OS Carb For A CMB 45 Evo 2

I have read topics regarding using an OS 9B or 40A carb on a lot of CMB engines, but from the posts I have searched they mainly discuss .67 and newer .45 engines. I have an older .45 purple head Evo 2 engine that has what looks to be about a 26mm bolt pattern for the carb. Is there and OS carb that will work on this engine? If so, what mods need to be done? I bolted an OS carb onto my 1/8 on-road CMB .21 engine and holy smokes what a difference!! Any help will be greatly appreciated.

JP

JayP said:

I have read topics regarding using an OS 9B or 40A carb on a lot of CMB engines, but from the posts I have searched they mainly discuss .67 and newer .45 engines. I have an older .45 purple head Evo 2 engine that has what looks to be about a 26mm bolt pattern for the carb. Is there and OS carb that will work on this engine? If so, what mods need to be done? I bolted an OS carb onto my 1/8 on-road CMB .21 engine and holy smokes what a difference!! Any help will be greatly appreciated.

JP

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JayI raced a .21 picco in a seducer mono with a os 8 b carb on it, took about a half lap to get the engine warm enough to hit the pipe, once it got on the pipe it was a lot of fun to drive, very fast, third channel,I won a ton of races with this set up.

Mobydickk

Yeah, OS carbs seem to work quite well on different engines. Does anyone know if a 9B of 40A will bolt up to the older purple head .45? It has a CMB black carb on it now with a round, knurled needle on the throttle arm side. I am assuming this is the low speed needle? I guess since there might not be an OS carb that will fit, I will give the stock one a go. I have read posts where some close the low speed and open it up 3/4 to 1 1/2 turns, and others who back it way out to where it is not effective. Where should this needle be set at?

JP

On my CMB .21's, I set it as I would on my nitro cars & trucks. Stable idle with a minimum (1 mm +/-) throttle opening at idle & smooth transition to full throttle. Set the high speed needle according to glow plug reading first. Just pretend it's a car with a propeller, works for me (-; Set correctly, it should idle smoothly from bench to pond, only requiring throttle to launch the boat. Ask some folks here in D-12 how slowly I can crawl to the line & jump on the pipe at the horn.

Jay,

I use an OS Max 9B, 12 mm (.4724") bore carburetor, on my Nelson .45 long stroke. A special machined adapter plate, visible in the photo, mounts the carburetor in the same way that the standard pylon racing engine carburetor would be mounted. Adjustments of the high & low speed needles can be difficult, however once they are adjusted properly, the carburetors throttling performance from low to high speed is "spectacular". The low speed needle is opposite the knurled high speed needle. Both needles can be adjusted independently. The carburetor has a hardened steel barrel with a hardened steel guide screw controlling the barrels movement in & out. All the parts in this carburetor are precision machined.

Jim Allen

Hi Jim,

You have a pm

Jim,

Got it. I'm looking for the prints. The back side of the carburetor needs to be machined & threaded with four 4-40 bolts to mount it to the flange. The flange or adapter is hand finished inside to give a smooth entrance into the crankshaft window. Also the same "O" ring is used on the adapter as would be used on the pylon racing engine's venturi. The carburetor's "O" ring is also used to seal the area where it is mounted to the flange. The teflon venturi is a snap on machined piece & according to our bench testing it improves the top end performance.

JA

Jim Allen said:

The teflon venturi is a snap on machined piece & according to our bench testing it improves the top end performance.

Click to expand...

interesting, does the teflon venturi insert come with this OS carb?

David Bryant said:

Jim Allen said:

The teflon venturi is a snap on machined piece & according to our bench testing it improves the top end performance.

Click to expand...

interesting, does the teflon venturi insert come with this OS carb?

Click to expand...

No. However, there is an aluminum venturi piece available from Tower Hobbies that could be adapted by using a machined sleeve for mounting. Also the aluminum flange piece sitting inside the engine's crankcase, which is used to mount the carburetor, has been carefully matched to the crankshafts window.

JA

Jim Allen said:

David Bryant said:

Jim Allen said:

The teflon venturi is a snap on machined piece & according to our bench testing it improves the top end performance.

Click to expand...

interesting, does the teflon venturi insert come with this OS carb?

Click to expand...

No. However, there is an aluminum venturi piece available from Tower Hobbies that could be adapted by using a machined sleeve for mounting. Also the aluminum flange piece sitting inside the engine's crankcase, which is used to mount the carburetor, has been carefully matched to the crankshafts window.
JA

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JimWhat's the holes drilled in the flywheel outer diameter for? What fuel & oil you using?

Alan,

The crankshaft in the Nelson .45 long stroke engine is .750" OD with a very large intake window. This creates a large out of balance condition that would cause the engine to vibrate a WOT RPM's. The holes drilled in the flywheel's OD are positioned to be placed on the heavy side of the crankshaft. The heavy side of the flywheel is lined up with the crankshafts window. A similar thing is done to the pylon racing engine's flywheel by relieving the back side of the flywheel's inside. Both types of counterbalancing have proven to be very effective in eliminating excessive engine vibration during WOT operation. If the counterbalanced flywheel is mounted incorrectly, or if it should move during WOT engine operation, the crankshaft will be broken.

Jim Allen

Note: Alan, I normally use 65% Technology Industries Inc. fuel with 5 ounces of Blendzall racing castor added to each gallon. The deck is set at .013" with the Nelson double chamber head & Nelson heavy duty plugs. Another excellent fuel for this engine is 80% nitro, 12% ethylene oxide & 8% oil. The oil content is 2/3 Klotz Super Techniplate & 1/3 Blendzall racing castor.