Jump to content

XR650 FCR swap (Finally done right)

Recommended Posts

After a couple months of trial/error and loads of research, I finally got my FCR 40 dialed in and running right! I learned alot along the way and saw alot of people running into the same issues I was. I figured I'd write up some of the information I picked up to help out someone who might be having issues. 

I snagged a used FCR40 from a CRF450x off of ebay for a steal in June. The first thing I did was tear in to it to clean and inspect the components inside. I had to replace the accelerator pump diaphragm and some o-rings. My first mistake was spraying carb cleaner on the slide. There's an o-ring on the slide itself that will swell and degrade when it comes into contact with carb cleaner. Also, watch out for the way the slide/floating plate go back into the carb body. The correct orientation is for the open circle on the plate to face the bottom of the slide and the plate faces the engine side of the carburetor. Anything other than that and you will be stuck with idle surging and stalling.

The hot start port on OEM carbs is another issue that needs to be addressed. My carburetor didn't come with the hot start knob so it was just a giant hole in the side of the carb. Blocking it off with a bolt won't fix the issue. There are two holes that need to be plugged inside the carb, one is at the top of the engine side of the bore and the other is under the top cap of the carb near the same area. I used silicon to plug it and then put a bolt through in the opening in the outer case.

The biggest issue that took me a long time to figure out was the idle circuit. On the OEM FCR the intake bell is flat but the aftermarket has a removable bell that smooths out the airflow and lets you use the regular air intake. On the OEM models the air intake has a section that lines up with the bell and smooths out the airflow. Without an intake adapter your carb will never run right at idle, I ran a 58 pilot and soldered/redrilled the air jet down to a 60 and it still hung up on me. I got the adapter off ebay for ~$42 shipped and went back up to an 80 air jet and she purrs like a kitten! I spent just as much trying to jet my crappy homemade adapter correctly as I did on the adapter.

Hopefully this will help someone who's trying to setup their FCR. The carburetor is worth every penny, smooth instant throttle response and I feel like she picked up some power too!


  • Like 1

Share this post

Link to post
Share on other sites
12 hours ago, ricky racer said:

What main pilot and needle are you running now? 

Originally I had an OBELT needle I ordered in the carburetor, it had a real bad lean spot off idle. The carburetor came with some kind of dual taper needle in it when I bought it, I measured the diameter and it was a Q diameter needle according to the keihin needle specification chart. I threw it in there just to see if it cleaned up the off idle response and it ran great! Not only did it clean up the bottom end but the bike pulled harder all across the whole throttle range so I decided just to leave it in there.

Pilot jet is a 58 with fuel screw ~2.25 turns out and a 165 main jet. Needle is set 3rd clip from the bottom.

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with: