Hello,
I have an alpha trainer engine. I need help to modify the needle valve to remove the stoppers. Why do they put them on it though?

They put them on there so a newby could start the engine and fly, the engine is pre-run and set up at the factory. There would be no advantage to removing the limiters.

We remove them after the engine has a couple of gallons thru it. I've noticed some run so rich they drown. After we remove them they are really pretty nice engines. The high speed only needs to remove the set screw. It will come off with the needle valve detent. Put the detent back on and go for it. The idle just needs the set screw removed and you're in business. If its new, run it with the limiters for a couple of gallons or so.
We had one that became very finicky until we removed them. Was great afterwards.
Edwin

Engine dies when i give it full throttle. I was wondering maybe it is way too lean? what could be the cause?
Thanks

Without hearing it, it would be hard to tell. My guess is it is probably rich. You would need to turn the idle needle valve in about a 1/16 turn at a time. This is with the restrictor off in this case. I have had some engines that could not be adjusted to transition smoothly with the restrictor. There was a hesitation before it throttle up. We left it like that until there was more run in time on the engine. In some cases a students plane would get better. But in all cases, after lots of run in time (usually about 30 flights or so) we removed the restrictors. If you throttle up and it just dies (without the hesitation), that would indicate to me the idle mixture is lean. I think that is doubtfull since they ship these things running rich, but it's possible. Better to have someone help you with the engine at the local flying field. They will be able to hear what is happening better than me just speculating. This of cource is, just my opinion.