The Norvel .074 is one of those rare, virtually perfect devices that come along, usually by accident. I have no idea why it works the way it does but over the years, I've found that port timing, compression ratio, glow plug heat range, nitro content all play a role.
You can bore out the throttle barrel a shade and get a few more revs without hurting throttling. Don't use a drill bit, that barrel is some pretty hard steel. I use a round, diamond file and work it in from both sides by hand. This minimizes burs all around but even then, you need to dress off the burrs that DO form very carefully. Use the fingernail test.
On this one, http://www.youtube.com/watch?v=7lM9fbsPaBA
I used a throttle made by GS for their ball bearinged, Cyclon clone. Superbly made with a considerably larger bore in the barrel. In this case, it DID need an adjustable airbleed. I put a GS pipe on it and got some pretty good numbers. Soon as I can, it'll be permanently installed on my Tsunami that came out a tad heavy. Can't fly this one in the park, need much more room.
Back to the stock, .074, LiteMachines had Norvel make them a special, .074 and the appealing feature was brass bushings at both ends of the crank. The LM heli uses an exhaust throttle and it's very efficient. So I got me a few when they were being sold off for 25 dollars and proceeded to add a throttle. I used the GS throttle mentioned and it had an adjustable airbleed. Despite that, the idle isn't as good as the original and acceleration isn't up to par. I used all the tricks I know to improve things to no good effect.
On the LM .074, the crank is shorter and the intake is rounder and not as oblong as the original. I haven't bothered to check the port timing though, that may be a factor. Which only illustrates again that the original, Norvel .074 is unique and a true gem in the world of small engines.
That .074 plugs are no longer available from Norvel is a bad sign that currently, it seems, Norvel is just selling off stock and will one day be gone. Sure glad I have their entire engine line including a half dozen .06s and .074s