Modifying Commando intakes for CR or FCR carburetion on P11

I lied about the Red velocity stacks that come with the 35mm FCRs. They are 1 3/8ths inches in height, not 2 inches. Lots of room for those.

So I shortened the intake manifolds and the air filter adapters by 2mm and radiused the flat surface of the opening of the air filter adapter. The adapter is not radiused by Keihin. Not sure why other than extra cost to have it done, and it probably makes close to no difference. I did it because I like the way it looks, and I think air will flow better without the square edge that is exposed by about 3/32nds of an inch inside the filter. Only the radius is exposed with the modification. It's like a mini-velocity stack. Probably good for .010 of a HP increase at some RPM I'll never reach.

Radiused adapter on the right. Carburetors upside down.


Both adapters radiused. Carburetors upside right.

Spigots arrived a little later than I expected. My fault not Sudco's. I swapped the last two numberals in my address and the USPS drove down the street with the parts, but could not find the address, so returned them to Sudco. Anyway, got em and I Installed the 35mm FCR carburetors on the Atlas in a Matchless frame P11. It was not a struggle getting them into the frame with the modifications I made to the Commando intakes. I didn't really need to shorten the air filter adapters, but it did make getting the air filters on easy with the carburetors mounted.

Looks like this. Excuse the scratches on the frame and wild ass wiring. Darn digital flash photography shows everything. Paint color of the bike is wrong too. Auto White Balance doesn't work that great in my garage.







Notes:

The Motion Pro push/pull throttle that comes with the kit is a 1/4 turn throttle. Might be a bit more than the old motor can swallow. Don't know yet. Weather is poop, and I'm a fair weather rider. And, I have yet to start it. I will update this thread or start another one about the running and riding experience with the FCRs.

The Sudco FCR kits come with no instructions, so a few decades of motorcycle tinkering experience might help with the install.

Straight down petcocks would work better than the 90's I have. I had to raise the tank 1/4 inch at the rear to get clearance for a clamp on the left side. A clamp went on over the hose easy, it was zero clearance though.

I reduced the length of the filter adapter to 1". Doesn't need to be done, but it will make installing and removing the air filters easier.

I used 1.5" ID automotive fuel tank filler hose for the intake manifold to carburetor interconnect. The length I'm using is 1.25". A longer interconnect of 1.5" could be used, but I didn't want to struggle getting the air filters on. A longer interconnect will push the carburetors further back toward the frame at the rear. In case that air filter comment wasn't clear.

Looking at the Commando spigot intake from the side and measuring from the center of the top lip on the head end mount surface to the center of the top of the carburetor end spigot is 1 7/8 inches. The measurement done the same way at the bottom is 1 7/16 inches. A straight cut between the two measurements was done, and then a groove was filed all the way around leaving 1/8" of flat surface from the end of the spigot. Hope that makes some sense to anyone who feels like fooling around with some bigger carburetion.

The FCR slides do have a hallow rattle. Given how much of a rattler my motor already is, I figure it's gonna sound like it's coming apart at idle.

Schwany said:

Spigots arrived a little later than I expected. My fault not Sudco's. I swapped the last two numberals in my address and the USPS drove down the street with the parts, but could not find the address, so returned them to Sudco. Anyway, got em and I Installed the 35mm FCR carburetors on the Atlas in a Matchless frame P11. It was not a struggle getting them into the frame with the modifications I made to the Commando intakes. I didn't really need to shorten the air filter adapters, but it did make getting the air filters on easy with the carburetors mounted.

Looks like this. Excuse the scratches on the frame and wild ass wiring. Darn digital flash photography shows everything. Paint color of the bike is wrong too. Auto White Balance doesn't work that great in my garage.

Notes:

The Motion Pro push/pull throttle that comes with the kit is a 1/4 turn throttle. Might be a bit more than the old motor can swallow. Don't know yet. Weather is poop, and I'm a fair weather rider. And, I have yet to start it. I will update this thread or start another one about the running and riding experience with the FCRs.

The Sudco FCR kits come with no instructions, so a few decades of motorcycle tinkering experience might help with the install.

Straight down petcocks would work better than the 90's I have. I had to raise the tank 1/4 inch at the rear to get clearance for a clamp on the left side. A clamp went on over the hose easy, it was zero clearance though.

