School me on FCR carbs

[250Renvy]

Are these things model specific? Every one I see for sale states multiple uses and the website shows only sizes of carb in relation to cc's.

Are these semi- universal? Meaning a 39mm FCR carb could work on a 400ex, z400, drz400 and other 350-400 cc machines and if someone had it on their YZ400 or YFZ450 it's still usable on another machine?

[GPracer2500]

I'll take a crack at it.


Originally posted by 250Renvy
Are these things model specific?

Yes and no. But mostly no.

There are downdraft and sidedraft versions of FCR. The difference is the angle of the throat thru the carb body (it's obviously different). The chances of accidentally ending up with a downdraft when you needed a sidedraft are basically nil. If you're talking single cylinder engines and FCR carburetors you're talking about sidedraft FCRs. Downdraft versions are far less common and mostly for multicyclinder road bikes. So don't worry about it unless you're buying a carb from an add that just says "FCR carb for sale".

Originally posted by 250Renvy
Are these semi- universal?

They all have the same fundamental slide and venturi design and they all have an accelerator pump. In that way they are all about same and semi-universal.

But there are many individual versions of Keihin's FCR carburetor. Aside from venturi size, there can be a multitude of detail differences between them. Some differences are a result of OEM's spec'ing a carb a certain way. Other differences come from Keihin evolving their design.

As long as it physically fits, just about any FCR can be made to do its job. As long as the carb has the features you need for you're application (size? TPS? choke? hotstart?) it should be a viable upgrade. As a general rule, the older it is the greater the chances it's a less desirable version.

If that answer satisfies you, great. If not, read on cause here's some history and additional info as far as I'm aware....









The predecessor to the Keihin FCR carb is the Keihin CR. FCR = Flat CR. The FCR's flat, rides-on-rollers slide might be it's most central feature. Introduced in 1990 (downdraft variant only), the FCR quickly became the carb to have in motorcycle roadracing. The sidedraft (horizontal) version came out in 1995. That's the style used on quad engines and other engine designs that draw intake air from the back instead of angled from above.

The FCR-MX variant was introduced around 2003. The MX version has added features such as a hot-start button, Air Cutoff Valve, float chamber baffle, and more. Within the last handful of years the the FCR's float bowl got a leak jet to help tune the Accelerator Pump. FCRs are available in 32mm (OEM), 33mm, 35mm, 37mm, 39mm, 40mm (OEM), and 41mm. Or larger if you modify one.

The 1998 YZ400 was the first mainstream bike to come stock with an FCR from the factory. There have been others (the Kawasaki ZX7-RR special edition comes to mind) but basically, the YZ400 was the first.

Since then FCR's have been used by all the manufactures as original equipment on a whole bunch of machines. So there are slews of OEM FCR versions. The differences between them could be minor spec differences like having replaceable vs. fixed orifice air jets. Almost no one is going to care about that. Or it could be something one might care about--like having a hot start or not.

There have also been many evolutionary changes as Keihin improved the design. For example, the AP has seen several design improvements. Some FCR carb bodies have replaceable, bolt-on intake "flanges" or "manifolds" or whatever you want to call them. On others the intake flange is cast into the carb body.

So you have an idea what I'm talking about, here's the intake flange supplied by Sudco for mounting an 41mm FCR to a Suzuki DR650.

http://img397.imageshack.us/img397/6015/p10100185vj.th.jpg (http://img397.imageshack.us/my.php?image=p10100185vj.jpg)

And of course, Keihin sells FCRs through the aftermarket. Many (most? all?) aftermarket FCRs have the bolt-on intake flange (helps them adapt to different setups). An aftermarket FCR could be an FCR-MX or not. For example, the FCR kit CurtisSparks sells for the 400EX is a "normal" FCR while their kit for the 450R is the newer, "better" FCR-MX variant. The earlier, non-MX FCR's still work (I have a pair of downdrafts on a NT650). At their heart they are still FCR's. And not all bikes can utilize a feature like a throttle position sensor.

Generally, the newer the FCR the better. Here's one from a YZ400. Yamaha had to add a separate hot-start mechanism because the FCR didn't offer one yet. Not visible (it's on the other side) is the AP rod located outside of the carb body. It was later moved inside since the rod was prone to corrosion. This version FCR also has the bolt on intake flange. This is probably not a great choice in used FCR since it's such an early version. It if was setup right and nothing was worn out it would still be one heck of an upgrade carb. But other FCR's might be a better choice if you're spending your hard earned money.

http://i34.photobucket.com/albums/d133/gpracer2500/th_P1070064Large.jpg (http://i34.photobucket.com/albums/d133/gpracer2500/P1070064Large.jpg)


I'd think any relatively recent OEM FCR that could be made to fit would be a fine upgrade in most cases. Or you can buy an FCR for most popular applications from Sudco and others. They take a "generic" FCR and add appropriate jetting, manifolds, throttle cables, etc. to match different applications.

[deathman53]

I know my ktm 450 uses a 39 fcr, crf uses a 41mm(I believe) and the trx450's use 40mm(I believe). Another difference is the diaphram for the accelerator pump. Many ktm guys buy a diagpram for the honda fcr, they say it makes better power and some say better fuel mileage. Another difference is the versions that are on quads have plugs(return cable) in them and a different linkege for the thumb throttle. Lots of little differences between them. My ktm doesn't have a hot start on it, crf does.

[woodsracer144]

so then when did they change the yzf 400 carb? i have one on my yanda and my buddy has one too... hes a 426 and i have a 400. he has a hot start i dont...

[scottyyfz450]

i say take you current carb in and get the guy at the counter 2 give ya the right one

[woodsracer144]

theres a spikgot with a jet in it... what the heck its on the out side too...

I know on the YFZ 450 that I just sold it had the FCR and man was that thumb throttle like butter. A flat slide would have less weight I guess making it that much better. When you hit the gas it was like EFI it felt so smooth almost hard to believe you were moving anything.