CMB 101 tuning issues

[Tom Foley]

Here's the deal , new 101 on 60% in my Seaducer mono that I have had many 90's in so I decided to break in this new 101 , same pipe prop etc. I started out rich and head clearance now at .016 " - MC 9 plugs ...as i begin the leaning down process to get it to clean out after several tanks of fuel , it starts to sound like its getting crisp and close to a good needle then takes out the glow plug element comepletely without turning the plug dark brown / black that you would normally associate with being lean . I'm wondering about the type of plug....I have always run the MC 9 's in all my other big block CMB motors with no problems . Any ideas's ??????? :unsure:

 

[Don Ferrette]

Tom I would start by fattening up the needle and putting a bigger prop or same one with more pitch. Sounds like you might not be loading the motor enough and when you start squeezing the needle down to get it to clear out you wind up too lean.

 

[Tom Foley]

Thanks , I'll give it a try ..makes sense .

 

[Piston1]

Open the head volume to 1cc. If you keep running it stock, you will burn the piston.

 

[Terry Keeley]

Also make sure it's not being over-cooled, need some heat to burn nitro!

 

[Joe Warren]

Open the head button up to .9 the piston has .1 in the top of it. you would like to end up with a total volume of 1.0cc. Mac`s 15cc pipe @ 11 1/2 to 12 X467 prop. you may want to reduce the dia of the prop or cut some leading edge to relieve the prop load.

 

[Piston1]

1cc plus the .1cc in the piston is what I run 50-55% nitro on a stock motor. If you cut timing or shim the sleeve up then you might have to reduce head volume to compensate trapped volume.

 

[tnrcboatracer]

My guess was also too small a combustion chamber and/or over cooled. CMB motors are not originally set up for high nitro. Overcooled will keep it from clearing out, so you lean it out to heat it up and take out the plug.

 

[Tom Foley]

All good answers here ...thanks a bunch...the motor was coming back with the plug gone, the case just warm and the pipe sizzling hot !

 

[Joe Warren]

All good answers here ...thanks a bunch...the motor was coming back with the plug gone, the case just warm and the pipe sizzling hot !

Tom I run full cooling, But I guess if the water is too cold to swim in I might consider restricting. what pipe are you running How long?? I run 60% NITRO.... ALSO..... MOST CHANGE THE INTAKE DISK VALVE CLEARENCE. New Clearence .006... Factory .014-.016. If your running it out of the box? It just wont run out of the box......

 

[tnrcboatracer]

I forgot about the disk clearance. I set all zimmerman disk motors to 0.005" (0.0025"/side). Too big of a clearance does not seal.

Doc's twin wouldn't clear out until we just about cut off the water when we ran on the 2nd. Of course it was 27 degrees at the pond with a water temp in the 40's or so. Thought about breaking out the prope.........

Tom, from your description it was lean due to over cooling for sure; warm case, no plug wire at all, sizzling pipe.... Sounds like you had to lean it to get it to heat up, and once it did, it was way lean. I'd try cutting back the water for sure, but the cc may still need to be opened up for the 60% fuel.

 

[Buckshot]

Drop to 40-45% Fuel, Head clearance @.014"(stock bowl), Pipe at 11.625", X-665 S&B. LET IT EAT.

The disk must have clearance reset!

With the lower % you gain a more reliable needle, and better fuel mileage. No need for big % in that motor for heat racing!

Buck

 

[Don Ferrette]

I forgot about the disk clearance. I set all zimmerman disk motors to 0.005" (0.0025"/side). Too big of a clearance does not seal.

Doc's twin wouldn't clear out until we just about cut off the water when we ran on the 2nd. Of course it was 27 degrees at the pond with a water temp in the 40's or so. Thought about breaking out the prope.........

Tom, from your description it was lean due to over cooling for sure; warm case, no plug wire at all, sizzling pipe.... Sounds like you had to lean it to get it to heat up, and once it did, it was way lean. I'd try cutting back the water for sure, but the cc may still need to be opened up for the 60% fuel.

Forgot about the cooling thing. Tom your 90's in the twin you got from me do not run any water in the lower passage of the water jacket. Pop the jacket off of that 101, tap the two small transfer holes between the bottom cooling chamber and the top chamber and plug them. Run your water in and out of the top chamber. Those motors just do not need that much water plus cooling the lower area can cause the sleeve to run tight and wear it out that much faster.

 

[Tom Foley]

All good answers here ...thanks a bunch...the motor was coming back with the plug gone, the case just warm and the pipe sizzling hot !

Tom I run full cooling, But I guess if the water is too cold to swim in I might consider restricting. what pipe are you running How long?? I run 60% NITRO.... ALSO..... MOST CHANGE THE INTAKE DISK VALVE CLEARENCE. New Clearence .006... Factory .014-.016. If your running it out of the box? It just wont run out of the box......

It is out of the box with the exception of lowering the bottom of the exhaust port a little so it is closer to the dome of the piston at BDC, and the sleeve relieved where intake charge comes in . Sure would be nice to have a motor run out of the box ..the Mac 84's I have run awesome and never even checked the head clearance since new ! :lol:

 

[tnrcboatracer]

Andy spec'd the MAC motors for the typical nitro % we run over here. Most were checked by Andy before they left his shop.

Per Buck's recommendation, if you lower the nitro, you can leave the cc alone. The big CC is required for the add'l nitro @ 60%. I agree that the 101 probably doesn't need 60% fuel to heatrace. Probably just dumping it out the pipe. You need to do a flame front calculation, but likely the flame hasn't made it to the OD by hte time the exhaust port opens.. That's another thing that can make the pipe sizzle; the fuel is still burning as it enters the pipe.

Tom, Does it have the 2 grooves and little oil holes on the sleeve OD below the exhaust port? Need to oil the skirt on the exhaust side! I think that Mark Bullard refers to this as the piston antistick thing. PM Mark for details.

 

[Joe Warren]

Open the head volume. OR you will get Passed. Many are running 60 Percent Nitro. some 65 Percent... it does matter.... Mark Sholund has a spec modified Octura X467 for that set up that works very very well. Time to take it all apart & start over........ :blink:

 

[Tom Foley]

Open the head volume. OR you will get Passed. Many are running 60 Percent Nitro. some 65 Percent... it does matter.... Mark Sholund has a spec modified Octura X467 for that set up that works very very well. Time to take it all apart & start over........ :blink:

Got it apart except for the crank assy in the case . sure looks like a lot in the way in there !! Gonna do the disc housing and head button tomorrow , thinking of cooling just the top chamber ??

 

[Terry Keeley]

My guess was also too small a combustion chamber and/or over cooled. CMB motors are not originally set up for high nitro. Overcooled will keep it from clearing out, so you lean it out to heat it up and take out the plug.

Ding ding ding.

Give that man a cigar.

ps: do yourself a favor & try it before you open the head, higher C/R advances the ignition point.

 

[BigChuck]

Tom

With out to much effort you can make that motor work fine. The stock head needs to go to

.019 - .021 on head clearnce. Stock 15cc pipe measured to 11 1/2 to the fisrt tuning band from the side of the motor. Stock X467 B/S. Now.............Truth!! It will get close to the transon of the mod motors that "Everyone is talking about." But it will mean that they and you will have to run the full 6 flat out to stay a head. Every package mentioned is good. It will depend on what you want to run.

 

[Tom Foley]

Really got to thank everyone who has commented . A plan has been developed and I will be maching three backplates from this motor and my 2 backups as well as the head button . I am learning a lot here , thanks !