I reduced the length of the filter adapter to 1". Doesn't need to be done, but it will make installing and removing the air filters easier.

I used 1.5" ID automotive fuel tank filler hose for the intake manifold to carburetor interconnect. The length I'm using is 1.25". A longer interconnect of 1.5" could be used, but I didn't want to struggle getting the air filters on. A longer interconnect will push the carburetors further back toward the frame at the rear. In case that air filter comment wasn't clear.

Looking at the Commando spigot intake from the side and measuring from the center of the top lip on the head end mount surface to the center of the top of the carburetor end spigot is 1 7/8 inches. The measurement done the same way at the bottom is 1 7/16 inches. A straight cut between the two measurements was done, and then a groove was filed all the way around leaving 1/8" of flat surface from the end of the spigot. Hope that makes some sense to anyone who feels like fooling around with some bigger carburetion.

The FCR slides do have a hallow rattle. Given how much of a rattler my motor already is, I figure it's gonna sound like it's coming apart at idle.

Click to expand...

Love em!

Fired the P11 up. Here's how it went:

I opened the petcocks and everything looked leak free for about 5 seconds. Then the float on the right carburetor stuck open and fuel barfed out of the air vent at petcock fuel feed speed. Arrgh!! I shut off the petcocks and started scratching my head thinking damn it I really don't want to pull those float bowls off. I looked at what it would take, and really decided I didn't want to take them off. So I removed the bowl drain bolt on that side and drained the bowl with the petcocks open again, then shut them off after a couple of seconds. Clean fuel, so I put the drain bolt (Keihin calls it a drain bolt) back in, and drained the other side. Fuel was also clean. I put the drain bolt back in and turned on the petcocks. No more barfing up fuel through the air vent. Float on the right side was working. Unfortunately, the accelerator pump on the right side was weak. I went through the bowl drain scenario again and worked the throttles until I got a reasonable amount of dribble out of the pump nozzle on the right side. Turned the petcocks on again, and worked the throttle with rags in the throats so I didn't fill my head and or cylinders up with fuel. I eventually got a good pump shot on both side. Seemed like all was good. That's when I decided to try and start it.

Fairly easy to start without a choke with the accelerator pumps, but without a choke it takes a few minutes for the idle to settle in. It's very low when cold until it warms up. Typical of old cold carbureted bikes. I could dial in more idle, but I am very fond of using motor brake to slow down initially. If the idle is too high once warmed up I always feel like I'm braking late. blah blah blah, who cares.

The FCRs are impressive blipping the throttle in the garage. Much better throttle response than the 34mm Mikuni carburetors.

I have zero regrets about installing the 35mm Keihin FCRs on my little Norton 750 P11.

Road test will happen next time I see the sun.

YMMV

There might be a few grammatical errors above. I didn't proof read it.

So... Are 35mm Keihin FCR carburetors too big (over carburetion) for old Norton 750 engine?

Not in my book.

Are they just cool looking add-ons with no real benefit over Amals? They are a lot more than just cool looking. Actually I think they are kind of ugly myself, but they sure do work right out of the box. Amals or Mikuni VM carburetors are not even close to working as well with quick throttle input.

Will the engine use more fuel? Definitely, but I didn't build my Norton for fuel economy. The pump shot isn't a big flood of fuel. It is enough to let a quicker throttle wheel work without hiccups

Will the power curve move up dramatically making the engine weak in the lower rpm range? I suspect the motor will make better power everywhere with the flat slide design and accelerator pumps based on what it sounds like. I haven't ridden it yet, but I do have a good ear for a tune.

The only issue I see with the FCRs is I'll need to upgrade the brakes.

Seriously though, they do have a lot of operational parts, and if I were riding a lot using the FCRs could raise the maintenance level on Norton ownership over the long haul. I don't put that many miles on my P11, so I'm not worried about it.

If you have the money, they are worth the cost of admission.

If you think small port heads and little Amal carburetors work better, ... good luck with that. See ya in my rear view. LOL

Schwany said:

So... Are 35mm Keihin FCR carburetors too big (over carburetion) for old Norton 750 engine?

Not in my book.

Are they just cool looking add-ons with no real benefit over Amals? They are a lot more than just cool looking. Actually I think they are kind of ugly myself, but they sure do work right out of the box. Amals or Mikuni VM carburetors are not even close to working as well with quick throttle input.

