CMB 101 tuning issues

[Marty Davis]

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.

I agree 100% with this. Saw it on Tommy Cordoza's mono. Put a larger prop on and it was perfect. That engine will NOT run well unless loaded a ton.

 

[dwilfong]

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.

I agree 100% with this. Saw it on Tommy Cordoza's mono. Put a larger prop on and it was perfect. That engine will NOT run well unless loaded a ton.

Tom stop messing around and slap some prop on it till it can't take any more. Then drop back one.

 

[Tom Foley]

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.

I agree 100% with this. Saw it on Tommy Cordoza's mono. Put a larger prop on and it was perfect. That engine will NOT run well unless loaded a ton.

I've always been intrigued by the "Why " behind the motor loading issue with big bore stuff . I've had three CR 500's , a Yamaha 450 -f , Ktm 400 , KTM 450 (current ) and they all like loading alot !! Big Block Chevy ..same deal ..

 

[Tom Foley]

More progress ...after work I machined the backplate to get the desired disc clearance . Ended up with around .004 - .005 " measuring with a caliper . thanks to Stu Barr for his time on the phone last night and Mark Sholund as well with some info on these motors . More to come on the bowl volume when i get to that tomorrow or Thursday . Thanks again for all the tips guys .

BTW if you are wondering why i turned the inner surface it's because my lathe cut .012 " when I dialed .010" ...so I got paranoid about the disc being too tight and took a .0015 "skim cut on the face of the back plate to loosen it back up a tad ......**** nitro motors !! :lol: :lol:

 

[Joe Warren]

If your making chips your making progress....... :huh:

 

[dr gary turner]

Tom, We have ran the RS 91's and 101's with great success. Most things recomended will help. A critical modification is on the sleeve to prevent sticking the pistons. We make a cut BELOW the exhaust port the width of the port about a 1/4 inch below the port through and through the sleeve. Then drill a series of small holes in the slit and then groove the OUTSIDE of the sleeve from the holes to the bottom of the sleeve. This forces fuel during compresion of the case up the side of the sleeve to cool the piston.

Also since you have the motor appart, Check the rotor timing. It can be improved upon. Gary

 

[Tom Foley]

Tom, We have ran the RS 91's and 101's with great success. Most things recomended will help. A critical modification is on the sleeve to prevent sticking the pistons. We make a cut BELOW the exhaust port the width of the port about a 1/4 inch below the port through and through the sleeve. Then drill a series of small holes in the slit and then groove the OUTSIDE of the sleeve from the holes to the bottom of the sleeve. This forces fuel during compresion of the case up the side of the sleeve to cool the piston.

Also since you have the motor appart, Check the rotor timing. It can be improved upon. Gary

You got a pic or two ?? [email protected] thanks .

 

[Buckshot]

Tom, We have ran the RS 91's and 101's with great success. Most things recomended will help. A critical modification is on the sleeve to prevent sticking the pistons. We make a cut BELOW the exhaust port the width of the port about a 1/4 inch below the port through and through the sleeve. Then drill a series of small holes in the slit and then groove the OUTSIDE of the sleeve from the holes to the bottom of the sleeve. This forces fuel during compresion of the case up the side of the sleeve to cool the piston.

Also since you have the motor appart, Check the rotor timing. It can be improved upon. Gary

You got a pic or two ?? [email protected] thanks .

E-mail Sent.

Buck

 

[tnrcboatracer]

That's the anti stick thing I was referring to. I understand the holes and grooves on the OD part of it; but the through-and-through part, I don't get. If the cut is through hte sleeve wall, how do you drill a hole?

Buck, can you forward a pic to me, too? Do you still have my email add'y?

 

[Andy Greene]

I have to admit -

This is one of the greatest places on the planet for information B)

But really guys, I gotta run against the Franken-Foley that will be a result of this mess :blink:

Let him figure it out for himself ,

so at least I can place a race or two

Andy

 

[TUNLBOTR]

I have to admit -

This is one of the greatest places on the planet for information B)

But really guys, I gotta run against the Franken-Foley that will be a result of this mess :blink:

Let him figure it out for himself ,

so at least I can place a race or two

Andy

Drew,

Franken-Foley? dude your killin' me. Wait till I tighten his bolts up and hook the jumper cables to him at The WinterNats.

Tom,

Set up for tight turns and xtra long straights, that course will be a challenge.

 

[Don Ferrette]

I have to admit -

This is one of the greatest places on the planet for information B)

But really guys, I gotta run against the Franken-Foley that will be a result of this mess :blink:

Let him figure it out for himself ,

so at least I can place a race or two

Andy

Franken-Foley, dude your killin' me. Wait till I tighten his bolts up and hook the jumper cables to him at The WinterNats.

