Hello, I would like some assistance with a ST90 breaking in procedure. I recently began witha new OS No. 8 plug and new fuel. The engine has been difficult to start and once running will not stay running when the battery is disconnected. I have left the start battery on to gain some run time and managed to get up to speed for a while (half throttle)and it ran a few minutes and stopped. I have the high needle valve at 4.5 turns. One tank thru but have now ruined the plug. What should I do next? Is the engine piston/sleeve tighten when the temp is raised? The fuel is fine. The plug was new. Thanks, Bob

Try a glowbar plug too help keep the plug illuminated and it sounds as though you are running the engine too rich. You might also try reducing the diameter of the propellor by 1 inch too reduce the load on the engine.

Sounds like it is all out of adjustment.

1. Open your throttle to its approximate idle position-should not be much.

2. Disconnect your fuel line from the carb attach a piece of fuel tubing to the carb long enough to blow into.


3. Close your low end needle completely, Open your high end around 3 turns.

4. Now blow into the fuel line at the carb nipple and at the same time, open the low end until you here a hissing noise. This should put you in the ballpark for starting it up. Hook it all back up and try it. if the low end is close to much it won't even start.

If this works, you can check things like this. While the engine is at idle, disconnect the fuel line. If the rpm increases before it quits, the low end is to rich-therefor close it a tad(1/8 inch). If it just quits, it is to lean-open the low end a bit. I found on mine the engine will overheat and quit very soon if it is too lean. I do the pich test and go for no change in rpm. If i get a quick rise and decrease, it is too lean.