Well I did it...... no, not the install, but the purchase. I got a 39mm FCR-MX off a Yamaha 450. It has the air-cut valve to enrichen the pilot circuit automatically on decel. Bye bye popping! I've done a lot of studying and feel the 39mm FCR-MX is the perfect carb for the DR650. The FCR's ingenious bearinged slide and floating slide seal is superior to a standard slide carb. Low end response and mixture accuracy is greatly improved with this set-up. That's what I'm most looking forward to is the low end response and torque improvement with this carb. I'm sure there will be gains everywhere but I wanted to focus on the area that the DR makes it's best power. I'm not a big beleiver in the "four-barrel on a moped syndrom" I sent many emails talking with Sudco (imagine that) about clearances and measurements as the MX version is a bigger carb than the standard FCR. I'm certain I can get it to fit. A new 39mm FCR-MX from Sudco with the air-cut valve is $530. I got the carb for $200 off eBay. I figure the most I'll have to spend on jetting is around $75 worse case senario. I'm figuring it will be less. I will have to make an aluminum air boot adapter for the back of the FCR to reach the DR's boot. No biggie... I've made many a machined adapter to graft carbs before. My bike isn't here from San Diego yet and the carb I just bought a few minutes ago so it will take a bit of time before I start playing around with the install. I'll post a full pictured install and the jetting process. Stay tuned!

<------ envious!

mx rob
I might be mistaken but I thought I read a post of yours in the last few weeks that you said you rode a DR that had a Mikuni TM 39 I think that was it but not sure. But anyway you said it was one of the best carburetors DR's you ever rode. If I am recalling it right why the FCR instead of the TM.

Bacon225 said:

Nope, not me. My bike has the stock BST40 and I feel it is one of the best carburated CV carbs I've got to work properly.... but I know the Keihin FCR pumper will be superior. With the CV carb there are so many concessions that have to be made in jetting to get decent throttle reponse (read richen everything up) I'm very confident I can improve on fuel economy while substantially boosting response and torque..., even top end, with this FCR.

the_aussiedr650 said:

Yeah, I know they think the 41 is the way to go. I just don't buy it... not for the DR650's state of tune anyway. It's just not a high tech high RPM engine. A 41mm is over carb'd for this bike's needs IMO. The stock BST40 with a butterfly valve smack in the middle of air flow after the slide would be dreaming of flowing as much air as the 39mm FCR so this is already going to increase top end. The 39mm carb will provide superior low to mid range velocity over the 41mm carb. This is where the DR is designed to operate.

didnt a dr 350 come with a 37mm pumper carb?

Awesome! I love hot rodding mods! I'd actually like to increase the top end performance on my bike as well, eventually. Cam, big header, free flowing exhaust(if I can make something that is quiet as well). Wonder if it would be better to go big on the carb? Either way, if this works out to be a better performer all around, I'll probably opt to go this way and not go too crazy on the rest of the mods. I like how this thing will probably live as long as me!

brianwheelies said:

Me too.  I can't wait to read the results.  

Well my bike showed up Friday and I received the carb today! You don't know how fast that DR came apart once I hit the garage with my Makita impact this afternoon! I'm getting more and more excited about this conversion. I was kinda holding my breath wondering if it was gonna fit even though the dimensions I got from Sudco said it would. They haven't even tried the MX version on the DR650 yet and were not sure if it would fit. I can tell you that it does!

Here's the list of cool things I know so far.
1. It fits into the working space! There's not a lot of room behind the carb at the top near the down curve of the frame backbone but there is enough clearance. The saving grace there is the shorter length of the FCR without a butterfly valve brought the carb forward enough that the frame is angling up away from the carb. Also the accelerator pump housing at the bottom of the carb is directly over the rubber cap for the starter wire.... Still enough clearance to not be an issue but those fancy Boyseen (and other mfg's) high capacity anodized accel pump housings are not an option. I'm confident we won't need that FARKLE anyway.

2. The stock intake spigot fits! Sudco thought it would work and it does. The FCR is actually almost 2mm bigger in OD on the carb spigot stub but it's not a problem for the mount. The stock clamp can open up plenty far to do the job.

3. I figured out a way to use the stock throttle cables! The OEM carb FCR-MX that I have does not have a set-up for the push cable in the carb housing but it does have the cam. The casting is the same as the FCR's with both the push and pull cables but the hole for the push cable is just not drilled. This couldn't be any better as I can now drill and tap the hole to thread the stock push cable into the carb. It will require a slot for the inner cable to slid through.... no big deal. The pull cable works perfect it will just require the adjuster at the handle bar to be extended out quite a few turns. There's still adjustment left though. Also the stock metal curves on the cables near the cam couldn't have work out any better if I plannned it. The FCR cam is on the right side of the carb instead of the left. The stock cable curves, now turned to the left, run the cable across the head and back to other side of the frame where the cables run normally so no rearanging of the cables is required. I can tell you this was a big relief as chasing cable and throttle attachment issues is a pain and can add $$ to the project. This way when you need new throttle cables you just order the stockers.