Set up for tight turns and xtra long straights, that course will be a challenge.

Franken-Foley, LMAO!!! :lol: :lol: :lol: :lol: :lol:

 

[carlnoki]

Doc;

On my 90's (yeah i still run those babies), I make a channel the width of the bottom of the exhaust port starting 1/4"

below the bottom of the port to the bottom of the sleeve on the INSIDE of the liner only.

I have never had a piston stick doing this.

Carl

 

[Michael Costanzo]

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.

I agree 100% with this. Saw it on Tommy Cordoza's mono. Put a larger prop on and it was perfect. That engine will NOT run well unless loaded a ton.

hi marty, is this also true for the cmb .91rs? thanks,mike.

 

[Marty Davis]

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.

I agree 100% with this. Saw it on Tommy Cordoza's mono. Put a larger prop on and it was perfect. That engine will NOT run well unless loaded a ton.

hi marty, is this also true for the cmb .91rs? thanks,mike.

Not sure about that. But it is in place on the older 91's and the 101's. I would assume that it is also true on the 91 RS, but not positive. All of these have the problem of sticking and transferrng piston aluminum to the sleeve and ruining the piston. The fixes below the exhaust ports seems to be effective.

I suspect the reason for the aluminum transfer is the cooling ring. You need to NEVER cool the engine below the head button. NEVER on the crankcase.

 

[Piston1]

I have run every CMB 90, 91, 1.0, 1.01 that have ever existed since the early Grand Prix series. The only time I have stuck pistons to the liner, burn holes through the top of the piston, etc., etc. is when I had allowed detonation to occur (my fault). All these motors needed head volume adjustments. Keep the head clearance at a reasonable .014-.016 and adjust the head volume to support the Nitro content that you use. 90 EVOs and 1.0 EVOs used to have the oil holes below the exhaust port for added protection. I wonder why CMB stop doing it for the RS series.

 

[Michael Costanzo]

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.

I agree 100% with this. Saw it on Tommy Cordoza's mono. Put a larger prop on and it was perfect. That engine will NOT run well unless loaded a ton.

hi marty, is this also true for the cmb .91rs? thanks,mike.

Not sure about that. But it is in place on the older 91's and the 101's. I would assume that it is also true on the 91 RS, but not positive. All of these have the problem of sticking and transferrng piston aluminum to the sleeve and ruining the piston. The fixes below the exhaust ports seems to be effective.

I suspect the reason for the aluminum transfer is the cooling ring. You need to NEVER cool the engine below the head button. NEVER on the crankcase.

hi marty, what is the best way to run the water lines to the head? i run a cmb .91 rs in a rigger and have a hole drilled in the head to exit the water. thanks,mike.

 

[dr gary turner]

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.

I agree 100% with this. Saw it on Tommy Cordoza's mono. Put a larger prop on and it was perfect. That engine will NOT run well unless loaded a ton.

hi marty, is this also true for the cmb .91rs? thanks,mike.[/quote

Not sure about that. But it is in place on the older 91's and the 101's. I would assume that it is also true on the 91 RS, but not positive. All of these have the problem of sticking and transferrng piston aluminum to the sleeve and ruining the piston. The fixes below the exhaust ports seems to be effective.

I suspect the reason for the aluminum transfer is the cooling ring. You need to NEVER cool the engine below the head button. NEVER on the crankcase.

Marty, I think in the larger bore motors, as the piston speed gets high and due to the amount of surface area of the larger bores, the coefficient of friction goes up and heat then goes up and aluminum is left on the liner. We have found the only solution is to reduce the friction with lubrication using the sub exhaust port slits holes and groves. BTW the slit alone is not adequate, becasue we really twist the 91's and 101's up using 60% nitro. Gary

 

[Tom Foley]

More progress , got the sleeve mod done for additional oiling of the exhaust side piston skirt . Today the head button and it's off to the lake . Also got the lower waiter cooling shut down with some 4- 40 stainless cap screws red loctited into the case . a couple of pics here .

 

[Norm Doerr]

More progress , got the sleeve mod done for additional oiling of the exhaust side piston skirt . Today the head button and it's off to the lake . Also got the lower waiter cooling shut down with some 4- 40 stainless cap screws red loctited into the case . a couple of pics here .

I would like to see the trapped compression ratio and blowdown numbers. I have been watching a guy who knows a LOT about this stuff and have seen some big positive changes in performance by just messing with the head. Fun stuff

Norm Doerr