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XR650L 40mm FCR-MX Conversion

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Here's the details of my carb swap.

 

Here are a few links that may help you out (I found this info when doing the research for my conversion).

http://www.thumpertalk.com/forum/archive/index.php/t-905879.html

http://www.thumpertalk.com/forum/archive/index.php/t-714088.html

http://www.thumpertalk.com/forum/archive/index.php/t-902618.html

http://www.thumpertalk.com/forum/archive/index.php/t-902618.html

In one of the links they discus the difference between the 40mm and 41mm and it was said that the difference is so little that you would not notice.

 

The 41mm "slant body" FCR for the XR650L sells for $$$$$$$$.http://www.power-barn.com/1993-2007-honda-xr650l-keihin-fcr41-carburetor-kit/

 

First a little back ground info on my bike, it has a stock engine, smog pump block off kit, UNI air filter, header welds ground and a White Bros E2 muffler.

 

I bought a complete carb with cables and throttle from a 06 CRF250R.

The CRF250R carb is easy-er and cheaper to find than the CRF450 carb, the 2006, 2007 and 2008 CRF250R came stock with a 40mm carb. This is measured on the inside of the carb body at the slide.

(I'm assuming the newer CRF250Rs also use a 40mm carb but I have no info to back that up)

CRF250R casting #s

37mm FCR01A.A - '04

37mm FCR01B.A - '05

40mm FCR01C.A - '06

40mm FCR01D.A - '07

40mm FCR01E.A - '08

Just be careful that the carb you get is indeed a 40mm.

 

I dis-assembled it, cleaned then re-assembled using a 2004 CRF450R needle Honda part #16203-MEN-671 (NCYR needle, clip 4th from top) 45 slow jet, 170 main jet, (this is the jetting I ended up with after a little test & tune for 600-800 ft above see level)

 

FYI: do not separate the 2 middle sections of the carb body (attached using 4 allen head screws) I heard the gasket for that is NOT available.

 

 

I used the Noss adapter http://nossmachine.com/fcr-carb-adapter-long/ You will have to file the lip off the carb intake (only remove the lip the adapter should fit tight) Do the filing before the carb is cleaned, seal the adapter with a LITTLE automotive grade silicone. DO NOT get in a hurry, let the silicone dry over night.

 

I also removed the air box snorkel and drilled 12 or so 1 inch holes in the top of the air box (I read in my research that these carbs "need lots of fresh air".

The CRF throttle housing will need the "point" cut off.

I removed the hot start cable, but left the plunger and spring in the carb.

I cut the wires off the throttle position sensor & left it bolted to carb to plug the hole.

I also installed a adjustable fuel screw like this one, or you WILL burn the HELL out of your hand while trying to adjust the fuel screw.

http://www.rockymountainatvmc.com/p/44/54/213/1151/-/13581/Tusk-Fuel-Mixture-Screw

 

The carb mounts right up with no issues (it is a tight fit) the bike fired right up and ran great.

By loosening the boot clamps you can spin the carb to change the jets, but I had to remove the carb to get the needle out to move the clip (some one makes a tool for removing the needle with out removing the carb I just don't have one)

Edit:

While I was jetting my new 35mm FCR on my 02 XR250R, I found a trick for taking the needle jet out of the carb without taking the carb off the bike.

Remove the cover, & the screw that holds the needle in, then twist the throttle to bring the slide to the top, with a pair of tweezers pull the needle out. Much easier than taking the carb off. :D

 

I can not give you a comparison to the stock CV carb, as my bike had a bad motor when I bought it. I just knew from a past experience with another bike that had a CV carb that I did not want to run one.

 

Hot start plunger and spring

3a8efced2221354e9615b44f6dec3b69.jpg

 

Hot start plunger and spring installed w/plug (use silicone to seal & keep plug from coming off)

cca8060bcb13469dcf53e141ba85d6ff.jpg

 

Choke pull (note the TPS w/wires cut off)

69f34b05101ff35ad587228e4b317a34.jpg

 

Choke pull w hose installed

807ac71dc67736b49856d87814ee96b3.jpg

 

The aluminum ball is attached to the end of the hose (ball is not a must have but makes choke easy-er to use)

6009dfc2de8bef2fa90d870fd1e95d20.jpg

 

Noss adapter

 

b8a1b060b7d7dc610f7479f8cf3b9dac.jpg86527c5e75c9520ea1c0896a2dc4b74d.jpg

 

Note the intake filed for the adapter

ba90691e4068d649d92e18ec3dc68af6.jpg

 

On final installation line up the 2 holes in the adapter with the ones on the carb (sorry no pic of this) but you can see the holes in the carb intake, and adapter in the last 2 sets of pics ^.

7a38f6dc70bd9848041d359842c75a81.jpg

 

Pic of the carb installed on bike

3d4295fc38ab6da7e8f04d6099badf98.jpg

 

Carb w/ aftermarket fuel screw installed

 

46fedf09aa42f3f2f0e897bb9aff5732.jpg

 

Stock CRF throttle housing w/point

047da593af0f3cb4935c48928b9af966.jpg

 

Throttle housing installed w/point removed

b0fee58fd24f712d587517ecdc17d4cc.jpg

 

Drilled air box

2d04e2c974726c798c2143cd4953167d.jpgdef4be96c078b473536f0f0349612fea.jpg

 

Top of drilled air box above filter

7ede37817c14cc348a9f300072ee0901.jpg

 

Top & right side of drilled air box

12e514e0718ae678fe868dc6e9ddfd83.jpg

 

 

 

Ask questions if I missed anything.

