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1977 25 to 35 hp conversion

J

Johnsonmitc

New member
Fearing the worse for my current 1977 25 hp outboard 25702h, i started looking online for a powerhead. I stumbled on a few powerheads for sale and read that powerhead is same on 25hp as 35hp. If i were to want to upgrade to 35 hp, is it possible? Ive read a few things about changing carbs but also read something that said intakes were different, and gearing too...? If I need to replace powerhead, then all internals would be there and intake, etc.. then may just need to buy different carb and adjust timing..... but if there is something that won't match up between lower and powerhead, ild like to know first! Don't know if this matters, but i have a jet foot on motor, instead of standard lower unit. I'm hoping someone has experience with these specific motors and can lead me the correct way! Thanks in advance!
 
Sure, you can sit down with the parts lists and figure out what parts it would take to make it a 35hp. But I gotta tell you that it will never perform like a 35 because of the very different lower units between the 25 and 35. And you have a jet drive yet. That makes it a whole different story. Me?? I say save your money and be happy with whatever you get.
 
Careful research is required.-----There mqay be an issue with crankshaft spline size if you want to do a powerhead swap.
 
Do you have any locations that would help me find this information. When i was looking up my motor crankshaft, it lists the 25hp crank then had a different crank for 35hp. I agree that there must be some difference, but i can't find any descriptionon or any document that explains what is different..
 
??----Did I say that the SPLINE may be different from 25 hp to 35 HP models.------But just for one or two years.----Shop carefully.
 
I know that you said the spline may be different and you said to carefully research.... so i was trying to carefully research... i thought i may be able to find some info that said a 25 hp would have a spline of xxxx and a 35 hp would have a spline of xxxx. It's hard to shop carefully when there are no specifics on descriptions and that was why i was asking if there is spot that you know of that would have more info than i have found. Thanks again!
 
Was at an antique motor meet yesterday.------Found a few bargains.------A gentleman sat next to me and started asking questions about getting an old 10 hp to work on .----After 20 minutes he shook my hand and said " thank you very much for sharing " ---Paid for my coffee and muffin.
 
The 1977 parts books show a different crankshaft for 25 / 35 hp. That, and the fact that that they have different drive shafts and lower units, are pretty good evidence that the splines are different. But I can't say that for certain. I don't know of any printed info on the specs either.
 
Thank you so much for looking! I'm at the point that if i need to replace the entire powerhead anyway, i would consider buying the 35 and upgrading carb etc... but it is starting to sound like there is a lot more to this than just an easy power adder!! i was looking for any help i can get with a 25 jet that only outputs abiut 18 hp... lol.... thanks again!
 
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