Saito 1.25 from glow to gas
I've had a Saito 1.25 on the shelf for several years, it being taken out of service for a couple of reasons. It was hard to hand start and would bark the fingers till they hurt. To use an electric starter required 24v and I'd have to drag up another battery to get it started. Consequently it didn't get flown often and that brought on another problem... the exhaust valve would be stuck from inactivity and that would require pulling the jug off. So, it got traded for a gas engine and stored.
The recent desire to build a Cub brought a need for the engine, as four stroke sound was a priority with a lesser desire that it be gas. So, the engine was given ignition to cure the starting difficulties with the hope that it would tune for gas. I knew the plane's cowl would yield full jug frontal air flow and that full power would be needed only a small percentage of flight time.
To my surprise, the stock carb showed promise on gas. The single difficulty was getting the idle low and smooth. Initial efforts yielded a rich and rough but reliable idle at 2,650 rpms. Leaning the LSN more would smooth out and increase the idle allowing the barrel to be closed to a more typical idle position and thus lowering the idle, but it brought spool up stumble problems. With guidance from Brutus (Bert) from the Netherlands, a one second servo speed was programmed in the transmitter. That stopped the spool up problem which often resulted in the engine quitting. Drawing the LSN out of the spray bar more slowly, prevented the dumping of fuel before engine rpms started accelerating. It worked. The current idle is likely too slow in combination with the 17 x 4w APC prop that yields strong braking for those no head wind landings. Idle will likely need raised some.
I've now eight flights on the Cub with the conversion and couldn't be happier. Running gas, I don't believe the exhaust valve will freeze up. And, fuel cost are considerably less than with glow fuel with a contrast of 25 cents per 10 min flight compared to $ 2 or more. And... there is little clean up. Best of all... the Cub has that wonderful Saito four stroke sound.
Fuel: regular gas w/ 15:1 Stihl HD 2 stroke oil
Timing: 28° BTDC
Ignition: RCexl
Prop hub: self made
Pickup sensor: mounted to a bracket mounted to the aluminum motor mount
Engine mounted: inverted
Fuel tank: 12 oz
Fuel consumption: .5 oz per minute
RPM: w / APC 17x 4w - 7, 950
Idle rpm: 1,700 - 1,800
Max temperature: w/ not more than 30 sec duration wot - 220F ( measured with sensor bolted to rear jug screw and sensor positioned behind the jug and out of the air flow)