Back to the Johnson 300…

[Tom]

I’m still pondering this Johnson 300. Quite a motor, it’s like an opposed twin version of an LT, AT, DT, or TD without the primer.

OK, so I got the carb off and started checking things out. There is a butterfly with a vertical shaft. Atop the shaft is a spring loaded cam to open the butterfly. The spring normally closes the butterfly. For the life of me I can’t figure out what opens the butterfly. There is nothing on the bottom of the mag plate that would move the cam enough to open it up all the way. There is no linkage or any hole for one. There is a brass plate, less than a square inch, that is mounted to the bottom of the mag plate, and the parts diagram indicates that this part is a carburetor control cam. But the cam doesn’t open the throttle butterfly in any position. The part matches the factory diagram, so I know it’s not broken.

Any assistance would be appreciated. I’d like to figure this one out before I get deeper into the motor.

Thanks in advance.

Tom Manley

[Mumbles]

I think I know what the problem is here as I got stumped on it to.

Everything sounds correct as you describe it but the problem is the butterfly itself. On any normal carb, the butterfly limits the amount of travel and acts like a stop to close at idle speed. On the two 300’s I have, it seems like the butterflies are too small and they spin right thru the carb bore making the linkage on top reversed. I don’t have any photos handy but I’ll try and get some later. The butterfly might have a slight bend in it and I might have adjusted the bend a bit and relocated it using the screw holding it in place. Can’t remember. Whatever I did, I got it to stop where it should and the throttle linkage works correctly now and the motor runs good!

https://www.youtube.com/watch?v=-wWAGHn … e=youtu.be

Thinking about it more, the spring may have had some bearing on the closed position to.

[Mumbles]

Here’a couple photos showing the butterfly in the closed position. The shaft turns clockwise when viewed from the top.

---

Attachments:

[Tom]

Mumbles, you da man! Exactly as you predicted, the butterfly was forced past its stop. It’s all good now. Thanks,

Tom

[Author]

Posts