Will the engine use more fuel? Definitely, but I didn't build my Norton for fuel economy. The pump shot isn't a big flood of fuel. It is enough to let a quicker throttle wheel work without hiccups

Will the power curve move up dramatically making the engine weak in the lower rpm range? I suspect the motor will make better power everywhere with the flat slide design and accelerator pumps based on what it sounds like. I haven't ridden it yet, but I do have a good ear for a tune.

The only issue I see with the FCRs is I'll need to upgrade the brakes.

Seriously though, they do have a lot of operational parts, and if I were riding a lot using the FCRs could raise the maintenance level on Norton ownership over the long haul. I don't put that many miles on my P11, so I'm not worried about it.

If you have the money, they are worth the cost of admission.

If you think small port heads and little Amal carburetors work better, ... good luck with that. See ya in my rear view. LOL

Click to expand...

Treat em kindly and they'll look after themselves!

Al's#5 said:

Treat em kindly and they'll look after themselves!

Click to expand...

Thanks for the encouraging words

They will last longer than I do, that I know for sure.

I do like to take care of my toys. I find keeping them clean is the best way to find loose fasteners.

I need to throw a couple of profile pics of the entire bike in this thread. I better get to getting to it.

Schwany said:

Fired the P11 up. Here's how it went:

I opened the petcocks and everything looked leak free for about 5 seconds. Then the float on the right carburetor stuck open and fuel barfed out of the air vent at petcock fuel feed speed. Arrgh!! I shut off the petcocks and started scratching my head thinking damn it I really don't want to pull those float bowls off. I looked at what it would take, and really decided I didn't want to take them off. So I removed the bowl drain bolt on that side and drained the bowl with the petcocks open again, then shut them off after a couple of seconds. Clean fuel, so I put the drain bolt (Keihin calls it a drain bolt) back in, and drained the other side. Fuel was also clean. I put the drain bolt back in and turned on the petcocks. No more barfing up fuel through the air vent. Float on the right side was working. Unfortunately, the accelerator pump on the right side was weak. I went through the bowl drain scenario again and worked the throttles until I got a reasonable amount of dribble out of the pump nozzle on the right side. Turned the petcocks on again, and worked the throttle with rags in the throats so I didn't fill my head and or cylinders up with fuel. I eventually got a good pump shot on both side. Seemed like all was good. That's when I decided to try and start it.

Fairly easy to start without a choke with the accelerator pumps, but without a choke it takes a few minutes for the idle to settle in. It's very low when cold until it warms up. Typical of old cold carbureted bikes. I could dial in more idle, but I am very fond of using motor brake to slow down initially. If the idle is too high once warmed up I always feel like I'm braking late. blah blah blah, who cares.

The FCRs are impressive blipping the throttle in the garage. Much better throttle response than the 34mm Mikuni carburetors.

I have zero regrets about installing the 35mm Keihin FCRs on my little Norton 750 P11.

Road test will happen next time I see the sun.

YMMV

There might be a few grammatical errors above. I didn't proof read it.

Click to expand...

I wonder if the flooding could have been down to the fact you inverted the carbs in an earlier post? Maybe the needle got stuck at an angle and couldn't shut off initially, then once flooded the float lifted enough to let it drop? All hypothetical of course. They look great anyway, and I look forward to seeing those profile pics!

I started fitting FCR's in the 1990's at around $1700 a pair by the time they were turn key.
It might not make to much different but am yet to see one mounted anywhere near the right angle on a Norton, perhaps the catch is even the side draught FCR is heading towards a down draught (space limitations) and found the float bowl plug level was to the floats liking.

Great job.

Only just stumbled across this thread, with me not being a P11er I don’t normally frequent these pages...

The carbs look awesome, great job !

I think you’ll find they perform great too.

I’ve been running them for 6 years and love ‘em.

Do be mindful of those accelerator pumps, it’s quite easy to over do it and wash your bores.

And, if you end up having to adjust settings, it’s a good idea to disconnect the accelerator pumps temporarily so you know what the jets are doing.

Final point, if the slide rattle bothers you, your pipes are too quiet !

cliffa said:

I wonder if the flooding could have been down to the fact you inverted the carbs in an earlier post? Maybe the needle got stuck at an angle and couldn't shut off initially, then once flooded the float lifted enough to let it drop? All hypothetical of course. They look great anyway, and I look forward to seeing those profile pics!

