640 FCR 37 install

[gunnerbuck]

Originally Posted by B-Rod

An FCR on a 640 would be fun fun fun. Maybe fire off a message or call Motolab. He might have some jetting suggestions, or even JDjetting. To me a 162 main at sea level sounds lean for the air a 625cc pumps through

162 is the closest of the 2 jets I have on hand, remember the SXC with a 41 FCR only runs a 165 in stock form... My airbox has been left stock so the current jetting shouldn't be too far off...

[B-Rod]

If you do need jets in your fine tuning, you'll probably find that any shop that sells dirt bikes will have a box o' jets somewhere handy. I've found everything I needed locally. I don't really know the difference between the long and short mains for the FCR, but I've successfully used either in two different bikes. Seems to work just find with either. Maybe I'll look it up one day.

[gunnerbuck]

Well, I found why the bike was stuck at half throttle... The fault goes with the motion pro cables... It turns out the cable is too short, more specific there is not enough cable end free of the sheath... So when it is put in place the throttle cam has a little tension on it and lifts the slide.. The sheath is set as short as possible and the cable end is still 3/8" too short so I will try to trim a bit off of the sheath to buy some more length on the cable exposure...

I have heard some complaints about motion pro cables, this is my first trial on one and so far I say they suck....

[gunnerbuck]

I shortened up the cable and gave it another test run... Was lean down low with a bit of a hang before the revs dropped to idle...So I swapped out the 42 pilot for a 48 and raised the needle one position... I tried it again and it idled nicely and throttled up cleanly...

The issue now is the bike will stall when I rapidly snap the throttle open.... I think a bit of play time with the A.P. is in order to fix the stalling, it may not be getting a big enough squirt... I ended up throwing the BST back on for now and will go back to playing with the FCR at a later date....

Running the BST again showed me how much more snap in my brief bit of testing the FCR 37 gave my engine...

[Droptarotter]

Gunner;
FCR's usually come with a 3 second squirt on the AC pump. That's usually way too much fuel.
I tuned mine on my 520 EXC to 1 second........that made a huge difference.

Cheers

[gunnerbuck]

Originally Posted by Droptarotter

Gunner;
FCR's usually come with a 3 second squirt on the AC pump. That's usually way too much fuel.
I tuned mine on my 520 EXC to 1 second........that made a huge difference.

Cheers

I believe some of the KTM FCRs dont have the power Jet {leak jet} which lessens the pressure to the A.P... The carb I have is equipped with this feature so it already doesn't have a full pressure squirt and I still have a larger leak jet to try.... The thing I have to figure out is if it is dying on the throttle snap because it is getting too much fuel or not enough...Between the plunger timing screw and the leak jet it should be solve able... But that is for another day as I have spent more time and money than expected on this project already, back burner it goes for a bit...

[Droptarotter]

Originally Posted by gunnerbuck

I believe some of the KTM FCRs dont have the power Jet {leak jet} which lessens the pressure to the A.P... The carb I have is equipped with this feature so it already doesn't have a full pressure squirt and I still have a larger leak jet to try.... The thing I have to figure out is if it is dying on the throttle snap because it is getting too much fuel or not enough...Between the plunger timing screw and the leak jet it should be solve able... But that is for another day as I have spent more time and money than expected on this project already, back burner it goes for a bit...

Gunner;
I have been watching your exploits on this carb swap intently as I was thinking of doing the same.
So.........as per your quote...........in hindsight, would it just have been easier to buy a new carb already set up for the 640??
Most of us like to tinker, but with some projects there comes a time when enough is enough.
Any comments?

Cheers

[gunnerbuck]

Originally Posted by Droptarotter

Gunner;
I have been watching your exploits on this carb swap intently as I was thinking of doing the same.
So.........as per your quote...........in hindsight, would it just have been easier to buy a new carb already set up for the 640??
Most of us like to tinker, but with some projects there comes a time when enough is enough.
Any comments?

Cheers

If you count the labor involved as an expense then yes buying a new carb outright is probably cheaper the first time... But if somebody does the legwork and puts out the info on parts needed, jetting and fitment issues then the carb can be setup for a one off install... Remember, even if you buy a new carb you will need to have the jets set up, get the proper airbox boot and buy a set of throttle cables unless your willing to bend up the bst units... Also a used FCR 37 can be had for $100 if you wait for the right deal as to where a new one puts you back $400-$500 right from the get go... Add an airboot and cables and there goes another $60-$100...

My biggest beef with installing the FCR is the size of the thing... It is really tight to fit it in there and you have to move some electrical and shift the hose routing to make it work... Also the throttle cables run right into the forward right frame backbone so you either have to route them above it or below it which kinda kinks the cables a bit... When I get to the final install I will likely run the cables under the backbone and think about running with the cover plate off so the carb will stand straight up....