Getting familiar with FCR-MX/adapting to XR250

[JohanChaves]

Hi All!

I just bought an CRF 150R carb (FCR-MX32mm)

The thing is that I am starting a project to install this carb on my oldie 1995 Honda XR250 and I know very little about FCRs

first questions I have are:

1-in the picture above, is the hot start the cable and is the choke the pull-type knob below?

2-If so, would I need them both on my XR250 once is adapted or would the choke be the only thing I need?

3-Can the choke and the hotstart cable/knob be interchanged?, say I want the choke in the handlebars and the hotstart on the carb body

Thanks for your time.

Johan

Edited April 8, 2013 by JohanChaves

[JohanChaves]

ah BTW, what do I do with the wire on the right of the picture (sensor) should I just cut it? what is this for anyways??

[FORMA MOTO]

Hi All!

I just bought an CRF 150R carb (FCR-MX32mm)

The thing is that I am starting a project to install this carb on my oldie 1995 Honda XR250 and I know very little about FCRs

first questions I have are:

1-in the picture above, is the hot start the cable and is the choke the pull-type knob below?

HOT START IS THE SWEEP 90 DEGREE CABLE WITH THE PLASTIC NUT. YOU WILL NEED A HOT START LEVER ON THE BARS

2-If so, would I need them both on my XR250 once is adapted or would the choke be the only thing I need?

YOU NEED BOTH, UNLESS YOU NEVER STALL OR NEVER FALL DOWN

3-Can the choke and the hotstart cable/knob be interchanged?, say I want the choke in the handlebars and the hotstart on the carb body

JUST KEEP IT STOCK. YOU WILL USE THE HOT START MORE THAN THE CHOKE

DO NOT CUT THE TPS (THOTTLE POSITION SENSOR) CABLE, JUST LEAVE IT ALONE.

Thanks for your time.

Johan

[JohanChaves]

Thank you krannie!

I all ready have the hot start lever too, thanks for the advise, I will work on this project tomorrow, I will keep you guys updated how I do.

[FORMA MOTO]

Thank you krannie!

I all ready have the hot start lever too, thanks for the advise, I will work on this project tomorrow, I will keep you guys updated how I do.

DON'T USE ANY CARB CLEANER on the carb! You will destroy the rubber stuff inside!

Use simple green, compressed air, spray brake cleaner, etc. ONLY!

[I6CJ7]

I'm really interested in how this turns out. I'm wanting to do the exact same thing with my 00 Xr250r.

Good luck!

[JohanChaves]

DON'T USE ANY CARB CLEANER on the carb! You will destroy the rubber stuff inside!

Use simple green, compressed air, spray brake cleaner, etc. ONLY!

Thank you for the advise man!! I was just about to do that

I just tried to fit the carb in and had a really good first impression (i thought there was too much to cut-weld-remove etc)

Basically I do have to modify the frame by cutting a small portion on the inside of the right frame tube.

Here is how it looks mounted, the FCR is 10mm shorter than the XR PD carb, it fits perfect in the intake manifold (maybe just misses 1mm of diameter)

The back of the carb also fits pretty good on the air box boot, how ever is 10mm shorter than XRs carb so my plan is to fit it to the air box boot and make an extension between the carb and intake manifold. Will let you know how it goes.

Here some pics:

Sizes:

[I6CJ7]

How much smaller is the FCR carb bore diameter than the XR carb? Looks at least a few mm smaller in the pics. Wonder if that will affect high end or wide open throttle performance? Or is the FCR just so many light years ahead of the old carb it won't matter?

And if you can, I would grind the carb body or see if you can clock it a bit before you grind the frame. Also just a thought, make sure the bike is completely vertically to see how the carb will sit when riding maybe?

[JohanChaves]

How much smaller is the FCR carb bore diameter than the XR carb? Looks at least a few mm smaller in the pics. Wonder if that will affect high end or wide open throttle performance? Or is the FCR just so many light years ahead of the old carb it won't matter?

And if you can, I would grind the carb body or see if you can clock it a bit before you grind the frame. Also just a thought, make sure the bike is completely vertically to see how the carb will sit when riding maybe?

Hi, I thought to clock it as you suggest but if I do not cut that area the carb wont align with the intake manifold, so seems that the only option is modify the frame, of course will talk to the welder about his opinion and maybe find the way to reinforce the frame in that area.

oh BTW I will post all measurements later today

[I6CJ7]

Looks like a cover goes over that area for the throttle cable, that probably adds a few millimeters to the carb width, and makes you have to grind out a little more, I didn't think about that. I can't imagine you'd have to grind or clearance the frame more than 1/8th of inch. I don't think you'll need to worry about strengthening the frame unless you have to take off more material than that. Maybe if you were jumping it like a Crf in motocross lol, then I might be more of a concern.?

