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2002 Drz with 470 Big Bore kit, 41mm FCR carb, 12.5:1 comp. Stage 2 hotcams, full Yoshi exhaust, 3x3
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Discussion Starter #1
I picked up a 2002 Drz400s the other day with a newly installed 470 kit, and FCR 41mm carb. The bike is currently running rich, bike accelerates pretty well with occasional stumble, however when maintaining a constant speed at about 25% throttle say 25mph it will stumble and run rough. It's not a bog like a lean condition would create. I removed the spark plug today (NGK CR8E) and the plug was pretty black. I had a fresh plug on hand and installed it. The bike will die pretty regularly if I downshift and pull in the clutch at higher RPM. Basically, as soon as I pull in the clutch at higher RPM and the bike returns to idle speed it will die. It doesn't die if I let the RPM,s go down and then pull in the clutch and shift down. Occasionally if I blip the throttle it will die but this is pretty occasional and usually only after the bike has been running for a bit and is pretty warm. My question is, with the 41mm FCR carb what jetting should I be running? I will post the build sheet below as well as my elevation and all that fun stuff. Just looking for direction with my jetting as well as anything else that could be causing this. I have not opened up the carb yet to see what the current jetting is, however in the kit I have a 160, 155, 150, and 145 main as well as a 42 and 48 pilot. Also, with proper maintenance what kind of reliability should I expect from this build? bike currently has 6,900 miles on it.


Build Sheet:
94mm Big Bore Cylinder
Hotrod 5mm stroker crank
Hotrod main bearings
CP 12.5:1 Piston Kit
Stage 2 Hotcams
Kibblewhite valve spring kit
Kibblewhite stainless intake & exhaust valves
FCR MX41 Flatside racing carb.
Yoshi RS-2 full exhaust
 

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Drop the needle a notch for a start. A bore and / or a compression increase will make the motor run rich , even with the same jetting that may have been OK before.
 

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Need to know a few more things. Leak jet in the float bowl. Accel pump timing and what needle it's running.

I agree with red about dropping the needle but I'd also investigate the squirt timing and amount from that ap. Maybe it's too much or too early.
 

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2002 Drz with 470 Big Bore kit, 41mm FCR carb, 12.5:1 comp. Stage 2 hotcams, full Yoshi exhaust, 3x3
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5 Posts
Discussion Starter #4
Okay, I asked the previous owner what jetting they were running. They said it was the jetting that came in the carb that the manufacturer suggested. I will have to pull the carb and let y’all know what I find.
 

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2002 Drz with 470 Big Bore kit, 41mm FCR carb, 12.5:1 comp. Stage 2 hotcams, full Yoshi exhaust, 3x3
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5 Posts
Discussion Starter #5
I have resolved the issue where the bike dies on occasion when I pull in the clutch. It would do this almost every time if I blipped the throttle. I adjusted my fuel screw about 1/4 turn to lean the bike out. This totally fixed any problem I had with the bike dying. It still stumbles a bit if I hold it at about 10% throttle to maintain constant speed but other than that the fueling seems pretty spot on. Any suggestions on what to do to resolve the stumble? The bike is rich when it is stumbling but after that is fine. I've heard I may need to drop the needle one notch which I guess sets the stage for my next stupid question.. every bike, car, etc.. that I have experience working on has been fuel injected so I'm pretty new to carbs and still trying to wrap my head around what all I need to do adjustment wise. In order to drop the needle on my FCR MX41, what do I need to do? Any help would be greatly appreciated!
 

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Needle seems like a logical step.

First things first. Record where your fuel screw is. Count the number or turns it takes to close it. Write that down and then turn it back out that number or turns. Depending on where that is you might want a different size pilot jet.

For the needle you will need to remove the top cap on the carb. Probably Allen screws. From there the needle will be sitting in the slide. Not that if you twist the throttle to raise the slide you will squirt fuel into your engine. The needle has clip positions on it. To lower the needle you will move the clip up toward the end of the needle. Most are 3 or 4 clips away from the end. The needles come in different tapers as well. Record the clip position from the end and if there are any markings on the needle. Depending on where the carb came from you might see a painted dot on the back or you might see some letters.

With FCR carbs on a drz there isn't much room to work. Depending on how this one was installed you might be able to remove the upper motor mounts and rotate the carb to remove the top. I remember the 39 being a tight fit. The 41 on my Ktm seems about the same height so I cant imagine it's any better.
 
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