Johnson QD12 10HP carburetor problem

[bob-d]

Have a newly redone QD12 with a carb problem. When you set the high and low speed needles for low and high speed stability, and run up thru the revs, at about 1/2 throttle the motor starts to break up. I can smooth it out by leaning in the low speed needle. When I come back down to an idle, with the lean setting, it runs poorly and stops. The needles are both in good shape, and the carb has all new gaskets.
Any ideas?
Thanks,
Bob D

[Buccaneer]

Is the nozzle gasket in place and in good shape?

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Prepare to be boarded!

[fleetwin]

Is the throttle butterfly and ignition synchronized properly? Or, is this one of the early ones with separate carb and ignition controls…

[fisherman6]

First, Make sure the selector switch near the tiller arm pivot is set to "OFF" before making any attempt to dial in carburetor adjustments on an early QD.

In my experience these carburetors on the early QDs (QD-10 thru QD-14) do not transition thru mid range as smoothly as the later carburetors. They do fairly well once they are dialed in but do not seem to have as smooth of a mid range as the later ones (assuming the idle and high speed are set for best running). This seems to just be a characteristic of these motors.

It seems that to smooth out the mid range the low speed ends up too lean for a smooth idle and the engine tends to stumble if throttle is applied too rapidly. Set it for a good idle and tune out the stumble and mid range gets rough again. I have three of these motors running well now (QD-10, 13, and 14) and they all exhibit this behavior. If anyone knows of a way to tune this out, I’m also interested.

-Ben

OldJohnnyRude on YouTube

[bob-d]

Thanks gents for the suggestions.
Buccaneer, the early QD’s are a little different setup than your picture. I’m going to drop the bowl and check everything again today. All new gaskets, when I did the rebuild. I also put the carb into my Ultrasonic cleaner.
Don, this model has the ignition throttle butterfly setup and it’s synchronized.
Ben, you are right, you can’t set up the carb with that unusual selector switch engaged….found out the hard way.
I’m glad that I’m not the only one who has a problem with the midrange setup for these motors. It almost as the low speed circuit doesn’t want to hand off responsibility to the high speed jet!
It idles great and it runs at full throttle great.its just the middle that rough.
Bob D

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[fisherman6]

Yes Bob, that is pretty normal behavior for these engines in my experience. They just don’t have a real smooth transition from low to high speed circuits. Irresponsible hand-off is a nice way to put it. 😆
-Ben

OldJohnnyRude on YouTube

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