cmb greenhead .21 tuning help needed
 

I bought a cmb greenhead .21 long stroke used. Seems to be in good condition. Good comp, bearing, liner looks near new. It has a black carburator that I am not familiar with. Seems to have a progressive mixture that richens as the throttle opens. I have it in a small cat and can't seem to get it tuned up. It seems either so rich it won't run or so lean it wants to run away at idle. Any help would be aprreciated, including an approximate needle setting. I'm not new to nitro, but this one is giving me fits. Running 30% nitro, with an O'Donnel purple plug.

RE: cmb greenhead .21 tuning help needed
 

Does this engine use a remote needle? Looks like it might. If so, I feel kind of dumb but at least I don't think I hurt it by trying to run it full rich. Is the needle on the carb only the low speed then?

RE: cmb greenhead .21 tuning help needed
 

Yes,it sounds like you have a single needle carb and will need a remote needle

RE: cmb greenhead .21 tuning help needed
 

Can you post a pic, I would guess it doesn't have a adjustment on it. It might just have a barrel stop adjustment. The Mac .45 I have doesn't have any adjustment on it , its all controlled by third channel needle. I am not sure but I think alot of the race engines don't have LSN they only use HSN, these engine aren't suppose to be idling (they go and go fast). If your engine was too lean it wouldn't want to accelerate when you hit the throttle, it would want to die out. On shore you should have to blip the throttle just to keep the boat running, it should seem real rich but when it hits the water and gets a load on it (it will lean out). But its hard to say what is the problem without seeing a pic.


Paul

RE: cmb greenhead .21 tuning help needed
 

I have a CMB 45 that I have never run. But I think you are right. I think the needle on the carb is a low speed needle, and you need the remote needle for use as a HSN. Attached is a pic of the carb on my .45. I am guessing this is the same type of setup you have (??). And the needle in the middle of the arm being the low speed needle?? Don't get me wrong...I don't know much about the CMB engines, but that is my understanding of how the are tuned...

Sean

RE: cmb greenhead .21 tuning help needed
 

Fast9,

I have a CMB LS Green head and running 35% Nitro.

This is the carb I have and the remote HSN (temporarily on my NR engine).

The HSN is between 3/4 to 1 1/4 turn out, this setting me be too lean for you if the motor is too easy to start richen the needle until it is rich and you can clear it with the throttle and the LSN is 3/4 to 1 turn out.

when you start the boat you want the engine to sound rich, and when you start bleeping the throttle it will start clearing out. If you can have her like this you will be good. Don't try to let her idle she will load up and die. once you cleared it let her settle for 10sec and clear her out again, always do the same cycle.

when your ready to launch make sure you clear her out again before than launch the boat and go.

RE: cmb greenhead .21 tuning help needed
 

I think, although without a photo, not sure, you will find that the carb has a remote fuel adjustment needle valve built in already. Might be this is where your problem is coming from?


Glenn

RE: cmb greenhead .21 tuning help needed
 

Another thing that these engines are notorious for is the O-ring around the high-speed needle goes bad and leaks. That can make setting the needle a chore. Some people have converted the carbs over to remote needles to cure the problem.

RE: cmb greenhead .21 tuning help needed
 

this is the set-up i got,(thanks to james clegg,TopFuel4430),if this is what you have also,you can rotate the 3rd channel servo arm till it locks and manually adjust the needle to about 3 1/2 turns out,I noticed these cmb's like to run a fat needle setting,and as mentioned above,it will take some blipping to 1/2 throttle just to clear her out......but will be ok,once it hits the water....;)

RE: cmb greenhead .21 tuning help needed
 

Sorry about lack of pics. Yes it does need a remote needle. Looks just like Sean Bowf's .45 set up. Thanks for the help.

RE: cmb greenhead .21 tuning help needed
 

in my clubb we run the same motors in our boats. no one is using the standard cmb carb deu to the tuning difficulties. replace your carb with a G61 SUPER TIGER CARB. easy to adjust and very reliable. tower hobbies do stock them. enjoy your cmb it is THE FAST AND THE FURIOUS of the water.

RE: cmb greenhead .21 tuning help needed
 

Thanks of the help. Put an extra OS needle I had in it and now it's running good. Still running on rich side and messing with pipe length, already running pretty fast.