Keihin CRS smoothbore compared to Keihin FCR flatslide

jseng1

jseng1

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Here's the WOT view of the CRS
Keihin CRS smoothbore compared to Keihin FCR flatslide


Here's the WOT view of the FCR (anyone have a better photo?)
Keihin CRS smoothbore compared to Keihin FCR flatslide
 
That's not the version of the FCR that comes in a Sudco Norton twin kit. I have no idea what the differences are between that version and the 35mm pumpers sold in the Sudco kit for the Norton.

CRS are good carburetors. I had them on a GSXR 1100. Definitely the fastest bike I ever owned. I did not have to do any tuning on them though. They worked perfectly.

I like the tuning feature of the FCRs. They are surprisingly good everywhere from low RPM to high RPM. They don't stall at stops. Some people disable the accelerator pump and still make them work. No clue about the flow characteristics compared to CRS carburetors.

I don't have any pics of the FCRs with the throttle open or the slides removed. Both the CRS and FCR have cuts for the slides in the bore flow path.

The 35mm FCRs can be too big for people that like to keep the revs really low and expect instantaneous wheelie throttle response when they whack the throttle wide open while riding around at 20mph in 3rd gear.
 
Some pics of the 35mm FCR WOT

Air goes in this end
Keihin CRS smoothbore compared to Keihin FCR flatslide


Air and fuel comes out this end and goes into the head.
Keihin CRS smoothbore compared to Keihin FCR flatslide


I'm not showing much of the cut in the bore for the flat slide, but the cut is a substantial ditch in the sides of the bore. I'm happy with the FCRs and how I have them setup. Glad I bought them when I did. Price is probably redamdickulous now and they are harder to get.
 
Unfortunately neither is legal in period 2 up here, but the CRS are good for BEARS... go figure. Was in Pittsburgh this past weekend, talking with the AHRMA guys about the tech rule differences across the border... sidecar driver quipped "another reason NOT to live in Canada" LOL
 
yea its staying together as long as I don't over rev it, or speed shift or....



those are vent lines
 
I'm wondering about the width of the channels in the side of the bore. The flatslides have a reputation of better partial throttle signal. But the CRS is supposed to have better WOT flow. Both styles are harder to get now that Sudco closed its doors (but they're still available and a little more expensive).

Keihin CRS smoothbore compared to Keihin FCR flatslide


Keihin CRS smoothbore compared to Keihin FCR flatslide
 
jseng1 said:
I'm wondering about the width of the channels in the side of the bore. The flatslides have a reputation of better partial throttle signal. But the CRS is supposed to have better WOT flow. Both styles are harder to get now that Sudco closed its doors (but they're still available and a little more expensive).
Click to expand...
There are two straight cut channels: An 1/8" wide channel closer to the rear, and a 3/16" wide channel closer to the head. Both are near the center of the throttle body.

FCRs are surprisingly good on a 750 putt putting around once tuned for the power plant configuration. I almost went the CRS carburetors, but changed my mind because of the height and angle of cable set up and height of the CRS bodies. The FCRs were a better fit in my frame.
 
Schwany said:
There are two straight cut channels: An 1/8" wide channel closer to the rear, and a 3/16" wide channel closer to the head. Both are near the center of the throttle body.

FCRs are surprisingly good on a 750 putt putting around once tuned for the power plant configuration. I almost went the CRS carburetors, but changed my mind because of the height and angle of cable set up and height of the CRS bodies. The FCRs were a better fit in my frame.
Click to expand...
The stock cable and bracket layout of the CRS carbs needs to be changed as shown below to give enough clearance.
Keihin CRS smoothbore compared to Keihin FCR flatslide



The FCRs are also a tight fit.

Keihin CRS smoothbore compared to Keihin FCR flatslide
 
jseng1 said:
The stock cable and bracket layout of the CRS carbs needs to be changed as shown below to give enough clearance.
Keihin CRS smoothbore compared to Keihin FCR flatslide



The FCRs are also a tight fit.

Keihin CRS smoothbore compared to Keihin FCR flatslide
Click to expand...
Jim those FCRs are the bigger bodied 37mm ones ! The 35s fit without any mods to the gusset etc.
 
jseng1 said:
The stock cable and bracket layout of the CRS carbs needs to be changed as shown below to give enough clearance.

The FCRs are also a tight fit.
Click to expand...
I had to modify the cable bracket a little on the 35mm FCRs to clear the head better, but it was a very simple change. The CRS bracket change is more work. I also prefer the push/pull dual cable arrangement. I don't have a Commando frame to deal with.

At the time I installed the FCRs I had to make my own intake manifolds out of Commando MKII manifolds. You weren't making manifolds for sale at the time, and I would have to modify what you are selling now if I were to use them. I could have bought Matt's pretty manifolds, but I knew I'd have to hack them up and that would be a shame. My motor is straight up, but I don't use straight manifolds. I'm probably going to make another set, because I know what works in my frame now, and I have a stronger assembly design idea. I think I've gone far enough off topic.

I'm not recommending FCR carburetors over CRS carburetors for Norton owners. I merely use FCRs successfully on a 750 Norton engine and this typing is therapy. :)
 
Fast Eddie - I was wondering why the gusset was removed in that photo. Either way - The Keihins are a tight fit, but they do fit.
 
Does anyone have a Mikuni TM-R or photos of a TM-R we can add to this? I know the VM mikuni when modified with a UFO flows almost as well as a standard Mikuni TM, and I have spoken with guys who I trust who claim that the vm would out flow the TM with out side gaps of the slide.

At WOT, I feel the CRs have an extra pull, but nothing scientific to back up my observation.
 
Hi Jim, Refering to the tight fit FCR picture, I have FCR 39's for other bikes in both a horizantal bore to the slide and downdraft about 20 or 30 degrees to the slide. The pictures not a good angle to tell but those look downdraft and may get more clearance with the others
 
maaseyracer said:
Does anyone have a Mikuni TM-R or photos of a TM-R we can add to this? I know the VM mikuni when modified with a UFO flows almost as well as a standard Mikuni TM, and I have spoken with guys who I trust who claim that the vm would out flow the TM with out side gaps of the slide.

At WOT, I feel the CRs have an extra pull, but nothing scientific to back up my observation.
Click to expand...

A single 42mm TMR (HSR) might be nice if the cable bracket would clear the downtube. I would imagine if they worked well on a Norton twin somebody would have tried it. I thought baz had one or tried one. But my memory is unique and makes stuff up.

The thing I like about the FCR is the tuning features. Amazing carburetor for a tuner. Youd like 'em. Maybe
 
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