Click to expand...

I thought of that as well. The carburetors have been upside right a lot more than upside down, but that's as good a theory as any. I'm just happy they work given the fluster cluck with the spigots not being installed in the carburetors from the get go. Anyway, the longer harder stream of fuel with the drain bolt out corrected whatever was wrong.

I'm heading out to the garage for the pics now. Will get them up in a hour or so, unless I get distracted.

Time Warp said:

I started fitting FCR's in the 1990's at around $1700 a pair by the time they were turn key.
It might not make to much different but am yet to see one mounted anywhere near the right angle on a Norton, perhaps the catch is even the side draught FCR is heading towards a down draught (space limitations) and found the float bowl plug level was to the floats liking.

Great job.

Click to expand...

I believe I got the drain bolt mount surface for the side draft FCRs close to level when the wheels are on the ground.

The jetting and air filters were turn key for the P11. The intake manifolds not so much. The jetting is real close, but many would find it richer than they like if they read their plugs after riding through town in stop and go traffic.

I'm into them for a lot less than $1700 using $0 for my hourly rate.

Schwany said:

I believe I got the drain bolt mount surface for the side draft FCRs close to level when the wheels are on the ground.

I'm into them for a lot less than $1700 using $0 for my hourly rate.

Click to expand...

Perfect.

It might have been more $.
Single FCR's were over NZ$700 each back then (I got a large discount luckily. (One DD and one SD)
Then there was front spigots and air filter adapters along with jetting off Sudco/USA.
Two 750 F1 rear manifolds, rubber intermediates along with machined O-ringed adapter wedges (free machining) to set the mounted angles.
Home made cables, K&N flat track type filters and a HRC throttle (NSR maybe via in the know channels)

Once set up they make most other carburetors look very average.

Fast Eddie said:

Only just stumbled across this thread, with me not being a P11er I don’t normally frequent these pages...

The carbs look awesome, great job !

I think you’ll find they perform great too.

I’ve been running them for 6 years and love ‘em.

Do be mindful of those accelerator pumps, it’s quite easy to over do it and wash your bores.

And, if you end up having to adjust settings, it’s a good idea to disconnect the accelerator pumps temporarily so you know what the jets are doing.

Final point, if the slide rattle bothers you, your pipes are too quiet !

Click to expand...

I probably won't adjust much of anything. Running the plugs on the darker side does not bother me. I don't get what appears to be enough pump shot fuel to wash the cylinders. Exhaust doesn't stink excessively rich when blipping the throttle no load. Anyway, that's my jam and I'll figure it all out.

In my situation the slides only rattle a little if kicking the engine over with the ignition key off. Vacuum makes them rattle a little. It's audible then, because nothing else is going on. When the motor is running, it is actually quieter. What? Yep the slide rattle fills in the gap between the tappets tapping and it's just one long machine note. That and my exhaust off idle is loud enough to bring my neighbor over because of its siren song. The exhaust is all that can be heard when turn the wick up.

Time Warp said:

Perfect.

It might have been more $.
Single FCR's were over NZ$700 each back then (I got a large discount luckily. (One DD and one SD)
Then there was front spigots and air filter adapters along with jetting off Sudco/USA.
Two 750 F1 rear manifolds, rubber intermediates along with machined O-ringed adapter wedges (free machining) to set the mounted angles.
Home made cables, K&N flat track type filters and a HRC throttle (NSR maybe via in the know channels)

Once set up they make most other carburetors look very average.

Click to expand...

Oops, not perfect. I screwed up. They definitely are a little low at the rear. I might be able to fix it, but will have to investigate further. Probably not a show stopper, but I thought I did a better job.

Yeah, it's a much easier Sudco kit install now. You had to work lot harder.

Here are a couple of side shots. I need a better background.

Schwany said:

Here are a couple of side shots. I need a better background.

Click to expand...

That is one cool bike

baz said:

That is one cool bike

Click to expand...

Thanks. It's my little low rider.

I might ride it today, but damn it's cold on the old bones at 33 degrees. Plus I don't think it hooks up that well on back ice. I'll probably come up with a few more excuses not to ride it until the weather warms up.

