Tuning a CMB .21 LS green head engine

[Sean Bowf]

I got this engine new this year. It does not really have a lot of fuel through it. I am not used to this quality of engine (ran a lot of RTRs and OS engines last year).

Anyhow, I am having problems getting this thing to pipe up like it should. I have it installed in a 33" SD3. The props I have tried are a X437/3 B/S, X440/3 B/S/detounged, and an M440 B/S.

I have tried a cooper .18 - .21 pipe, and the AB .21 SS pipe (I think it is the SS...it is the shorter of the SS and ST). I think I have tried everything from 8-9 inches for a tuned length on the pipe.

I have a third channel needle on it, so I can adjust it to where it slows down in either direction (lean and rich) without much help. The boat just never really seems to scream.

Someone told me that the LSN on earlier versions of this motor used to vibrate in on their own and cause problems...so I screwed mine out far enough that I have to pinch the fuel line with some forecepts to get the engine to start...with no help.

I am really at a loss with this motor/boat. I put this thing together for my kids to run, so they can get used to something a bit faster than their RTRs...but I really don't think this thing is going but 30sh MPH. And I can run it at that speed until the tank runs dry.

The motor does not have a lot of pinch, but never did new out of the box. It does have more pinch than the .18 OS engine in my RTR that pushes my RTR over 40 MPH.

Does anyone have an input (inputs) of what to try on this thing?? I might take it out and tear it down...just to see if I can see anything going on with the inside of the motor...but really can't see that being the problem...

Here is an odd note...I stuck the boat into a bouy today (first time ever)...and it kept running. I could give it some throttle and push the bouy around like it had a lot of torque. It seemed odd to me that it appeared to have that much torque at a stand still when it won't turn the prop that well at speed....??? There was obviously more pressure on the prop like this then there should have been at speed...

Thanks in advance for your help,

Sean

 

[john v]

did you check the head clearance?. some times they get shipped with high numbers

 

[Nigtmare]

And use 60% nitro ^_^

 

[Sean Bowf]

What should the head clearance be??

I am running 40% nitro right now...got a couple gallons left...thought of going higher next year...but been running 40 this year, up from a mix of 30 and 40 last year. My CMB .45 HR is running good on the 40...

Sean

 

[john v]

i think 8-10 thousands, you may get a lot opinions on that one. my greenheads came though with no bushing on the wrist pin side of the rod and as they wear your head clearance is all over the place. i had josh from precision boats bush my rods, i am not sure if he is around anymore. i had some motors rebuilt from john otto at technology hobbies and he does a great job (813) 651-1840

 

[Sean Bowf]

current head clearance is about .0145...shim is .007 ...so head clearance without shim is .0075...is this going to work, or do I need to go find a REALLY thin shim to make up the difference??

Also, will the difference of .007 between what it should be, what and it is make the kind of difference we are talking about??

Sean

 

[john v]

with 7 thous. head cl, thats kinda tight, everything has to be in order to run that clearance(pipe length,prop, hull&hardware) i would say go to 10 thous and start from there

 

[jetpack]

Hi Sean,

If your still keeping glow plugs, you might want to try a warmer plug. The right plug can add up to 2,000 rpm in some motors. If your engine drops rpms when you take the battery off, and you still havent richened it up before launch, then you might be running too cold of plug.

If your engine is running too cool, this can have an affect also. These motors do need to keep some heat in them, it is easy to over-cool one. The warmer the better without going too hot.

As long as you have your head pulled, take a close look at it. Hopefully, it is still smooth along with the top of the piston. If it has a gritty look to it, then I would suggest backing off on the compression a bit, clean the head and piston smooth again and start over. Careful not to hurt the lips or corners when doing this.

My method with a new engine is starting with an extra shim, warm or medium plug and lower nitro along with the suggested break-in suggestions. This assures there wont be any head damage from pre-ignition. From there, I'll remove the shim after break-in and work on heat range of plug, and keep watching the head condition.

I put a link to my photobucket to a sheet I keep handy, it will give great examples of what to look for if your unfamiliar with what the head and plug should look like, and suggestions what to do to correct it.