I am looking for an engine for alittle project. I built one using a 5 hp Honda now I am looking to go bigger. I researched all the fan cooled 2-strokes for simplicity. Then I ran across a couple SEA doo jet skis with 650 motors in them. I don't think I am going to be able to use them due to the cooling mods and the huge tuned exhaust pipes. I would have to have a seperate water pump, and custom tuned exhaust made for the 2 stroke.

So now I am going four stroke and need your guys help.

With the fourstroke I can route the exhaust easier and a snowmobile engine should have a self contained water pump. I want a carbureted one so I can play with the jets since it will be used in a warm environment. We don't have any snow down here in South MS so I am not familiar with all the models of snowmobiles.

I am looking for 60-80 hp and the best compromise of efficiency, weight, reliability, and cost?. Which ski should I be looking for for the donor engine?

A yamaha motor for sure... best most reliable motors out there in the snowmobiling world.... You can get a 500cc 4 stroke with about 80-90-100hp somewhere out there its out of a phazer.

If you want to play, I'd just get an old 340 or 440 Polaris fan. I don't think you can find a simpler two-stroke engine and they are nearly bulletproof. You can literally tear the engine down and put it all back together in an hour (once you've done it a time or two anyway.) They aren't real fussy, not being highly tuned. They won't give you the 50-60 HP you're looking for, but they'll certainly be way more juice than the little Honda you been using.

FWIW, I've run many, many Polaris fans. One of my current ones, a 488 fan, has over 23,000 miles and the engine is untouched except for rings and gaskets at 13,000 miles.

I was originally looking at all of the fan-cooled two strokes. I was leaning toward the 485 yamaha since at the time a couple of months ago they were plentiful on ebay. However I can't get past the exhaust. I know that two stroke uses a specialized expansion chamber to control the port timing so that it runs best. For my application I have to modify the pipe too much and I would be scared that it wouldn't run its best.

The 5hp honda is on a smaller version and I will keep it as it is, but I am building a bigger version and would like to have atleast 60 hp.

You might be able to find a Cat 660 on the cheap? They've been around a while. They came both normal and Turbo. Don't know if you've considered that route or not. I think you're on the right track avoiding the 2s pipe issue...

You wouldn't happen to know the approximate weight of the 660 would you? It looks like they are reliable but I havn't been able to find out how much they weigh.