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1948 Johnson HD25 25 HP slow speed issue

Y

youngster

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"Hi.

This restoration proj

"Hi.

This restoration project starts and runs well at high speed. No amount of carb adjustment will get it to run lower than at the "Start" position. I finally borrowed a compression tester (the universal kind with the rubber tip) and I get only 50 psi in both cylinders after a couple of pulls. I'm told it should be closer to 80psi to run properly. I'm confident that the head is torqued adequately and gasgets are new. Am I looking at piston and/or ring replacement next? I also question the 50psi reading. Could the motor start and/run this good at high speed with compression that low? It's an old tester, and I'm wondering if it's bad.

Any input would be appreciated.

Thanks! Chuck"
 
"Your "belly button" a

"Your "belly button" adjustment in the middle of the front of the motor may be your problem. It controls the high speed fuel mix. The thumb wheel on the starboard side of the tank- controls the idle mixture. But they both work together. If you change the idle mix, it changes the main mix. So get it running and throttle down. If it tries to die, push down on the thumb wheel and that will pump fuel till you get the idle mix right. After that get the main mix adjusted again.

Next tankfull- add some "ring free" from yamaha, and your compression should improve. If you can get to 60# your there!!!"
 
"Thanks for the input William.

"Thanks for the input William. I think I follow your directions except that the "belly button" controls the low speed and the thumb wheel the high speed according to the original docs.

Also, I bought a new compression tester, the kind with the threaded end. I get compression readings of 67 psi with it. Are you saying that it should idle with compression of 60psi or greater? If that's the case, I will try your instructions, otherwise, I was thinking of purchasing a differnt power head. I was told that main bearing wear out was often the problem on these motors.

Thanks for your help. Chuck"
 
"You mean I've been adjust

"You mean I've been adjusting these motors backward for 20 years? See it is never to late to learn a new trick Woof!!

Should run fine on 60#. don't waste any money on high test gasoline thought.

They did have bronze main bearings which required the 24/1 fuel mix. And a laveranth seal between the cylinders, so a worn out main bearings would make them unable to keep a fuel load to one cylinder. Try to move the flywheel from side to side and see how much play is in there.

The worn mains will also ruin the points settings. (you have the tower points right?) They are hard to set and the fiberglass rod that runs them liked to cut a groove in the cam lob. Also the center round magnet would sometimes get loose on the crank shaft, and beat the coils away from the magnet."
 
Thanks! Great information. I&#

Thanks! Great information. I'll try all of your suggestions. The points are the tower variety but they seem to holding their adjustment ok. Your comment about the center round magnet has me thinking. It does seem to be looser that it was when I started working on the motor a year ago. Can you tell me anything about the slow speed mixing valve (belly button)? The original packing for the adjustment screw is long gone. I have tried other packing material arrangements but not sure they are working. How critical is that to proper performance? I found some teflon bonnet packing for plumbing applications. Might try that.

Thanks again - Chuck
 
I've taken that apart and

I've taken that apart and found an "O" ring in there. So would try that. it seemed to work. When you adjust the "O" ring down- over tighten it a little then bring it back as the rubber would torque it out of spec. Use grease on the "O" ring to help it settle in.
 
"Thanks. I have an "O"

"Thanks. I have an "O"ring in there now so I'll assume that that piece of the puzzle is ok. I'll check for play in the crankshaft, indicative of worn mains. If that checks out, I'll start it up and try your slow speed adjustment technique.

Thanks much. Chuck"
 
"Hi William,

I tried your

"Hi William,

I tried your slow speed adjustment technique withouth success. When it tried to die I pumped the plunger but it died anyway. I'm beginning to believe that there simply is not enough compression at slow speed to suck gas through the slow speed valve during the compression stroke. I know the plunger works and the line is unobstructed because it spits gas out of the bellybutton. I'm shopping for a powerhead and/or pistons/rings. If you think of anything else, please let me know...I'll get this thing running yet!

Thanks.

Chuck"
 
"Before you spend a bunch of m

"Before you spend a bunch of money on it, I would get some of Yamaha's "ring free" and mix it double strength with a tank of your 24/1 mix and run it in a trash can till it is gone. The water in the trash can will slowly warm up and help the process.

I don't think that engine even has a bottom seal just a real long lower crank bearing (bronze bushing). So if it is worn egged shaped, you may be out of luck."
 
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