Johnson 9.9



I've just bought a Snapdragon 23 with a Johnson 9.9 o/b. The engine has two rotary controls - one on the side, one on the front - with no indication of their function. The one in the front seems to connect to the carb, the other seems to be some sort of stopper on the throttle control.

Can anyone enlighten me as to their function and prpoer setting?

The engine has two problems:
1 - the water output is intermittent - is this a repair job or am I missing something obvious?
2 - the engine occasionally jams on full throttle after the cut off has been pressed when in gear and the engine then restarted. is this a fault or something to live with?

I'd appreciate your advice


A two stroke twin cylinder. I'm not sure of its age - from the physical condition I'd guess over ten years.

90% of the time it runs well - but the other 10%and those mystery knobs worry me.

thanks for replying


New member
31 Jul 2001
The control on the side is an adjustable throttle stop. If the engine is ticking over okay then leave it alone because it rarely seems to work. The control on the front of the engine is the slow speed idle jet adjuster, again if the engine is ticking over reasonably then leave it alone. Screwing it in weakens it and unscrewing it richens the mixture. I think it is placed there to allow for variations in the oil fuel ratio, the tickover is not brilliant on these engines as after a few mintues they tend to start choking unless you blip the throttle, slightly hotter plugs will help this provided that you are not running at full throttle all the time. The intermitent water could possibly be caused by the thermostat sticking as I had this problem a few years ago and the new thermostat cured it. The throttle is operated by a cam from the magneto base plate and is a spring loaded roller that opens the throttle when you twist the throttle and moves the cam (which advances the timing). I would think the roller is not remaining in contact with the cam. A squirt of WD40 may be all that is needed. I hope that this helps.