nitro newb needs help

Dave Gilles · Nov 11, 2024

first time running a 3rd channel. Got an old CMB .21 from this forum in a rigger. The thing barely wants to start at all. then when it does absolutely will not take throttle like its lean as heck. but will not start with it any richer on the LSN. Now im using 15% plane fuel for bench testing cause im out of nitro for the season. is that what the issue is maybe?

What do I do with the LSN on the carb? open it up and let the remote needle control all fuel flow? I have no idea.

Danny King · Nov 11, 2024

You can richen the LS or remove it entirely (suggested).
Make sure the plug is good and don’t be afraid to try 2 plugs. You can get bad new plugs that look fine or run a plug to the point it isn’t the same.
A lean nitro not loaded screams rpms.
You only need the HSN. Thats it.

Mike Hughes · Nov 11, 2024

What Danny said. To set the needle by ear, you have it at idle and pull to full throttle qucily. It will run up to full song in about 1.5 secs. This will get you in the ball park. Then run and tweak 3rd channel from there.
Mike

Jeff Barnes · Nov 11, 2024

Old 21 cmb I would definitely put bearings in it

Dave Gilles · Nov 12, 2024

ok so help me understand how this works. The fuel flow is the same at idle as it is at WOT? or do i constantly tune the needle?

Mike Hughes · Nov 12, 2024

The amount of available fuel is consistent. With a low-end needle, it is not. It changes as the throttle is closed. This allows for lower throttle positions to run leaner, since there is less air. Not really worthit.

For RACING, we run most of the time during the race at wide open throttle. That is what you tune for. Once you have a good tune, I would just race it that way. Too many other variable during a race to concentrate on.

Dave Gilles · Nov 12, 2024

ok thanks. I think i got her finally running. I put more shims in it since im playing with 15%. I had it set to like .006". kinda found the needle and tightened the carb. a lot better now.