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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New status on this crazy project is I am ignoring all intelligent advice regarding over carburetion on a 750, and putting the Keihin FCR 35mm pumpers on when they show up. 10 or 15 days is the current estimate if Sudco can get the throttle cable setup from Motion Pro soon.

Sure hope the carburetors fit under that backbone. It'll be tight. The tank is not an issue with the throttle wheel on the P11.

Regarding over carburetion. Since I have a stock 750 head to compare to my Fred Barlow Spares ported head, I might get away with making the larger carburetors work with the Fred head. The porting in the Fred head is considerably better than the stock head, which frankly looks like crap to me. Maybe I can make some of that big Norton lawnmower engine 50+HP with the FCRs. Either that, or it'll fall flat on it's face when I turn up the wick.

If I think about this too much, I'll bail on the project. Best to stay delusional and move forward.
 
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Still waiting for the carburetion to show. Bike is more or less ready. I might have to swap the petcocks so they point forward, and shorten the intakes a little, but it's ready for a test fit.

Took a while, but I eventually realized people have been putting Keihin FCR carburetion on Commandos for over a decade, so nothing new by any means. However, if somebody wants to know if it can be done on a P11, that is what is happening in this post.

 
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I'm a simple man. I'd have to guess at what you mean by fixtures. So here's a story and and some pics:

The exhaust is obviously not P11 scrambler style. I got my P11 in crates from my brother in the early 70's. Many parts were missing. The exhaust was one of the many missing pieces. It was flat track raced prior to my bother getting it, and had 1 1/2-inch ID spigot style adapters screwed into the head exhaust ports so that TT pipes could be installed. Initially I had what I think were N15 header pipes and short mufflers on it, but slid off the road and wrecked one. I had a very little luck finding any kind of exhaust that would bolt up after that happened, so I had a 2 into 1 BUB exhaust built when BUB Enterprises was a small shop in San Jose California.

I use Triumph 1 3/4-inch exhaust clamps on the header pipes. The header pipes go under the motor into a 2 inch diameter collector section, and then into a 24-inch long megaphone. The megaphone is gutted and I pulled the SuperTrapp setup out of a Honda XL600 slip-on I had, and installed it in the end of the megaphone. It has 23 plates in it. It is loud when the throttle is twisted. Not too annoying for the general population at cruise around 35MPH. When on the pipe it sounds like half an F1 motor.

The exhaust did not work that well until I installed the SuperTrapp and extended the section between the collector and megaphone. It'll be interesting to see how the Keihin FCR 35mm carburetors work with the exhaust as it is.



If I could weld, this next connection would look better. The original setup had the springs up top and the springs hit the sump drain plug, so I redneck rigged these spring hooks underneath and turned the collector over. Pipe has threaded inserts in it for the allen hex head screws. Need to put stainless button heads in there, or learn how to weld.


Added a 6 3/4-inch section between the collector and megaphone. This extends the pipe by 3 1/2 inches through the 2 inch section.


 
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Yes I was wondering about how the pipes are attached to the head, Thanks for the reply. I also run a 2 into 1 exhaust that was made by BUB. My 72 combat also has a custom pipe coming from the collector to the can. Really nice looking bike you have there, I have always liked the P11 and the other off road Norton's.
 
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Thanks. It's that Arrest me Red makes it look nice.

For a dirt bike, the P11 with the light weight Matchless frame actually works pretty good on the road as long as panic stops are avoided. It is a surprisingly nimble and stable road bike for it's age once you get used to how quick it turns in. Unfortunately, the brakes are 7-inch single leading. Basically it brakes like an old motorcycle with little brakes.
 
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Mind if I ask where you got those rubber boots that connect the carbs to the intake manifolds? The ones that came with my JS Motorsports 33mm CRS carbs are starting to dry up/crack. Thanks!
 
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That rubber section on the intakes is 1 1/2-inch ID automotive fuel tank filler hose. It comes in various thicknesses and ID's. I don't know what the long term durability is of the stuff being sold today, but the stuff I bought 27 years ago held up great. It works well enough, and can be custom cut to any length. I think Jim uses something similar, so I don't know if you'll be happy with it.

Mikuni makes a rubber spigot to spigot clamp on click lock straight intake boot for VM30-VM34mm carburetors.
 
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Thanks very much and good luck with your project.

I don't want to take over your thread so if anyone else has any other suggestions to me about replacing the boots then please shoot me a PM. Thanks!
 
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Maybe post the question in the Commando forum. Automotive, marine, and aircraft fuel filler hose are probably the more common choices for custom spigot to spigot intake connections. I haven't really looked into other possibilities, because what I use works for me.

You could also ask Mark at CNW what he uses with the CNW Keihin FCR kit. The FCR carburetors use the same spigot design as the CRS carburetors. It looks like the Mikuni spigot to spigot part to me, but I'm not sure.
 
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