the 90 and 115.

Sosmerc, Why wouldn't you expect a 20% increase in HP improve hole shot?

Fishguts, what would we do if we didn't have all winter to think about how to improve our toys? I'll bet that a 17P black max would be your lowest cost alternative to inmproving hole shot. If you go that route, please post the results.

posted 01-17-2006 07:55 AM ET (US)            

On 2 totally different engine applications I consistently saw significantly better low end performance from the lower horsepower version of the same block. The only marine application was a 90 and 115 OMC on the same hull. With both propped well the 90 had a much better hole shot and we preferred it for skiing. It was spinning a prop with 2 inches less pitch. Obviously the 115 won on top speed but the 90 went faster than we could comfortably ski at anyway. For 7 years I ran a Polaris XLT snowmobile, 600 cc with three 38 mm mikunis on it. The more popular configuration for the XLT was the same 600 cc block with three 32mm mikunis. I probably raced against the 32's over 100 times and never beat one to 50 mph. Over 50 mph it was no contest and I would walk away from them. I was clutched right, shifting out at 8200 rpm. I asked a top notch racing mechanic to fix the hole shot and he said the answer was very simple, the larger carbs just dumped more gas on the holeshot and it took time to burn it off. He was not talking much time as the 0 to 50 time was well under 4 seconds.

Just try different props until you get the holeshot you want. Remember that a stainless prop doesn't guarantee a better hole shot. I have a 1988 Mercury 70 on my 15 CC and it has a lousy holeshot with a 22 Laser 2 but is a rocket ship with a 21 Black Max. Dave

posted 01-17-2006 06:38 PM ET (US)            

The difference is going to come only at the top end - there comes a point in the 75 when the throttle plates are wide open that the carb jets become the limiting factor of how much fuel can get into the cyliders. Until you get to that point, you will be developing the same hp at the same rps on either motor. The only difference is that the throttle lever may be farther forward on the 75 than on the 90.

The main difference is that the 90 has a higher top end rpm. On a given engine, HP is directly related to RPM - If the 75 has a max of 5250 RPM, it is a good bet that the 90 is going to develop 75 HP at 5250 RPM as well.

So bottom line is I agree with the others that you aren't going to change hole shot by making these mods - actually, you will hurt it because by the time you get propped right for the top end, you will be turning a steeper prop that will hurt your hole shot.

If you want to improve hole shot, try a prop like a high 5, or whatever they renamed it.

posted 02-05-2006 10:08 AM ET (US)            

Don't ask me where I got this info from (and I refer to the older +-1994 75Hp Two stroke), but this model had a restrictor plate on it, on entry to the exhaust tube which, I have been told (an don't quote me) is the source of the power loss on the 75hp. Might be of interest. Could be totaly wrong.