Modifying Commando intakes for CR or FCR carburetion on P11

I tried my cold startup with 3 squirts leaving the throttle wide open for each squirt and kicking through on each, ignition Off. Turned the ignition On, 1 kick started right up. The way I was cold starting the motor before was messed up, and a hangover from starting the motor with the Mikuni chokes. Probably did wash the cylinders a little. I'm going to try 2 squirts tomorrow when the motor is stone cold. Maybe I will be able to get it down to 1 squirt using a smarter startup procedure.

After warming the motor up and letting it idle for 10 minutes with the occasional mild blip, the plugs look unbelievably good compared to how they looked with the Mikuni carburetion and with the out of box FCR settings. Quite a transformation in how well the motor runs now. Still need a road test.

In the spirit of doing stuff wrong to see if it might just be right, I'm using Accel 416S plugs. Why? I have a lot of auto plugs for big valve aluminum SBC heads with the right thread and reach on the shelf, and I wanted something hotter than the NGK 7 series. The plugs seem to get along well with the Boyer MKIV, Podtronics, a little battery, a 12V dual epoxy filled coil, and the old 2-wire alternator. The Accel plugs are a simple copper core plug with a center cut ground electrode, and use a 5/8th wrench. Bigger flame supposedly. Probably burn holes in my pistons. Time will tell.

Finally was able to do a short road test with the tuning changes I made a while back. I also moved the balance tube ports just above centerline of the intake manifolds. I filled the lower balance tube ports, and did a little more smoothing inside the intakes. The motor is much easier to start with a smooth intake floor. I did not ride hard at all. I wanted to see if the motor would load up riding like the traffic laws matter. No problems so far. Plugs are a little darker than I'd like for round town putting, but will probably look much better if I can get out and get the motor wound up for more than a few feet. The difference on the street under load between the out of box settings and the tuning settings outlined in a previous post is significant. I don't want to jinx myself, but it ran better than it ever has. I'm looking forward to riding a little more this summer.

Raised balance tube. The port tubes are threaded and epoxy sealed into the intake manifolds. The tubes have a larger inside diameter than the tubes that were installed in the floor of the intakes.

Where did you source the tubes for the intakes?

MichaelB said:

Where did you source the tubes for the intakes?

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They were in a drawer where I have been keeping small brass fittings for years. I'm not sure where they came from. I got them with a kit for something else entirely or for another project and pack ratted them a long time ago.

They are straight no barb with a small shoulder on them near the middle. No threads. Probably originally for coupling/extending/repairing small diameter tubing. I may have got them at a hardware store. If it was Orchard Supply Hardware, they are not around anymore. Ace hardware still carries odd things in brass, but you have to walk into a brick and mortar store to see what they have. Dig around on Amazon?

Should be really good for wheelies , with those on it .

Matt Spencer said:

Should be really good for wheelies , with those on it .

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ha Not with me on it. I'm getting a little long in the tooth for that.

I doubt it will do a power wheelie on a level surface. It's geared up in the primary. Might be able to get the front wheel up WOT in 2nd coming over a rise in the road.

It runs surprisingly well with those big things on there. I did it for the heck of it. Not for everyone. It would cold start easier with 30mm Amals on it, but it would not run as well as it does with FCRs on it once warmed up.

For those not doing a 100% restoration and/or interested in getting away from Amals, the 33mm CRS carburetors would make more sense than FCRs for most riders and are allowed in AHRMA Vintage Racing venues. The FCR pumpers are not allowed in AHRMA heavy weight twins racing. I think there is a class that does allow pumpers, but not a class that a 750 twin would be competitive in without spending wheel barrels full of money on.

Schwany said:

ha Not with me on it. I'm getting a little long in the tooth for that.

I doubt it will do a power wheelie on a level surface. It's geared up in the primary. Might be able to get the front wheel up WOT in 2nd coming over a rise in the road.

It runs surprisingly well with those big things on there. I did it for the heck of it. Not for everyone. It would cold start easier with 30mm Amals on it, but it would not run as well as it does with FCRs on it once warmed up.

For those not doing a 100% restoration and/or interested in getting away from Amals, the 33mm CRS carburetors would make more sense than FCRs for most riders and are allowed in AHRMA Vintage Racing venues. The FCR pumpers are not allowed in AHRMA heavy weight twins racing. I think there is a class that does allow pumpers, but not a class that a 750 twin would be competitive in without spending wheel barrels full of money on.

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hi Schwany, just been reading your thread again, How close ( centre to centre) laterally, can the FCRs be mounted please?

An ’old’ Commando ports are 66mm centres according to my elastic tape measure.

The carb centres on mine are a tad more than that, approx 67mm. At that, the tops of the carbs are just / almost touching.

Of course, you have a little ‘wrestle room’ with them being rubber mounted.

What are the port centres on the head you’re looking at Jan ?

jan nelder said:

hi Schwany, just been reading your thread again, How close ( centre to centre) laterally, can the FCRs be mounted please?

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Not sure how close together the FCRs could be assembled since the Keihin 35mm FCR's in the Sudco kit came fully assembled for use on a Norton 750/850 head. I did not have to think about it. That's my favorite kind of thinking. Taking a look under the fuel tank it looks like they are about as close together as they can be and not touch at the top. I'd have to take them off to give you an accurate measurement. Not planning on doing that until maybe spring 2024 when I do a gearbox swap. Nigel's 67mm number looks good based on measuring the spare early 750 Commando head I have. The head the FCRs are mounted on now is essentially an Atlas head, but I doubt it makes much difference.

If your intake centers are closer than 66.5mm use a heat gun to soften the rubber tube mounts when mounting. I personally use a light smear of an automotive tire dressing with silicon in it around the ID of the rubber mount along with the heat gun. Makes it easy to mount the FCRs.

One of the nicest features I've found of Keihins, especially with today's ethanol gas, is their small tapered float bowl drain screw!
A quick turn with a screwdriver and fuel in the bowl empties out beneath the bike via the attached bowl hose (and not onto your crankcase as with the bottom hex plug).

I don't know if your twin FCRs have it, but the FCR MX and many OE Keihins do.
Great thread and great carbs.

That hose setup sounds handy.

The bolt-on (more or less) Sudco 35mm FCR kit for Norton 750/850 uses the aluminum hex drain plug with a rubber or nitril o-ring seal. I have a small shallow plastic container I use when draining the bowls. It's still a circus act not to spill on the cases, but I get by and have a lot of red rags for clean up.

To be pedantic, the cylinder heads were designed in imperial units, the inlet port nominal distance between centers is 2 5/8" ( 66.67 mm)

hybridracer said:

To be pedantic, the cylinder heads were designed in imperial units, the inlet port nominal distance between centers is 2 5/8" ( 66.67 mm)

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I came up with 66.69 mm when I got out the digital caliper, but didn't want to contradict anyone with a quick measurement. Hence, I said 67mm mentioned by Nigel was close enough. I could probably get a fairly accurate measurement of the minimum for the FCR centers at the rear without taking them off, which would be relevant to the actual question about how close the centers could be set on the 35mm FCRs.

hybridracer said:

To be pedantic, the cylinder heads were designed in imperial units, the inlet port nominal distance between centers is 2 5/8" ( 66.67 mm)

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To be even more pedantic the Imperial (i.e. correct) spelling is "centres"