CMB 90 RS EVO engine mods

[RobBinnie]

I have a dead stock CMB RS EVO as per attached picture

I have engine in one of Andy’s SGX 90 Hulls

What engine mods are you guys doing to these for heat racing on 50% nitro
Any help would be appreciated
Rob

 

[Steve White]

Head, Pipe, Prop, Hull.

 

[RobBinnie]

Steve

I am just not familiar with the CMB 90 RS EVO
Squish clearance for 50%
Do I need to make a new head button to suit 50% if so what cc’s
Do you cut Zimmerman disk
What does this engine like?????

 

[Steve White]

I've never done anything with a CMB. Do have a 1.01 that a friend has and is putting bearings in it for me.

Looking at the specs, if you got it to run at spec performance levels it outta be real competitive. What's it take to get there? Don't know yet.

Here's some tuning 101 strategy. Get a solid baseline before you change anything. Check head clearance and head volume. Lots of ways to check head volume, I do it both on the head button itself and installed on the engine after checking clearance.

How is it running? Is it propped correctly? Running in the desired RPM range? What about glow plugs? How do they look after running on the nitro percentage you are using? Pipe length good?

There are many schools of thought on tuning. I used to alter timing back in the day - today's engine designs have lots of timing, so no more chopping or shims under the sleeve for me. I just optimize the compression ratio, pipe and prop selection.

I'm sure you'll get lots of advice.

 

[Buckshot]

Here’s what I would do:
Set head clearance to .014-.015”.
Remove the low speed needle.
Excalibur Pipe at 11-1/2”
1667 at 5.8”
Go have fun

 

[Danny King]

Set the head clearance to .012”-.014” and clean up the case and sleeve and run it. They run pretty solid. If your not already using 50% they run great on 40%

 

[David Ashcroft]

Head button bowl 1cc ,,open intake side run rich smell the power

 

[RobBinnie]

Here’s what I would do:
Set head clearance to .014-.015”.
Remove the low speed needle.
Excalibur Pipe at 11-1/2”
1667 at 5.8”
Go have fun

So you convert this carb to a hydro style do you leave the remote needle in or make a spray bar
What size hole in spray bar?

 

[Buckshot]

So you convert this carb to a hydro style do you leave the remote needle in or make a spray bar
What size hole in spray bar?

Not sure the carb with attached needle will fit in an SGX.

 

[RobBinnie]

What carb do I use then ???

yes main needle is to wide

 

[Jeff Lutz]

So you convert this carb to a hydro style do you leave the remote needle in or make a spray bar
What size hole in spray bar?

I believe you will be able to unscrew the needle and plug/modify that carb to work no problem. If you want a cleaner look you could buy a new spray bar. On mods that are basic but needed on that engine , I would add that tightening the disc clearance down to .004” will be beneficial. Jeff Lutz

 

[Jeff Lutz]

I will add that the piston/liners are holding up very very well on all CMB engines.

 

[Buckshot]

What carb do I use then ???

yes main needle is to wide

CMB has Hydro Carbs that come with a remote needle.
Or Speedmaster makes a nice after market carb but you’ll also need to buy remote needle.

 

[ALLAN YUEN]

So you convert this carb to a hydro style do you leave the remote needle in or make a spray bar
What size hole in spray bar?

That is ok but need strong servo for remote needle because sometimes needle stuck then servo will burn out Good luck!

 

[David Ashcroft]

...........

 

[Timbo Whalen]

What carb do I use then ???

yes main needle is to wide

Speedmaster smooth bore .530" ID

and Stu Barr 3rd channel needle assy.

SPDC-012 Smooth Bore $85.00

 

[RaceMechaniX]

Not sure this was mentioned earlier, but thoroughly clean the motor. All too often I have found small machining chip and debris inside the motor. If you have the patience pull the sleeve out and use a small Cratex bit in a Dremel or Foredom wand tool and deburr all the ports. The chrome plating leaves a sharp edge which can break off and be ingested into the engine.

 

[Tim Kish]

I haven't had a chance to go through mine yet but what I've heard is disc clearance and no water in the lower cooling jacket.
Pretty much what we did with previous RS engines.