convert merc 1992 75hp to 90 hp - The Hull Truth

damien moran · 09-22-2009, 03:31 AM

I want to convert my 1992 merc 75 to the 90hp, i believe the only changes are the jets venturis and a small timing adjustment. Most merc mechanics dont want to know about it but one is keen to do it im just worried he will miss something he has overlooked and it will be a costly excersise resulting in no power increase or it wont run at all! Has anyone completed this procedure and know the ins and outs of exactly what has to be done? Also wondering about compression for these motors, i did a quick check today, and no i didnt warm the engine up or open the throttle! i got 125 psi in all 3 cylinders is that low? or about right. Thanks peoples

A1 Outboard · 09-23-2009, 04:30 AM

You need 3 carbs and it will run fine and pick up mybe 3 mph ...Buy a 115 pick up over 7 mph and gobs more power or if your boat isnt rated for 115 get a Johnson their 90 and 115 are the same looking motor...Sr

damien moran · 09-23-2009, 11:02 PM

have since been told the exhaust ports would need porting as well and its not worth it, the guy i spoke to says he has seen plenty of prople try it but they never get the same power as the original 90, sounds to me like he couldnt be bothered. Max hp on the hull is 110, a new or near new 115 is an option though i wouldnt touch a johnson, i am limited to merc and yamaha unless i want to drive 4 hours just to get a service etc.

A1 Outboard · 09-24-2009, 06:31 PM

Quote:

Originally Posted by A1 Outboard

You need 3 carbs and it will run fine and pick up mybe 3 mph ...Buy a 115 pick up over 7 mph and gobs more power or if your boat isnt rated for 115 get a Johnson their 90 and 115 are the same looking motor...Sr

OBTW....the ports are the same in the 75 and 90

BigShrimpin · 11-05-2009, 10:33 AM

Quote:

Originally Posted by damien moran

have since been told the exhaust ports would need porting as well and its not worth it, the guy i spoke to says he has seen plenty of prople try it but they never get the same power as the original 90, sounds to me like he couldnt be bothered. Max hp on the hull is 110, a new or near new 115 is an option though i wouldnt touch a johnson, i am limited to merc and yamaha unless i want to drive 4 hours just to get a service etc.

Merc used to make a 4 cylinder 100 which is very close to the 4 cyl 115