Hey what’s up brethren…. I’m attempting a conversion on 1973 25hp evinrude to 30/35hp using bigger 25/30hp(?) carb off early 80’s Johnson and increasing intake manifold orifice size. I’m learning about engines and particularly these outboards 1 engine at a time as I trade and buy them and fix them to be in good working order and either keep myself and sell my old motor or sell the newly refurbished motor and take proceeds to buy the next one and hopefully pocket a little scratch. Ok I’m reposting my original post from here:
Hello good folk. New to posting, but have greatly benefitted from the information exchanges in this forum so would like to say thank you all for contributing and very thoroughly and digestible more times than not. Just wanted to share my experience in this endeavor. I’m currently working on a ‘73 rude 25hp 2-stroke with an electric start up-fitting. I have the motor running and reaching decent high rpm while in neutral but under-load it won’t wind past 1/2 max. I’m pushing a lightweight 14’ aluminum flat bottom but it’s normally weighed down by trot lines, anchors, my buddy, my dog and, I think, myself. Estimated weight? I dunno, touching on 800 lbs max possibly. So I had a 198?Johnson 25hp motor I bought for parts and I noticed the carb in it was substantially larger in terms of volume and carb air intake/ throttle body opening size etc. so I cleaned it and blew it out and didn’t see need to replace the gasket as it seemed to had been recently replaced. I attached photos comparing the old to newer carbs. The holes for the studs I thought were exactly the same but upon removing and holding intake ends face to face realized a different if maybe 1/8”, the newer carb having slightly wider mounting holes. I used a die grinder to make a bevel on the hole going into my manifold that is the size of new carb opening down to maybe an eighth inch bigger diameter than what it originally was, then sanded smooth as I could. I will have to trim the ID of the gasket to the new hole size or I believe the whole intended purpose will be for not and I had to ream the newer carbs mounting holes in the eight inch or so to fit onto the studs. I am aware that a new larger 30 or 35hp manifold would be ideal to put on but I’m reserving that option in the likelihood that this doesn’t work out. Thoughts, criticism or suggestions are greatly appreciated.
Update: Today I did have success in getting motor cranked and sounding hungry at 1/2 throttle after completing afore mentioned mods. It was struggling to get calmed down enough to idle with choke open but started to act right as I closed the choke along with lowering throttle. I wish I could send video of it running to get some input and help. I’ll try to redo file sizes and post images and video of where I’m at thus far.
Hello good folk. New to posting, but have greatly benefitted from the information exchanges in this forum so would like to say thank you all for contributing and very thoroughly and digestible more times than not. Just wanted to share my experience in this endeavor. I’m currently working on a ‘73 rude 25hp 2-stroke with an electric start up-fitting. I have the motor running and reaching decent high rpm while in neutral but under-load it won’t wind past 1/2 max. I’m pushing a lightweight 14’ aluminum flat bottom but it’s normally weighed down by trot lines, anchors, my buddy, my dog and, I think, myself. Estimated weight? I dunno, touching on 800 lbs max possibly. So I had a 198?Johnson 25hp motor I bought for parts and I noticed the carb in it was substantially larger in terms of volume and carb air intake/ throttle body opening size etc. so I cleaned it and blew it out and didn’t see need to replace the gasket as it seemed to had been recently replaced. I attached photos comparing the old to newer carbs. The holes for the studs I thought were exactly the same but upon removing and holding intake ends face to face realized a different if maybe 1/8”, the newer carb having slightly wider mounting holes. I used a die grinder to make a bevel on the hole going into my manifold that is the size of new carb opening down to maybe an eighth inch bigger diameter than what it originally was, then sanded smooth as I could. I will have to trim the ID of the gasket to the new hole size or I believe the whole intended purpose will be for not and I had to ream the newer carbs mounting holes in the eight inch or so to fit onto the studs. I am aware that a new larger 30 or 35hp manifold would be ideal to put on but I’m reserving that option in the likelihood that this doesn’t work out. Thoughts, criticism or suggestions are greatly appreciated.
Update: Today I did have success in getting motor cranked and sounding hungry at 1/2 throttle after completing afore mentioned mods. It was struggling to get calmed down enough to idle with choke open but started to act right as I closed the choke along with lowering throttle. I wish I could send video of it running to get some input and help. I’ll try to redo file sizes and post images and video of where I’m at thus far.
