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Hey all, I’m trying to jet a 2008 FCR-MX 40mm off a CRF250R onto my 201) DR650. As I’m working on getting the needle tuned right, I’m noticing some questions...

 

1) Do the fuel screw and pilot jet always work hand-in-hand? Like, if a 40 pilot is a little too rich so I turn the fuel screw all the way clockwise, the idle might be perfect. But since the jet is richer than necessary, once I get on the needle at 1/8 throttle, is the fuel screw still helping compensate for that larger pilot jet?

2) Is it possible that needle changes can have a tiny effect on idle richness? I’m pretty sure my slide is fully closed. When I go from a Yamaha NFLR to Keihin EMP I see the idle becomes maybe slightly richer, although it could have been a fluke due to temperature variance between tests. 

3) I think I’m seeing the EMP is too rich off idle, and the FMQ (will try EMQ but theoretically it’s the same needle root diameter anyway) is too lean off idle. Has anyone else noticed the needle root diameter causes too great a change in richness? Using an AFR meter I’m seeing a difference of 4-5 AFR e.g. “P” diameter 11 and “Q” diameter 16. I can’t use slide cutaway as a tuning parameter since Keihin doesn’t sell those for the FCR. Can I lean out the off idle by using a smaller main jet? Does the main jet effect 1/8 throttle at all? Currently my main jet is too rich anyway.

 

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1) The fuel screw is a 'trim' for the pilot circuit at idle or just above idle. It is designed for the lower vacuum pressure at those rpms. You determine pilot size by fuel screw turns: with a hot motor, you put the fuel screw at 1 turn out and set the idle as low as it will go and still run. Then turn the fuel screw to raise the idle as high as it will go. That should be about 1.5 turns out. If it's less, you pilot is too big. If it's more, it's too small.  Once you raise the slide with the throttle, you are on the needle, and pilot jet deminishes. You can prove this by actually removing the pilot jet and bump starting the bike. It will run, but only at about 1/3 throttle and above.

2) Yes, the needle does affect idle richness by a few percent, but remember, it's about vauum levels. With the slide dropped, the majority of the vacuum only works in small venturis like the pilot jet.

3). There are tons of other needles to try. Each brand of bike had needles made by Keihin specifically for them, and they typically do not have the Keihin part numbers, but the brands part numbers.

KTM had a lot of 'strange' needles for their RFS motors with the massive low rpm vacuum, similar to the DR.

 

You have to remember that the DR has an extremely restrictive head and small valves, so the vacuum pressue is not as high as you think it would be.  A 40mm for that motor is probably to large....

You might try a JD Jetting 'blue' needle.

 

 

 

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Go to Procycle, and buy the kit to adapt that carb to your DR. All the details have been figured out by MX Rob.

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8 hours ago, KRAYNIAL said:

1) The fuel screw is a 'trim' for the pilot circuit at idle or just above idle. It is designed for the lower vacuum pressure at those rpms. You determine pilot size by fuel screw turns: with a hot motor, you put the fuel screw at 1 turn out and set the idle as low as it will go and still run. Then turn the fuel screw to raise the idle as high as it will go. That should be about 1.5 turns out. If it's less, you pilot is too big. If it's more, it's too small.  Once you raise the slide with the throttle, you are on the needle, and pilot jet deminishes. You can prove this by actually removing the pilot jet and bump starting the bike. It will run, but only at about 1/3 throttle and above.

2) Yes, the needle does affect idle richness by a few percent, but remember, it's about vauum levels. With the slide dropped, the majority of the vacuum only works in small venturis like the pilot jet.

3). There are tons of other needles to try. Each brand of bike had needles made by Keihin specifically for them, and they typically do not have the Keihin part numbers, but the brands part numbers.

KTM had a lot of 'strange' needles for their RFS motors with the massive low rpm vacuum, similar to the DR.

 

You have to remember that the DR has an extremely restrictive head and small valves, so the vacuum pressue is not as high as you think it would be.  A 40mm for that motor is probably to large....

You might try a JD Jetting 'blue' needle.

 

 

 

So I already have the FCR-MX 40mm. Others have got it working on the DR and report good results, even compared to the 39. Should I really be very concerned about this? I'm a little worried based on what you've written. In any case, even with imperfect jetting it seems to be running pretty well, although kind of low RPM on startup. Maybe I need a different starter jet. I have a 95 in there but it came with a 68. Others seem to run an 85 so maybe I should go for that. I was reading a lower starter jet will increase the startup RPM.

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The starter jet is the choke circuit. It has no effect on how the bike runs. 

All bikes run at low rpm on start up with the FCR, UNTIL IT GETS HOT.

You can turn up the idle for the warm up period. That is the simplest solution.

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The 40 works fine. But it needs significant changes to get it to work on a DR.

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1 hour ago, nobade said:

The 40 works fine. But it needs significant changes to get it to work on a DR.

Haha good to hear. You realize it’s already adapted and running on my bike right? I’m just trying to find the perfect needle. I have a couple options that are passable but imperfect so far. 

1 hour ago, KRAYNIAL said:

The starter jet is the choke circuit. It has no effect on how the bike runs. 

All bikes run at low rpm on start up with the FCR, UNTIL IT GETS HOT.

You can turn up the idle for the warm up period. That is the simplest solution.

Very good to know. Do you know what’s an acceptable cold start idle RPM? My old carb actually idled higher on cold start choke on. I assume the best way to find the ideal starter jet is to start the bike and turn the fuel screw? If clockwise increases idle speed, go to a smaller starter jet, and if counter clockwise increases idle speed, go to a larger starter jet?

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