Modifying Commando intakes for CR or FCR carburetion on P11

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Something for the performance oriented, or something for those looking for something to belittle. Bonus

I got a hankering to install some Keihin carburetion on my P11. I already have 34mm Mikuni carburetors on it, and they work well, but I wanted to see if I could do it more than anything else. Not done yet, but here's the story.

Based on some measurements Jim of JS Motorsports was kind enough to send me it looks like Keihin CR carburetors could be installed on a P11 with Triumph straight spigot manifolds, but they would need to be cut short, real short to prevent the carburetors from hitting the top tube on the very tight fitting Matchless frame. My motor and exhaust configuration like a longer intake. So I took a set of aftermarket Commando MkII spigot mount 32mm intake manifolds and modified them to bring the carburetor extension tube mount location down about 15 degrees from where a straight manifold would put it. When completed, the carburetors will be mounted very close to horizontal and should clear the top tube by a few thousands.

For the sake of brevity, below are a few pics of my hack work. I still have to get the carburetors to finish the fit, but I'm confident I can make it work.

And so it begins:

Close to finished






I'll use insulators against the P11 Atlas 750 head these manifolds will be going on.
The head pictured and used for mockup is a stock 750 Commando head.

For those interested, the initial cut angle was determined by putting the head with studs installed on that piece of 3/4 inch plywood on a work table and placing that little carpenter's square in the background left next to the mounted intake and drawing a line on the intake with a sharpie. That is how I roll, Ultra high tech. It's nothing special to accomplish, but tedious, and making the groove for a better extension tube seal by hand with a round file takes some finesse. A Dremel with a few cutters was also used.

I'll update this tale of mad science when I get the carburetion installed.
 
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I won't be installing Keihin FCR carburetors (Sudco part number 016-860) to find out if they fit in a P11 frame. I did get some FCR measurements from Matt at CNW and they should fit. However, I decided if I was going to try something that may not work, I might as well spend a little less money for two carburetors that cost more than the first new car I bought, and I won't be able to return once the box is opened. So the plan is to install Keihin CRS carburetors (Sudco part number 016-3234). Going to be a while though. Keihin is still trying to catch up to demand on all their products after a Covid shutdown.

The 016-3234 Keihin CRS dual carburetor kit is for a Yamaha SRX600, XT600, or TT600. The carburetor cable stay setup and throttle wheel have to be modified, and the carburetors have to be re-jetted for a Norton 750. For the P11 you'll have to come up with an intake manifold solution. The bore center to center spacing of the Yamaha setup is 66mm, rather than the Norton intake center to center spacing of 67mm. That shouldn't be a huge problem with clamp on rubber intake tubes.

JS Motorsports sells a Keihin CRS kit that has the modified throttle wheel/cable stay, and re-jetting done. The kit is sold with Commando intake manifolds, which would angle the carburetors funny on a P11. Short straight spigot mount manifolds could work with the least amount of effort. What I did with the Commando manifolds could also work with the manifolds JS sells, if there is enough meat on them.
 
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My gut feeling is the dual Keihin CRS 33mm carburetors will be slightly better at idle, off idle, and through the upper mid-range, and more than likely make it much easier to get the motor started when cold than the old dual 34mm Mikuni setup I have now. I'm not expecting a huge improvement, but some.

Given the backorder status on the 016-3234 kit, I doubt I'll know anything before the start of 2021.
 
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