[FORMA MOTO]

Man, how can you work on parts that are soooo dirty?

You can't tell what you are looking at, and dirt gets into everything......

[mdewet]

^{I am starting the same deal with a FCR from a KXF250, onto my '81 XR500R. will have to work the frame as well, but i need to change the "sub-frame" anyways, as i am doing a C&J type modification on the bike, to make it into a propper VMX machine! I want to see how the big power 2-smokers compare to this evolved Thumper, after 15 minutes on the track!}

[JohanChaves]

Man, how can you work on parts that are soooo dirty?

You can't tell what you are looking at, and dirt gets into everything......

cleaning is in my to do list!! ?

[JohanChaves]

ok after a few days of cutting, mounting, taking off and cutting yet a little more, I have finally mounted the carb on the XR250

I still have lots of tuning to do but first impression is WOW!!! some details...

DIMENSIONS:

PD FCR

Overall length: 92mm 82mm

intake inside diameter: 33mm 32mm (oval)

Intake outside diam: 40mm 39mm

Back inside diam 43mm 34mm

Back outside diam 48mm 47mm

Here the portion of the frame had to be cut for the carb to fit, at this point there is more to be cut, it has to be reinforced some how and of course painted

Now since the FCR is 10mm shorther than PD I had this adapter made so it fits the space between intake manifold and air box boot.

The thing is that the piece fits it pretty tight but when the bike is running it seems like there is an air leak between the white part and the carb, is there any thing you guys suggest for a gasket?

here some more pics of how it looks

Now to fine tuning questions.

The carb has the o-ring mod already, should I remove it or should I leave it? what do you think?

I am running 42 slow jet and 140 main, I had to set the pilot screw 0.5 turns out and seems ok, I will trough a 40 pilot/slow jet to see the difference.

Ah BTW engine has 75mm piston, head lightly ported, UNI, Leo vince can, stock header and I ride at sea level.

Thank you for looking and for taking the time to leave your wise suggestions and opinions.

[William1]

Leave the oring mod. for now. If you have no bog, you can replace it with wiring or better, a stiffer spring

You seem on track elsewise.

[I6CJ7]

Awesome man, I'm thinking, though it would've cooked the paint off... You could have used a acetylene torch to hest that frame up and gently push or hammer it in to have clearanced it. But that's hindsight..

I'm guessing as the engine heats up that plastic or Delrin material? Is expanding too much and causing the air leak, and tuning will be impossible until that's fixed. When the engine is heated up, trying spraying some carb cleaner at it, and see if the idle changes... There yah go. I saw another guy had a piece of aluminum or stainless, not sure... Machined that was ever so slightly smaller than the carb piece it was to go on. He then took a propane torch and gently heated it up so it expanded and it went on. Once it cooled... It wasn't going anywhere ever again!

So, a machine shop might be the ticket. But if not, I would recommend either red high temp silicone and put a bead around the base of that plastic adapter. Or, they also make a silicone repair tape that you can find at home Depot and it's made for sealing all kinds of things as well, just peel of the backing and wrap it on.

Since you have to have the frame welded, you're going to have to take the carb back out anyways, the machine shop is probably the way to go for durability in the long run.

Any chance you can post some pics of that Leo Vince can you have? What's it from?

[JohanChaves]

Hi I am back with an update and questions.

So previously I had a 42 pilot and the pilot screw set at 0.5 turns out, at this point it was hard to start the bike once it was fully warmed (15+ kicks) even with hot start So I went down to a 40 pilot 1-3/4 turns out and the bike starts easier both cold or fully warmed, still may need the hotstart when warm. So I guess I have the pilot circuit sorted? still, the exhaust smells a little like gas when idleling, would this mean the pilot still too rich?

Also, what is next?

Johan

[William1]

Ignore the 'gas smell'.

It doues sound like you have the pilot/fuel screw done. Now ride the bike, under load, 4th gear, various throttle settings and note how smooth it seems. Than later, you can do some WOT runs to figure out the main.

[JohanChaves]

also the previous owner of this carb a jetted it with a JD Jetting kit he even gave me with the carb, I havent seen the needle the carb has in it but I assume is the one marked with blue provided by JD and stated for low altitude/cool temperature, I have here in the kit one that looks like it hasnt been used masked with red and JD instructions state high altitude/high temperature, the other one looks like the stock and has this letters: NMQS

[I6CJ7]

You might smell gas on startup but if you're continually smelling it while idling, I'd be careful that you're not getting cylinder wash down, or more fuel than you can burn.

While idling, cut it off, and pull the plug to see if it's wet.