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FCR-MX 41 jetting

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434 big bore, NSHC stage 2, single layer base gasket, MRD Z-Pro shorty, 3x3 mod done. FCR-MX 41 new from ThumperTalk. JD red needle in the 4th or 5th clip, I don’t remember, 168 main jet, 200 MAJ 100 PAJ, O-ring mod. Sea level, very hot and humid climate.

I’m working on the pilot jetting currently. I started with a 45 pilot, fuel screw could turn all the way in and it would still run. Moved it down to a 42, just about the same. Can still be turned all the way in without stalling. Idles worse after 1 turn out, stalls at 1.5 turns out. Do I go down again to a 40 or do I need to look at something else before I do that?

Edited by Center_Sumo

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I run a 45 in my fcr41 with 440 kit, stock cams, and FMF exhaust. 42 was a bit lean for me. I was having a hard time adjust pilot circuit until I installed an EMR needle on 4th clip, that smoothed things out a bit. Verify all vaccum ports are sealing on the motor and then make sure there are no leaks on carb. Spray some starting fluid around carb boots while engine is running. If RPM changes the you have a leak.

Edited by Methotic

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Thanks guys.

The JD kit I ordered was listed as being for the 41, came with instructions for a ktm 625 if I remember right. I do think I have an EMR needle if that would be more appropriate. 3rd or 4th clip?

Edited by Center_Sumo

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No change on the fuel screw, guess I’ll go down to a 40.

Put the EMR in and it’s acting like it’s starved for fuel after it’s been running and giving it gas, wants to bog and die

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No change on the fuel screw, guess I’ll go down to a 40.
Put the EMR in and it’s acting like it’s starved for fuel after it’s been running and giving it gas, wants to bog and die
How high is the idle? It has to be set extremely low otherwise it will run. It's pulling fuel past the needle at a fast idle.

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When I’m adjusting the fuel screw, I have the idle turned down pretty low. It’ll die at 1.5 turns out with the low idle, but keep running with the screw all the way in. I’ve been keeping the screw at about .5 a turn and the idle turned up when running around testing jetting and otherwise.

I didn’t mess with the float from the factory. Is there any easy way to measure it with it still on the bike?

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When I’m adjusting the fuel screw, I have the idle turned down pretty low. It’ll die at 1.5 turns out with the low idle, but keep running with the screw all the way in. I’ve been keeping the screw at about .5 a turn and the idle turned up when running around testing jetting and otherwise. I didn’t mess with the float from the factory. Is there any easy way to measure it with it still on the bike?  

 

 The baseline 41mm jetting from the Eddie days is 175, EMR 4th clip, 48 pilot. 

I started with a 48. The bike wouldn't respond to the fuel screw but there were no running issues what-so-ever. I settled on 45 and dropped to 42 when it became a stroker. I don't believe your bogging issues are related to the pilot. Nor do I think you really need a 40.

 

The float level changes the overall jetting. You need to make sure it is set to 9mm. Regardless of how it was set from the factory the carburetor has traveled half way around the world before reaching your front door. Unfortunately it is not possible to set on the bike. It must be done off the bike while held at the correct angle. The tab on the float must touch the spring loaded pin in the needle but not compress it. Getting the setting correct is a bit of trial and error. Also take the time to verify accelerator pump timing.

 

Also worth noting, be extremely vigilant that the carburetor is actually in the airboot around the 5 o'clock position. It's difficult to see , you will need a mirror, but it is easy for someone without much experienced with wrestling the FCR to miss. This carburetor won't "pop" on like most oem setups.

Keep in mind the bike isn't finicky. If it continues to give you problems than in all likelihood there is some sort of mechanical issue or installation error that you are overlooking.

 

 

 

 

 

 

 

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The bogging actually didn’t start until I went to the EMR. It was actually running fairly decent I thought with the 42 and the red needle. The only reason I was still messing with it was because the fuel screw was so far in. I hadn’t gotten far enough to run a plug chop yet, so the main may be off, but I hadn’t started playing with that.

I’m fairly sure it’s all the way in place. I had a ton of trouble getting it there initially, but your tips helped, so it should be fully seated now. I need to spray some starter fluid around just to make sure, but I’m fairly confident it’s there.

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The bogging actually didn’t start until I went to the EMR. It was actually running fairly decent I thought with the 42 and the red needle. The only reason I was still messing with it was because the fuel screw was so far in. I hadn’t gotten far enough to run a plug chop yet, so the main may be off, but I hadn’t started playing with that.
I’m fairly sure it’s all the way in place. I had a ton of trouble getting it there initially, but your tips helped, so it should be fully seated now. I need to spray some starter fluid around just to make sure, but I’m fairly confident it’s there.


I wouldn't put much faith in a plug couple. Works fine on 2 strokes but so much in a 4 stroke with 2019 fuel.

You set the EMR at the 4th clip counted from the top? Bogging when opening the throttle is caused by the accelerator pump. If it continued to run erratically at steady throttle than that could indicate jetting. Make a note of what percentage of throttle.

After float height the main jet is the next jet that needs to be set. It does have some affect on the needle.

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Float was close, maybe 8.25-8.5mm? Hard to tell when you’re just trying to site a pair of calipers across the top. Set it as close to 9mm as I figured I could get it.

I’m now at a 40 pilot, 170 main, 4th clip red needle, O-ring mod, and probably 1/2-3/4 of a turn out on the fuel screw, so no real change there. But it runs fine, no bogging anywhere, seems to pull good. There might be a very slight hesitation when easing the throttle open when just cruising, maybe going from 1/8-1/4 throttle to 3/8. But romping on it or WOT feels just fine.

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Float was close, maybe 8.25-8.5mm? Hard to tell when you’re just trying to site a pair of calipers across the top. Set it as close to 9mm as I figured I could get it.
I’m now at a 40 pilot, 170 main, 4th clip red needle, O-ring mod, and probably 1/2-3/4 of a turn out on the fuel screw, so no real change there. But it runs fine, no bogging anywhere, seems to pull good. There might be a very slight hesitation when easing the throttle open when just cruising, maybe going from 1/8-1/4 throttle to 3/8. But romping on it or WOT feels just fine.
There is a tool for every job.

I wouldn't settle for "slight hesitation". I still think you are having to compensate for a needle that is too rich on the diameter with a smaller pilot. There is no advantage to running a smaller pilot. Screenshot_20190831-121625.jpeg

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I didn’t know such a thing existed, I may have to pick one up when I come to terms with pulling the carb off again lol.

It really was very minor. It just felt like when slightly increasing throttle at lower rpm cruising speed there was a half second before it got into the power, almost like the feeling on 600cc sport bikes right before you hit the powerband, just much less pronounced than that. I’m just not exactly sure where to start with trying to get rid of it. The bike didn’t like the EMR needle. I need to change the oil and go ride it around some more to test.

Edited by Center_Sumo

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I didn’t know such a thing existed, I may have to pick one up when I come to terms with pulling the carb off again lol.
It really was very minor. It just felt like when slightly increasing throttle at lower rpm cruising speed there was a half second before it got into the power, almost like the feeling on 600cc sport bikes right before you hit the powerband, just much less pronounced than that. I’m just not exactly sure where to start with trying to get rid of it. The bike didn’t like the EMR needle. I need to change the oil and go ride it around some more to test.


I use the depth function on dial calipers to set the depth when using the tool. Much more accurate.

Your bike may not like the full accelerator shot. Is it a running issue or is the power just a little pipey?

I don't get the EMR issue. Worked fine for me in a 39 and 41 as a 434 and 462. Using the JD needle it was pig rich off the bottom unless I used a 40 pilot.

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