4. The stock choke cable and plunger mech fits the FCR perfectly! So if you have one of the big gas tanks that hang way down you can keep the stock choke so you don't have to fumble for the choke. Of couse when I send the carbs out they will come with the little stock snap action plunger choke so you'll have the option of running either set-up without having to do anything but unscrew one and screw in the other.

Work to be done:
A. OK, as I mentioned the stock BST is a good 10mm longer than the OEM FCR-MX. I will have to make an air horn adapter to have the FCR fit in the exact same space as the BST and connect to the stock air box boot. I've already drawn a print of what I need and I will be talking to a machinist in the morning!

B. And this is probably the biggest pain for everyone. I have to put a fitting in the stock rubber intake manifold for the vacuum petcock. It's not a big deal for me but I will need the manifold from each person that wants to be able to use the vacuum petcock so I can modify it. If you don't use the stock petcock then nothing will need to be done to the manifold. I'll be reviewing whether there is a way I can put this fitting on the carb to eliminate that issue. Stay tuned for further developments.

Is it finished yet?

The Tuesday update: I was able to have the adapter made today!. I just completely disassembled the carb to inspect conponents and install the adapter. I did the push cable modification so all stock cables will work. So far this is going very smoothly... but a jetting fest is just around the corner. I plan on doing a jetting session with the stock muffler as well as the FMF-Q2. My Air/Fuel monitor will be a valuable instrument once again.

As far as getting my bike dyno'd goes.... I'll look into it. To be honest though it's not even possible for this FCR-MX not to out perform the BST in every way. The technology between the two carbs is worlds apart. Besides, my butt dyno has been recently calibrated and meets ISO9002 standards.

mx_rob said:

Rob if all goes well and the price is right I too am interested anything vacuum sucks I've had enough vacuum issues with 80's chrysler and Jeep vehicles and know that once the vacuum is gone there is lots more HP to be had. great posts!

The Thursday update: Sudco has sent out my first set of jets for the FCR. I based the range on already proven set-ups used by one of the forum members and Sudco's own DR650SE set up. The two differ substaintially.   I'll be testing both of them and a lot more before all is said and done. My needle and jet pile is ever mounting.

Was this carb of a 450 Yamaha originally? How do you think you will have to change the jetting for the 650? There could be arguments for leaner, richer and left the same.

What OE applications use a 39 FCR? and are they all the same? I was wondering about a 37 here at 5000'. Sounds like everything is going better than expected.

Rob, I put a 41mm fcr on my DR650 and had really good luck with a James Dean kit,  I really like his needles and I got the bike really dialed in.  I realize the 2 carbs are different in some regards but if you are interested in my jetting specs let me know.  Ed

bignick said:

I've ordered a good cross range of needles and jets that I think will cover what the bike needs. I have edjo69's specs for the 41mm standard FCR that he put on his bike but the 41's velocity will be lower throughout the range so his specs may not help me. The strange thing is how differently Sudco went about it's jetting for the same bike. I'll be testing soon as my jets and needles just showed up a few minutes ago! The last sticking point is the vacuum petcock fitting. After reveiwing the manifold there is nothing but a metal plate cast into the rubber so there is no place to properly thead the tube in. It look's like I'm going to use a cast boss the comes undrilled on the left side of the carb ahead of the slide for my vacuum point. I have to come up with a way to blind drill for the fitting without breaking though the inner carb wall. The 1/4" threaded hole on a radius like that would be way too big if it went all the way though and would cause some nasty turbulence just past the slide. I'll have to set up a fixture in a drill press to keep an acutrate stop for control. I'll then drill a small hole angled in the flow direction towards the bottom of the blind tube to prevent any fuel build up in the fitting. Should be fun.

edjo69 said:

Yep, you sent me the specs a couple weeks or more ago. We'll see how the 39 differs from the 41 on specs very shortly. This FCR-MX is one cool carb. I really like the huge float bowl capacity with the internal baffling. It's also nice that all of the mechanical moving parts are under cover unlike the standard FCR with it's exposed cam and accel pump shaft. The guys that like to get really muddy and dirty will like it too. Of course the air cut valve is the cat's meow for anyone with a modified exhaust system. You'll actually be able to set the pilot mixture screw where it should be instead of way too rich trying to  make up for decel popping.

mx_rob said:

Would a Pingel petcock get around all this hassle?  I'd be willing to think about upgrading the petcock as well if it would save grief.

mrbing73 said:

Yes it would but I know there are a number of people, myself including, that would want to retain the services of the stock vacuum petcock. I like a little  challenge to keep this old man's mind in tune. Certainly anyone wanting to try this carb option on their own could opt not to mess with the  vacuum fitting. I just want to come up with something totally turn key for the masses that could fit any needs/wants.

mx_rob said:

You need an IMS tank you do