RCU Forums - ys 91 AC problem!

Sounds like its surging after it gets hot. My suggestion is to look at air leaks and gaskets.

The airbox gasket, the valve cover gasket, carb and backplate gaskets. You said the regulator is clean...I would recommend another check of the passages in the reg...I use a can of compressed air to blow them out.

If the motor was working fine when new then it started acting up it sounds like dirt somewhere...but it could just be a heat thing. Another issue that can be of issue. If the motors are run lean it can damage the pistons. The hammering and or backfire pop...this is really bad for the motor. The pistons can get out of round and then they don't seal properly. You should not run them out of gas in the air for this reason. I'm guilty of this too...but the newer 140 pistons are much more robust than the older ones.

I would look to gaskets first and then send it to YS performance for a check up.

There is also the chance that you are lean...The motor will surge (rpms up and down) when the motor is too lean. Regulator screw should be flush with the case. Low speed air bleed screw say 2 turns open from full shut, and then the high end needle start at about 2-2.5turns open. Set the motor with a Tach to peak then back off 300rpms on the high speed needle. Then go to idle at 2000rpm...not 2200 and not 1900...2000exactly with your TX trim tab if needed. Then watch the motor for surging. The motor should hit 2000 repeatable...goose the throttle and make sure it is coming back to 2000 each time...ten in the first 30 secs watch for the motor to surge. It will speed up to 2400-2500 then back to 2000. You need to do this with a tach. If this surging occurs then its too lean on the air bleed screw. The air bleed screw on the throttle barrel is in for rich and out for lean.

Go about 1/4 turn at a time and then fine tune once you see the changes....Rich at idle means it will load up in this first 30 secs...2000-1900-1800-1700 and so on to eventually die...If the motor maintains the 2000rpm for 30-45secs then loads up to the 1900-1800-1700 and so on you are set correct. This means you are a little rich on the low end...If set this rich you will not have any problems with loading up in the air.

Then see how the midrange is going...set the throttle stick so you are at 3500-4000 and look for surging here....If it does the lean surging thing then the reg screw should be opened up CCW about 1/8th to 1/4 turn then re-set the high end and the low end as before....And check the mid-range again.

For all practical purposes the regulator adjustment as long as its close you will be fine...Its not as critical as the high end needle and the air bleed screw. Very few motors need a tweak off the starting location of flush with the case housing.

Another thing check the regulator body if you have had it off. The body of the reg should have a small nipple or raised mark on it...It goes pointed forward! If it was put on backward you could have similar symptoms to what you are having.

That's enough for now...try it and if you need to just send me a PM and I can help you out.

Troy Newman