Keihin CRS smoothbore carbs for Norton

jseng1 · Dec 11, 2014

Check out the excellent fabrication and carb installation job by "Matchless". These Keihin 33mm CRS smooth bore roundslide carbs look really good. They have chokes and are legal for AHMRA Bears and F750 classic racing. They come pre-syncronised which simplifies things. The fit is not as tight as the FCRs and they cost less. The only problem is you have to make up a new throttle wheel and bracket for the cable. I'm working on it.

BrianK · Dec 11, 2014

Do they have accelerator pumps Jim? (Purely curiosity on my part as I'm already running FCRs.) Tx - BrianK

jseng1 · Dec 11, 2014

No accelerator pumps - better gas mileage.

Holmeslice · Dec 11, 2014

jseng1 said:
...are legal for classic racing.

From the AHRMA rulebook:

9.7.14
CARBURETORS
a)
Flat-slide or Lectron-type and injection-type carburetors are not permitted. “Power
jet” and “pumper” type carburetors are not permitted unless such mechanisms are
disconnected or removed. Fuel injection is not permitted. Period carburetors are
encouraged. Smoothbore carburetors with concentric float bowls are not permit-
ted, except where they were fitted as original equipment or supplied as factory kits
on that model motorcycle. Modern replacement smoothbore carburetors are not
permitted.

acotrel · Dec 11, 2014

God-damn, another cheat stymied ? It would have been worth at least 10 BHP. Do AHRMA do fuel checks at all of their meetings ?

Holmeslice · Dec 11, 2014

Holmeslice said:
More from the AHRMA rulebook, specifically in the rules section for F750:

j) Keihin CR Special carburetors are allowed, maximum 31mm for four-cylinders and
39mm for twins.

nortriubuell · Dec 11, 2014

jseng1 said:
Check out the excellent fabrication and carb installation job by "Matchless". These Keihin 33mm CRS smooth bore roundslide carbs look really good. They have chokes and are legal for classic racing. They come pre-syncronised which simplifies things. The fit is not as tight as the FCRs and they cost less. The only problem is you have to make up a new throttle wheel and bracket for the cable. I'm working on it.

Very, very cool carbs 8) will you be offering this setup for sale ... I hope ??? Cheers.

74ok · Dec 11, 2014

jseng1 said:
They come pre-syncronised which simplifies things.

but still needs to done on bike?

Matchless · Dec 12, 2014

Ideally they need to be balanced on the engine to which they are fitted. Once this is done, they should stay in tune for a lot longer than a cable per carb. set up. From then on, they will only need the occasional tweak. With a fresh well balanced engine, they would be fine out of the box.

Martyn.

jseng1 · Dec 12, 2014

Holmeslice said:
Holmeslice said:

More from the AHRMA rulebook, specifically in the rules section for F750:

j) Keihin CR Special carburetors are allowed, maximum 31mm for four-cylinders and
39mm for twins.

Click to expand...

I contacted AHRMA and they said they were legal as below:

"AHRMA F750 is 31MM for 4 cylinders and 39MM for 2 cylinders - see page 43 of the 2014 rulebook. Bears follows the F750 rules - see page 52"

So they are legal in AHRMA Bears and F750.

Nortribuell - as I said above - I'm working on it. It depends if anyone actually wants them and is willing to pay the price.

Matchless - My biggest problems with the PWKs was from customers who had their carbs all out of sync and couldn't tune them because of that. I'm hoping the mechanically linked CRS carbs will change all that. They will at least start with the synchronization very close and should be able to tune them and dial them in from there.

marksterrtt · Dec 12, 2014

Jim, nice looking carbs, what is the center to center spacing on this 2 carb rack? And does the body size stay the same for larger bores in case some one wanted to go bigger? Thanks, Mark

jseng1 · Dec 13, 2014

marksterrtt said:
Jim, nice looking carbs, what is the center to center spacing on this 2 carb rack? And does the body size stay the same for larger bores in case some one wanted to go bigger? Thanks, Mark

The center to center is about 67mm and matches the Norton cylinder head ports pretty well. The carb bodies and spacing are the same on some different sizes. Sizes bigger than 33mm are more expensive.

marksterrtt · Dec 14, 2014

jseng1 said:
marksterrtt said:

Jim, nice looking carbs, what is the center to center spacing on this 2 carb rack? And does the body size stay the same for larger bores in case some one wanted to go bigger? Thanks, Mark

Click to expand...

The center to center is about 67mm and matches the Norton cylinder head ports pretty well. The carb bodies and spacing are the same from 31 up to 37mm. But sizes bigger than 33mm are more expensive.

Thanks for info Jim, those CR's look nice and would be another good option, would it be possible to post a picture from just a wee bit further back to see overall size and fit in the Commando frame/engine? Mark

850commando · Dec 15, 2014

would also need rubber adaptor and curved intake, looks nice, would be a nice way to exocise the amal demon... Jim have you got the jets etc all sorted out? and how much $

BritTwit · Dec 15, 2014

Jim,

They are great units
I had a quartet from Yoshimura on my Kawasaki race bike back in the day.
Definitely interested.

Carbs, Manifolds, cables, whole kit 'n kaboodle - how much do you estimate the price will be?

Any clearance issues with the stock frame?

jseng1 · Dec 15, 2014

BritTwit said:
Jim,

They are great units
I had a quartet from Yoshimura on my Kawasaki race bike back in the day.
Definitely interested.

Carbs, Manifolds, cables, whole kit 'n kaboodle - how much do you estimate the price will be?

Any clearance issues with the stock frame?

They require custom made short manifolds so nothing will rub or interfere, balance tube for the manifolds, special long life heavy duty rubber boots, twist grip, cable, aircleaner, filter & plumbing kit, custom made throttle wheel and cable bracket that won't interfere with the tank.

I have the jetting instructions that Matchless has worked out and I will be doing my own testing. They look to be a very good option and a high quality carb upgrade for Nortons.

warpedscout · Dec 17, 2014

Im trying to get an 83 550 Nighthawk and just finished the rebuild of the Keihin carbs on it. Could you not use one of the throttle wheels and maybe throttle setup from a Nighthawk since they are probably close to the same carbs? Mine has the push/pull wheel so you dont have to have much spring tension on opening and closing.

I was also looking at using the style Keihin on the Nighthawk on the Norton when I get back to her, but did certainly notice new manifolds would need to be worked up!

Good luck and keep us posted!

jseng1 · Dec 17, 2014

warpedscout said:
Im trying to get an 83 550 Nighthawk and just finished the rebuild of the Keihin carbs on it. Could you not use one of the throttle wheels and maybe throttle setup from a Nighthawk since they are probably close to the same carbs? Mine has the push/pull wheel so you dont have to have much spring tension on opening and closing.

I was also looking at using the style Keihin on the Nighthawk on the Norton when I get back to her, but did certainly notice new manifolds would need to be worked up!

Good luck and keep us posted!

Can you post some photos of the Nighthawk throttle wheel etc?

warpedscout · Dec 17, 2014

Ill try to remember and see if I can get a pic of my parts carb.

jseng1 · Dec 21, 2014

Here's what I worked out for manifolds and balance tubes. These manifolds were originally made by Quaife engineering - the gearbox people. They are shorter than others and that allows the necessary clearance.

Note the stepped balance tube between the boots. A small hole is cut in the rubber boots with a special tubing drill.

Keihin CRS smoothbore carbs for Norton

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