Yamaha 2 stroke 50hp problem

[SERG]

I have used 2 stroke Yamaha 50 (no information of age), compression OK!
Carburetor KEIHIN BCK choke model.
Use 95 EURO with oil mixed in tank 1:75.
No problem with start, 800 rpm steady.
After 3 min of heating, no problem with running up to 5300 rpm.
But then later I reduce to idle, or about 1000 rpm, (slow speed), then I start to increase, (at 2000 rpm about) engine starts knocking and vibrating, smoking.
After switch of engine, only shifting carburetor and throttle links, some time problem gone, some time remains.
Please, any advice.

[fl_skiff16]

Is fuel tank vented...how is the primer bulb on your fuel line

[Ken]

SERG,
Your outboard could be going into rpm reduction mode due overheat condition - but that should be accompanied by the alarm buzzer going off - but if the alarm buzzer is not functioning, you would not know it.
Most Yamahas have an overheat sensor, called a thermo switch, that is mounted in the cylinder assy - my C115, which is a V-4, has two, one for each bank of cylinders. Your 50hp probably has only one. The thermo switch is designed to close at around 180 degrees f. When this happens, the ignition control puts the spark into an rpm reduction mode, and also sends a signal to the alarm buzzer (horn). Usually when this happens, you throttle back to neutral and allow the motor to cool and the alarm stops - assuming the water pump is doing its job.
The thermo switch can be easily removed and tested with a digital voltmeter and thermometer and heated water. I have seen them go bad and close as low as 130 degrees - giving a false overheat condition to the control electronics.
If you have an overheat condition, it could be due to bad water pump, a stuck thermostat, or water jacket clogged up with salt/sand/mineral deposits.
If it has been at least three years since last water pump repair, you might want to consider it.
You can remove the thermostat and test it in hot water just like you do the thermo switch.
You can flush the motor out with something like "Salt-Away" or "Salt-Terminator" - but I have found this is not very effective for clogged up water jackets.
You really need a Yamaha shop manual to do this kind of work if you are so inclined. Go to www.yamahapubs.com to mail order.
Good luck,
Ken K