I'm certain you have a tuning problem. The Saito is a very reliable engine, even mounted inverted. Are you sure it isn't a YS? Ha Ha. First question I would ask...Where is your tank mounted. It needs to be forward up against the firewall. If it is there then I would suspect you can lean it down just a bit on the low end if you are still at the factory settings. The low end (the screw in the black throttle arm) can be leaned down a lot from the factory settings. If you've already done that than check your idle speed. We don't want speed sitting around idle so we need to check it. It should be for this exercise about 2200 RPM. After you determine reliability you can probably slow it down to under 2000 if you want but I always try for about 2000 RPM. Obviously you should make certain your glow plug is good, maybe try a new one. And last, take a razor knife and pull out part of the glow plug spring a very small amount. I'm talking about 1/32 of an inch. Just a very small amount. This helps get the "glow" down into the combustion chamber and makes for a more reliable idle. If all of that doesn't work you just as well buy a YS, as you're in for a tuning nightmare.
Thanks
Barry