CMB 90 Detonation marks on pistons?

[Steve White]

I picked up a pair of CMB 90's some time ago for a twin build. Engines were represented to be new. While at a race, decided to test run them individually. Mounted them up in an outrigger with a stock Octura 1667 prop. Pulled heads off of them today and don't like what I saw. Ran a tank of 25% nitro fuel through each of them.

They didn't light the world on fire power wise - I expected a little more grunt out of them. The top end seemed muted, like they hit a wall.

Looks like they were detonating. Plugs didn't scramble or melt, they were ran rich. Pistons on both engines are identical looking. I haven't cc'd the heads yet or checked clearance on them - this is just the initial inspection. No prior experience with the dual plug CMB's.

Comments?

Thanks.

 

[Danny King]

Never run an engine before setting the clearance.
These burn patterns on the pistons aren’t concerning. Although you would be much better off with single plug heads. The engines require a lot of work to utilize 2 plugs. And it’s easier to burn up pistons with 2 plugs.
and some of the cmb 2 plug heads had crazy low volume for a Stock engine.

 

[Steve White]

^^^ Agreed on setting head clearance prior to running. Engines were setup by someone that's considered very experienced by some. At the time I purchased them and ran 'em I didn't have a well equipped hobby shop.

Yes, the heads look to be very small in volume.

Single plug heads - that's the feedback I'm looking for. Looking at the pistons, concerned me. Never saw them kind of etching with so little run time before. Most tear downs at that point look pretty much unaffected or shiny new.

By the way they ran, I suspected the static compression may be a bit too high. I'll check head clearance and volume soon.

Thanks.

 

[Danny King]

I don’t care who builds a motor. Always check the clearance. It’s too easy to be off. And too easy to be catastrophic. And the clearance should never be tight when new. Break it in and check em again. Safe is better then Sorry.
it isn’t thecompression that’s a issue. It’s the lack of support to recharge.

 

[Danny King]

A single plug head will be more suited to the engine both volume-wise and 1 plug bs 2 is like using a smaller fuse.

 

[ALLAN YUEN]

Depend on what kind of pipe ?

 

[Terry Keeley]

What fuel was it?

 

[Steve White]

Prather 9045 pipe, open stinger.

25% nitro, custom mix don't know oil type or mix ratio.

 

[ALLAN YUEN]

Prather 9045 pipe, open stinger.

25% nitro, custom mix don't know oil type or mix ratio.

the Prather #9045 is very good but fuel seem bad oil ???

 

[Terry Keeley]

Maybe a lot of castor in the mix. Are they just brown marks? Or is there some frosting when you wipe them clean?

You sure don't want to end up here:

Or here:

 

[Steve White]

^^^ Exactly my concern. If those engines were leaned down from fat 4 cycle crackle that could be the end result. Going to make up some single glow plug heads for them and rock on.

Thanks.