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Help Needed. 1989 Johnson 25 hp converted to a 30/35 HP
- Thread starter sr522re
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Quick update:
Just checked compression: 120 psi in both cylinders. Spark jumps 1/2" on both leads.
Also, I primed the fuel line bulb yesterday till the carb bowl was full. Bulb became rock hard and no fuel passing through the primer solenoid. I checked this morning (24 hrs later) before compression test and fuel line bulb was still solid and bowl was full which is good. Process of elimination... I'm guessing a dirty path in the carburetor or a timing issue. I'm getting close, but need warmer weather to get it back in the water for more testing.
Just checked compression: 120 psi in both cylinders. Spark jumps 1/2" on both leads.
Also, I primed the fuel line bulb yesterday till the carb bowl was full. Bulb became rock hard and no fuel passing through the primer solenoid. I checked this morning (24 hrs later) before compression test and fuel line bulb was still solid and bowl was full which is good. Process of elimination... I'm guessing a dirty path in the carburetor or a timing issue. I'm getting close, but need warmer weather to get it back in the water for more testing.
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Keith Kirchoff01
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thill
Well-known member
Sounds like an air leak somewhere or a fuel imbalance.
Are you 100% sure the 35 carb is absolutely clean inside? I would go through it again. Make sure you run a carb cleaning needle through the little idle-pickup straw and spray it out carefully. Make sure the little holes in the top of the carb throat and the paths to them are absolutely clean. These are common issues that cause what you describe.
Is your idle fuel mix needle damaged? That makes it VERY difficult to adjust, as it goes from lean to rich in 1/4 turn, depending on the damage.
Does your idle mix screw have a red or black keyhole-shaped grommet? If that gets a tear in it, you can get fuel starvation.
It's possible that your sneeze is actually a small back-fire from poor ignition. In that case, pop in new spark plugs and double check that primary and spark plug connections to your coils are clean and greased, just to eliminate the possibility.
If you check the spec sheet, you might find that RPM's at WOT on the bigger motor is higher. You may have to open the max timing setting a bit to reach that higher RPM. But check the book, as I'm just going off memory that is not specific to your particular motor. Just throwing some thoughts out there.
I hope you figure it out.
Are you 100% sure the 35 carb is absolutely clean inside? I would go through it again. Make sure you run a carb cleaning needle through the little idle-pickup straw and spray it out carefully. Make sure the little holes in the top of the carb throat and the paths to them are absolutely clean. These are common issues that cause what you describe.
Is your idle fuel mix needle damaged? That makes it VERY difficult to adjust, as it goes from lean to rich in 1/4 turn, depending on the damage.
Does your idle mix screw have a red or black keyhole-shaped grommet? If that gets a tear in it, you can get fuel starvation.
It's possible that your sneeze is actually a small back-fire from poor ignition. In that case, pop in new spark plugs and double check that primary and spark plug connections to your coils are clean and greased, just to eliminate the possibility.
If you check the spec sheet, you might find that RPM's at WOT on the bigger motor is higher. You may have to open the max timing setting a bit to reach that higher RPM. But check the book, as I'm just going off memory that is not specific to your particular motor. Just throwing some thoughts out there.
I hope you figure it out.
Keith Kirchoff01
Member
I WOULD LIKE TO READ MORE ABOUT ABOUT 25 TO 35 CONVERSION BUT CANT FIND IT. DONT NO WERE TO LOOK
LaqueRatt
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I think one reason you can't find a whole lot online about conversions like this is because the parts to do it are quite valuable and often the motors are not. At least not if we're talking about older motors. When I've looked for the pieces to convert my 9.9 to 15 I found that complete donor motors are the cheapest route, but then what do you do? Take apart a running motor of the HP you want and put the parts on the other one? Makes no sense. You almost have to stumble upon the parts you need to make it worthwhile.
Pleading ignorance here, Is the 25 block the same block as the 35? Going back to the 70's OMC used a bigger block or at least greater displacement for the 35 vs the 25.
I just got this off of a google search: The 25 hp motor is 30.5 cubes, where the 35 hp is 34.5 cubes, so you can't do a jetting or reed valve change to make ten hp.
I just got this off of a google search: The 25 hp motor is 30.5 cubes, where the 35 hp is 34.5 cubes, so you can't do a jetting or reed valve change to make ten hp.
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MDCrappie
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Keith Kirchoff01
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I HAD A MECHANIC TELL ME THE ONLY DIFF BETWEEN 25 AN 30 WAS THE CARB THIS WAS YEARS AGO I MOVED TO ANOTHER STATE HE RETIRED OR I WOULD ASK HIM ABOUT IT. NOW I HAVE 2 MOTORS 25 HP THAT AER SAME I GOT INTERESTED IN THIS BUT I THINK IT IS DEEPER THAN GUST CARB AN INTAKE. I CANT FIND ANY SOLID INFO ON DOING THIS BUT IT WAS INTERESTING READING THE INFO I FOUND.
MDCrappie
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Did you read the link I posted? It's 10 years old but might be the most in depth one as far as research does with the differences. Pretty much explains it ALL.
LaqueRatt
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Well my memory has never been stellar and it's been awhile since I researched this stuff, but my 25 HP 'rude is a '77. I *think* this was the year the displacement was bumped. Pretty sure it's the same size as a 35. Your mileage may differ, but I found a ton of info online about this. Maybe you should try varying your search terms if you're coming up dry. There's also a site called LeRoy's Ramblings or something like that that is a goldmine of info for OMCs.