Long rambling post

I took the P11 for a ride Saturday without changing any settings on the FCR carburetors. With the engine warmed up and a load on the motor, it was not pretty. Rich would be an understatement on my Norton 750 engine. My engine is not fresh, so I wasn't expecting my plugs to look that great, but they looked like they had been sprayed with black velvet matte paint everywhere. The motor misfired here and there with a load on it. I can see why people think it is over carburetion.

So... I took the fastest route to making it run a little better, and I consulted google. Google brought me back here to Brian's FCR settings thread. I made the adjustments Brian used except for the main jet change. The difference in idle quality after making the slow speed adjustments was significant on a warmed up engine. I also moved the clip on the needle to the second grove from the top. I was going to move the needle clip after looking at the plugs anyway, but good to see that it worked in Brian's example. Idle is good, no load throttle response is great, but deceiving in the garage. Unfortunately, it's raining, and I don't ride in the rain, so have not had a chance to put a real world load on the motor to see how much difference the new settings make riding the bike. My optimistic fantasy of being able to use the out of box settings didn't last long. Not surprising.

Brian's settings information is listed below for anyone that hasn't already seen it. The settings should be good for a mild 750.

Slow Air Screw 2 turns out (This adjustment is made with the left hand screw at the air filter side of the carburetors with the air filter adapters removed.)
Slow Idle Mixture Screw 5/8-3/4 turn out (This adjustment is made at the intake manifold side of the carburetor from the bottom in front of the float bowls.)
Needle Clip Position 2nd clip from the top (Leaner setting than the as shipped 5th position from the top.)
Main Jet 145 (Optional before mounting, because it is easy to change later if you have room under the carburetors after they are mounted. Smart money would be to change mains before mounting. Order the main jets at the same time as the Sudco carburetor kit.)

After the test ride I also took additional material off the intake manifolds around the O.D. so I could move the connector boots closer to the head, I shortened the boots to 1 1/16". I could explain why, but it's too much babbling.

It's a lot of fiddling to get the Commando spigot intakes to work on an motor mounted with the cylinders in the vertical position, and it's not necessary now that I've done it, and see what the actual clearances are like. Short straight spigot manifolds would work with the FCRs. I'm not sure about getting the CRS carburetors to fit with straight spigot manifolds, because I have not done it.

It's not hard to cold start with the modified Commando intakes I hacked up, but requires a goofy procedure because the manifolds are not downdraft enough. What seems to work best is blip the throttle 3/4 to WOT about 5 times to squirt the accelerator pump, then with the key Off open the throttle all the way and kick the motor over 3 times, then turn the key On, and give it all you got with a little bit of throttle. I don't worry that much about washing down the cylinders on my Atlas setup, because the intake manifolds are very close to level before entering the head. Fuel from the accelerator pump doesn't flow right into the head easily without a lot of vacuum (motor running). When stone cold it doesn't want to get there at all just kicking it over, if I don't perform the goofy start up ritual. If I installed the straight intakes that put the carburetors at an increased downdraft angle, I would more than likely change the cold startup procedure.

My rationalization for not using a set of straight spigot intake manifolds in the first place is I did not think I could make the CRS carburetors I originally planned to use fit under the backbone of the frame. I figured the modified Commando spigot intake manifolds would provide more clearance. I then changed my mind and got the FCRs because they are not as tall above the intake as the CRS carburetors, and I knew I could make the FCRs fit. Now with the FCRs installed, it looks like I could make straight manifolds work with the FCRs. I could also install CRS carburetors on the Commando intake manifolds I just finished and the carburetors would be mounted at a proper angle for the CRS design, and the standard push pull cable setup would fit under the P11 tank.

5 squirts for cold start... I previously used two squirts and still suffered suspected petrol wash, so now stick to one squirt only.

Fast Eddie said:

5 squirts for cold start... I previously used two squirts and still suffered suspected petrol wash, so now stick to one squirt only.

Click to expand...

Wish that I could pull that off. Plugs are bone dry without a hint of fuel odor with 1 squirt. I might be able to do it with 3 squirts, and 4 WOT pre-ignition kicks. Not a chance with 1 squirt though. Weak pump shot and low vacuum kicking it over is what I suspect is part of my cold start problem.

I had to perform a cold start ritual with the Mikuni carburetors I took off... My kick start hardware has had a rough life.

Might be time for a compression check.