Is 8hp conversion possible???

[jcshep]

I have a late 80s 8hp evinrude and have been told it was and also wasn't possible to convert into 15 hp Any knowledge appreciated!!!

 

[racerone]

Forget about trying that.----Not sure where these tall tales come from.

 

[jcshep]

Ok thank you very much!!!!!! I think the stories start with a few too many Colorado Kool-aids.......Thank you for the reply. do you know what prop it would take? It has a tiny 2 blade prop that isnt much more than my trolling motors prop

 

[Chinewalker]

Prop is determined by what you're using it for. If it's an auxiliary motor on a large boat, a big ear low pitch prop would work best. If running on a 12-foot tinny with a 120 lb. kid at the helm, then a smaller diameter, higher pitch prop might be in order.

 

[F_R]

That's a general question, so gets a general answer. Example: A 1988 stock prop is 8-1/2" diameter X 9" pitch. Unless it is a sailboat model, which gets 9-1/4" X 6-1/2". Optional props are available for special needs.

 

[racerone]

I thought those came from factory with a 3 blade ?--------Do you have a model # for this 8 hp motor ?----Picture of that 2 blade prop on the lower unit ?

 

[thatone123]

I just heard today..ironic, that a 9.9 carb will fit on the 8hp...This is from a knowledgeable former OMC dealer. It stands to reason that a 15 carb would fit also as that model is a 15 but with the smaller 9.9 carb...a defector 9.9. A 15 carb would be stressing the 8hp so not a good idea. But the 9.9 carb on the lightweight 8 might be a real winner.

 

[jimmbo]

A 9.9 or 15 carb may very well fit, if it has the same spacing for the mounting studs. Doesn't mean it will work well or properly.
A bigger carb can let an engine inhale more air, but only if the engine can exhale the extra air. Over carburation is one of the leading causes of poor hole shot, low and mid rpm surging/roughness.

 

[raczekp1]

i had carb in my hands taken from 8 hp and 9.9. carb is almost the same( choke mechanism is different) but venturi is the same.
but 9.9 motor with 8hp carb starts only for few seconds

 

[racerone]

Some will argue.-------The 8 hp reeds and ports can only process so much air.-----So do we all understand the term -" mean effective pressure "- when it comes to power output of a motor ?-There is more to it then just adding a big carburetor

 

[ondarvr]

If the venture is actually the same size, then there isn't anything to be gained by switching.

 

[Vic.S]

ITYWF that the 9.9 and 15 hp engines are a much greater capacity that the 6 and 8 hp engines. 216 cm? compared with 164 cm?
That being so swapping the carbs will not uprate an 8 hp to 15 hp

 

[ondarvr]

If the venture is actually the same size, then there isn't anything to be gained by switching.

​Spell check changed it from venturi to venture, and it even keeps trying to in this post.

 

[thatone123]

But changing just to a 9.9 carb may increase the hp at least a little, maybe not. The 9.9/15 is a really great motor/outboard, but I have always felt the lower unit and gear case could have been made smaller and lighter and then it would have been a world shaker in a lightweight 15. In that era I believe the Chrysler 15 was quite a bit lighter. It was the lightest 15 even made I think. The lighter bottom end did the job it seems. Yes, It was not thru hub exhaust. The beauty of the 9.5 was that it weighed in at only 60lb. Then the next generation 9.9/15 was 74 lb. I have a 7000 rpm model I can't wait to try out this spring. Not many old hands even ever knew about the 7000 rpm model. My passion is the search for the lightest 10/15 possible. I had a few of the Tohatsu 9.8's which are very fast but will not handle big loads like even the 9.9 OMC. They just have it all over the Tohatsu Torque wise. I also had a Yamaha 8 which hard to beat for the smooth reliability easy starting and power for it's size. But...the next in line for them hp wise was heavy also.

 

[jimmbo]

The gearcase on the 9.9/15 was a good design, much stronger with lower drag than the gearcase on the previous 9 1/2.

If the carb venturis are the same size along with the same size throttle bore, the chances of seeing any increase in power is not very likely. What happens often when carbs are swapped, is a hesitation due to leaning. When the engine recovers from it stumble, the operator thinks a horsepower boost happened.

 

[raczekp1]

i only tryed to instal 8 hp carb on 9.9 motor. it worked only few sec. fuel was seeping threw vent hole in 8 hp carb.
the other way around i didnt try.

 

[thatone123]

I acquired a 90's like new 15 Merc this winter. It has a smaller LU and weights in at 71lb. Can't wait to try it out this spring.

 

[racerone]

Lower units on those Mercs are known to be somewhat weak.---------Lots of folks are looking for them or had to replace them.---Some would even say they are junk.-------Look very sleek in the showroom.

 

[thumbnut]

Right on those merc lowers. Helped my neighbor replace one. The lower only had a bushed pinion gear and a bushing somewhere else..can't remember. Let the oil get low in the lower and an occasional splash of oil is not enough for that bushing.

 

[flyingscott]

I acquired a 90's like new 15 Merc this winter. It has a smaller LU and weights in at 71lb. Can't wait to try it out this spring.

what does this have to do with the OPs question