First off the 120AC doesn't have the air bleed screw. That engine was before the air bleed screw on the throttle barrel was introduced.

As for the problems you have tuning the engine....It could be fuel related....I don't know what fuel you are trying to run now. Also I think you might have a tied engine and the 15-10 is a bit heavy of a load for it on only 20% nitro. I don't know....If the previous owner ran all Castor in the engine....then the exhaust ports, muffler, headers or what ever are likely all plugged up with baked on carbon. This is why I recommend not running Castor. This can also make the engine labor as it starts to close up the passages with the baked on gunk. This stuff is really hard to clean off.

When I ran the 120AC's I used 30% nitro on the 15-10.

But the engine could be time for e a bit of a rebuild. If its a worn out ring it can act like describe...Combine that with a heavy load...that a new one can haul but a worn out one will really labor.

I would recommend sending the engine to Richard at YS Parts and Service for some rebuild and cleaning work. I would bet this would solve your problems.

When the Castor starts to build up that baked on gunk its like choking the engine off...Thing of putting a banana in the tail pipe of a car. I have seen header pipes closed up all but 1/8" dia....The fuel is the culprit on that problem....

Plus being a well used 120AC....its likely time for a new ring. I usually put a new ring on the engines at about 250-300 flights....this is where it starts to fall off a little. You could probably go 400-500 flights before it really causes a major loss in performance....But when it starts to loose its seal and the gap in the ring gets way too big...the engine not only works on the top side compression and fire....but the downstroke of the piston works the super charger effect and runs the pressure system. So a worn ring here will affect the supercharger and the pressure system....This means not only low on power but the settings will wonder and become more difficult to setup.


Troy Newman
Team YS