 

Thanks to all the guys at Thumpertalk, your posts were helpful and gave me the inspiration and info for my swap.

 

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Good write up. 
I have a 39 and a 41. And theyre jetted the same.


Thanks, this was originally posted by me in January 2012 over on the SuperMoto Junkie forum.
I was doing some searching here yesterday and seen a couple of carb threads that had limited info & no pics so I thought I would put my conversion out there.

Notorious, if you don't mind can you tell us what year & model your bike is and mods you have (engine, exhaust)? Also comparison between the 39mm and the 41mm carbs would be great!!
Thanks
Jonny

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My fcrmx41 is on a 95 xr600r.Stock bore, stock cam.  45/160. Xl headers, powercore4 muff. Uni pod filter, NO airbox. This was my only difference for jetting. Why the 160. Thise bike is all street. 15/45.

The fcrmx39 is on an 88 xr600r. Bored to 100mm and a higher comp piston. And a Webcam. Stock airbox with a couple inches of Schedule 40 pipe as my coupler. I put 45/160 as my starting point. Felt rich after 1/4 throttle. Put a 155 in and its perfect. Xl headers and powercore4 muff. This bike is all dirt. 14/48. 

So I feel the difference is the gearing, A, and the motor mods B.

Carbs are exactly the same to me...Id have to try the 39 and the 41 on the same bike to give a more exact comparison. So I really cant say theyre the same. But I do think its a small factor in getting it right for an xr

 

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Jonny. I've seen this post over on Supermoto Junkie. Always wondered where you got the sweet red port block off caps & plugs to store your spare carb dust-free?

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Jonny. I've seen this post over on Supermoto Junkie. Always wondered where you got the sweet red port block off caps & plugs to store your spare carb dust-free?


From work.
Do a google search for "cap plugs" and you will find several places that sell them. They do come in very handy for a wide range of uses.
In the pics I was using them while media blasting the carb.
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One addition... Take the hotstart cable, attach the top end to the stock XRL handlebar choke lever... swap the choke plunger from your stock CV carb onto the other end of the cable and then screw it into the choke port. You now have a bar mounted choke for your FCR. Iv'e been running mine this way for several years. Works just like it was OE.

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One addition... Take the hotstart cable, attach the top end to the stock XRL handlebar choke lever... swap the choke plunger from your stock CV carb onto the other end of the cable and then screw it into the choke port. You now have a bar mounted choke for your FCR. Iv'e been running mine this way for several years. Works just like it was OE.


Thanks Thump.
Someday if I'm feeling frisky I'll give that a try. I only run the choke for 20-30 seconds, so using the knob isn't that difficult.

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Thanks for the detailed write up! I'll be gathering parts to do this conversion on mine in the very near future.

Can you tell me if you used the short or the long adapter?

Edited by OttawaBRP
added question

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I wonder if the holes in the air box are needed.

There is a lot of water and mud where I ride.

I'll try a run up block with the side cover off.

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This is my concern as well. I really don't want to compromise my water and mud capabilities just to get more ponies.

I ordered my CRF throttle assembly and cables last night so now I just need to figure out the carb adapter question and I'll be one step closer to running again.

 

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I doubt it's required to port the airbox with more holes.  I haven't done so, due to water concerns; the stock air box is a good design for channeling water away from the air filter.

You can easily experiment by removing the left side cover during a test ride; that's a great increase in air flow.  If the power difference is worthwhile, then port the airbox top toward the rear of the box.  That preserves the internal splash baffle's function.

Edited by heart_of_darkness

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I kept the hot start function, by connecting its cable to clutch perch with built in small secondary hot start lever.  I've found it handy for restarting a flooded engine after a tipover.

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Sorry for the delay in my reply’s.

 

 

Thanks for the detailed write up! I'll be gathering parts to do this conversion on mine in the very near future.

Can you tell me if you used the short or the long adapter?

 

Your welcome, I’m using the long adapter.

 

I wonder if the holes in the air box are needed.

There is a lot of water and mud where I ride.

I'll try a run up block with the side cover off.

 

When I was doing my research a lot of people were saying there was a need for more air flow, my bike is in SuperMoto trim so the only water it sees is the occasional rain shower and I’ve had no issues with water in the air box.

 

Doing a couple of test runs with the side cover on & off should let you know if the holes are needed. Please report back with the results of your testing.

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:facepalm:  You can run modified airbox or not , as long as it's jetted accordingly. More air needs more fuel , more of BOTH means more power. No extra holes , less fuel is needed. If you're jetted for a stock airbox , of course it won't run better just taking the sidecover off , you're suddenly giving it more air without more fuel to go with it.

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I guess that I’m going on the assumption that one is doing the conversion on the quest for more power, in stead of doing the conversion because he has to much money in his pocket & to much time on his hands.

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Certainly , but without proper jetting , the bike would likely run worse with the sidecover off.

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Certainly , but without proper jetting , the bike would likely run worse with the sidecover off.

 

Sounds like you need to do a conversion write up seeing that you know more than everyone else!

 

In case your wondering, guys like Joe is the reason I haven’t been around lately.

I don’t mind trying to help people out but I don’t have time for the BS!

You guys are on your your own from here on.

 

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