Johnson HD 25 carb problems

[burnt-clutch]

Hello all! I’m working on a Johnson HD 25, S/N 618801 that won’t run on its high speed carb circuit. It will run fine on the slow settings, but it cuts out when I push the magneto lever past start. Sometimes it surges and I can’t move the magneto lever further to the right without killing the engine. It doesn’t matter if I lean or richen the high speed needle; the engine still acts the same. I have the Johnson shop manual and they mention to make a needle and install it in the fuel passage in the carb and I have done that with no effect. This engine is new to me, I bought it from a guy cleaning out his Dad’s garage and he didn’t know the history on it. I have installed a new impellor, and Mr. Lautner’ s primer pump washers, and I’ve gone through the carb with an initial cleaning. For what it’s worth, the motor has Champion J6C spark plugs installed, and its compression readings are about 72 Psi for each cylinder. Has anyone else run into this problem? Please give me some pointers and tips if you have any. Thanks in advance for your help!

[frankr]

It would seem that there must be a blockage somewhere in that carburetor that you are missing.

[1946zephyr]

The knob on top of the tank is the high speed. What is your setting?

[garry-in-michigan]

Watching the float pin can tell you a lot. If the needle drops at the higher speed, then the restriction is in the tank and the fuel line to the carburetor. It could even be the air vent in the gas cap is plugged. If it comes up when the fuel is turned on and stays up at the higher settings, the fault is in the carburetor. I had one come into the shop that lost a screw that plugged the end of a fuel passage. The leak was immediately noticed, however the hardware store screw the owner replaced it with was too long. It stopped the leak, but shut off the fuel.

Good Luck . . . 😉

[frankr]

Yeah, I hadn’t thought about those brass screw plugs

[steveh]

It could also be the synchronization rod between the mag plate and the carburetor is missing, bent, or broken…

[burnt-clutch]

I got to thinking that there isn’t much up and down play in the float needle, and then I remembered that I used a Sierra plastic float (part # 18-7208) that I was using on my TN 28 to replace the original float to this carb. I compared the original float to a TN 28 float, and it is noticeably smaller. I’m leaning towards getting some fuel proof dope, strip and reseal the original float and then use it and see what I get. I’ll run some wire or guitar string through the passages in the carb while I have it apart and see if I can find any blockages. I’ll post my results when I get this all done. Thanks for all your replies so far!
-Karl

[burnt-clutch]

Here’s the latest – I finally got some fuel proof airplane dope and I sanded the original float down and gave it 4 coats of dope. I re- assembled the carb and tested it in the water barrel. When I press down on the primer gas squirts from the float needle in the top of the Carburetor cap – (not sure if this is normal – I thought that the gas was to pump into the slow speed yoke on the front of the power head). The motor starts, but it will only run fast if I open up the slow speed needle. While I have it running fast, I can adjust the high speed needle in and out (rich and lean) and this has no effect on the engine speed. This leads me to believe that the high speed circuit is still blocked. While I had the carb apart, I ran some light gauge wire through the passages in the carb from the float forward, but I didn’t run any wire through the primer cylinder, because I didn’t think that was part of the high speed circuit. I also checked the passage between the intake manifold and the rest of the carb and I was able to blow air through. Is there a passage that I’m missing that might be keeping the high speed circuit from working? Thanks for all of your replies so far!
-Karl

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