ORIGINAL: NM2K
Being a military configured engine, the ''problem'' could be anything from porting to the carburetor. The civilian versions throttled well, just like any good R/C engine would throttle. I've seen these engines on the auction sites for as little as $30. A new carb is going to cost you that much. See if you can get one with the bushings installed and low run time. Then you can figure out your engine, or sell it and keep the good one.
Ed Cregger
Randy at Mecoa said the problem is the carb and to replace it. I don't feel this is necessary since its the same design as the regular old sportster. Rumor has it that there was a run of Mis-machined spraybars or something and it is rumored that these defective parts went into the military engines - though this is only speculation from some other members, its not confirmed to be fact.
ORIGINAL: madman75
Does your spray bar have a small slit where the hole is in the spray bar? I looked at the disc/spray bar assy. on my 65 so I know there is a very small slit cut where the spray bar hole is. The only way I know to measure the holes near the disc and the hole in the spray bar is to check them with drill bits. Which ever one(s) fits the holes is the size of the holes.
I'm guessing your problem is the spray bar either not slit correctly or not slit at all. If the slit not there or screwed up, this would affect the low end adjustment. The slit helps the fuel draw at low speed/idle. The rotation of the disc changes the location of the slit in relation to the air draw thru the carb.
Okay. I don't have numbers on the size of the holes, but I do know the spraybar itself only has a round hole in it. The inner brass tube (coming from the mix. disk) has a slit cut into it with a small hole drolled in the middle of the slit. The Mixture disk has 2 holes drilled for fuel flow between the two o-rings.
The engine runs well at idle and off-idle rpms (up to about 4500) And once it "cleans out" for the first time, it will throttle fine as long as you keep the rpms up past 4500-5000rpms or only let it come back to idle for just a couple seconds. I will take pictures of my carburetor parts and post them when I am home from work. I dont know if I have any drill bits small enough to measure the holes but I will try to zoom it in enough to see the detail and compare to yours.