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Old 04-15-2019, 06:19 PM   #1
HadesOmega
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Bike stumbles half throttle runs better with choke

So I'm installing a KX65 carb in my KLX140. I managed to get everything installed and bike fires right up. It idles great doesn't need choke. When I rev it however about half throttle it it stumbles and if I roll more it'll eventually start revving. Same when I took it for a ride. When pull the choke out it runs better the stumble is gone. I run it with the airbox cover it runs a little better.

I'm thinking this means I'm running lean in the middle circuit (needle). So I move the needle one clip down which raises the needle to richen the mid throttle up. I fired it up and it still kinda runs the same.

Any one know what these conditions mean and what I could do to get it running well? I'm thinking it could be the main also. I have the next main size up. I'm not a carb tuning genius =/ It sounds like its running lean because with choke it runs better and by putting airbox cover on it restricts the airflow making it richer right?
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Old 04-15-2019, 06:33 PM   #2
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Yes, it needs more fuel. Go up a couple sizes on the main jet and maybe drop the clip on the needle a couple of notches and report back.
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Old 04-16-2019, 02:18 PM   #3
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Ok so I went from a 150 to a 155 and it runs a little bit better. Same story with the airbox lid still runs worse without. At 75% throttle it will sputter a bit then start revving again. Seems I am close.

I don't have anymore jets to throw at it though going to see if I can get a 160 and 165.
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Old 04-16-2019, 02:20 PM   #4
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aren't 2 stroke vs 4 stroke carbs significantly different in most cases?
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Old 04-16-2019, 02:31 PM   #5
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The adjustment screw is different and the choke is different. The 2stroke has an airscrew and a 4stroke has a fuel screw. This is supposedly the poorman's carb upgrade for the KLX =P
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Old 04-16-2019, 07:50 PM   #6
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Go further on the main. Trying a larger pilot jet
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Old 04-17-2019, 04:49 PM   #7
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Ok tried the 160 and 165 main jets. It's getting better. Still a tiny bit of stumble/hesitation at that 75% throttle. I hammered it down the street and shut the bike off as I went into my driveway and popped the sparkplug and it's grey maybe a tad whitish. So I think I'm really close, I think I would ride it like this but I want a little richer to be on the safe side so I'll put the 170 jet in.

When i take the airbox lid off it still runs like crap. I can't believe how much air this carb flows compared to the stocker. With the stock carb you could barely feel much different with the lid on or off. It's like night and day. It's no wonder the bike this on last was running a 195 main. I thought that was a big but maybe not.

I think I'm going to stop at the 170 I kinda have to because I'm out of jets but just threw it on there and going to test ride it now...
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Old 04-17-2019, 05:56 PM   #8
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Test rode it with the 170 main and its pretty good I almost don't notice the stutter. Sometimes it will do it. I ran it down the block in 2nd gear WOT and coasted to my garage and cut the engine and pulled the plug 2 times here's what the plug looks like:

#1


#2


I'm thinking that looks a little lean? Light grey? Should I keep on going up in jet sizes? Ideally I want it a little rich because I'm going to be going WOT most of the time on the track I don't wanna blow the engine up.
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Old 04-17-2019, 06:08 PM   #9
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That's pretty white. Did you try a larger pilot
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Old 04-17-2019, 06:56 PM   #10
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That still looks very lean to me. You mention this is the poor mans performance boost for a KLX140. What range of jets have other folks successfully used in it? You said that this carb came out of a different bike and I assume it wasnt a KX65. What bike did it come out of? Obviously all bikes, even the same model and same carb sometimes require different jetting. But these might give you some clues.

Close the airscrew completely (as its a two stroke carb) to richen it some more. Where do you get the highest idle with the air screw? Closed or out?
This will tell you if you can add a larger pilot. If it still starts and idles easily, you may be able to go bigger on the pilot. But I suspect you still may need to go richer on the needle and main.

Many folks have different approaches, to jetting a thumper. I always start with the pilot and tape off my throttle grip so I know exactly where my throttle openings are and then work on needle and main next.

Plenty of good tutorials on thumper jetting, google them. Thumpertalk had one of the best if my memory serves me well.

If this were my bike and I had absolutely no idea or guidance on a jet range that works, I'd probably make some big steps on the rich side to see where those limits are and work back from there. This would also be a good learning experience for you as you will be able feel the difference between a lean bog and a rich stumble then and not second guess yourself as much.

What you are calling a stumble is actually a lean bog btw based on what you have shared thus far. Good luck!
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Old 04-18-2019, 12:02 PM   #11
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Many bikes will run like crap with the air box lid off. My ST1100 would barely idle with it off and if you gave it any throttle, it would fall flat on it's face. Even with dialed in jetting.
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Old 04-18-2019, 12:24 PM   #12
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I did not change the pilot I think I got the pilot jet right on the first try. The bike fires right up no choke and it runs really well with light throttle. No popping on deceleration. Air screw is at 1.5turns out.

I bought 4 more jets today will keep popping them in I have 150-195 now. So funny the guy at the cash register was like "have fun with those" haha he knows what's up.
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Old 04-18-2019, 06:14 PM   #13
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Rode the KLR650 down to GP Motorsports today and grabbed more jets. I now have jets 150-195. I skipped from 170 to 180 and popped the plug the the stumble is pretty much gone. It's a little darker now. More like a normal grey.


Decided I wanted richer so I put the 190 in. 195 is the highest I got. It runs great no stumble at all. Popped the plug and it's little bit darker. I think I got it right on the money it's probably stoichiometric now I would like it a little richer but I think think this will do. If anything it's a tad rich. It was 85F today though so the bike may have been running richer. Hopefully in the cooler weather it'll still be fine.


So that's it went from 150 to 190 main jets. I should have just left that 195 in there I don't know how people are running 150, maybe I got a different carb or something.

Final settings are (if anyone with a KLX140 is interested)
190 main
20 pilot
2nd from bottom clip position on needle
1.5 turns out air screw.

Thanks everyone for the help. I'll let you know if it runs great or I nuke the engine after the race this weekend.

This is what I'm working on if your wondering.






KX65 Mikuni 24mm Carb






KX vs KLX carb it's bigger than the KLX carb by like 5mm in every dimension that counts.





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Old 04-20-2019, 01:28 PM   #14
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First off put a new spark plug in. looks to rich now. But till you get a clean read on a new spark, it's hard to tell anything. you may need more main jet, but from what i can see. I think most of the problem is in the mid-range of the fuel curve and that involves the needle I would try raising the needle up .030 or if it has an E clip on the needle have the needle go up in the carb 1 clip, and see what that gives you. That should get you closer than where you are now from what I see on the plug its running muddy. You may have fixed the mid range stumble at the cost of the rest for the fuel curve. .

Quote:
Originally Posted by HadesOmega View Post
So I'm installing a KX65 carb in my KLX140. I managed to get everything installed and bike fires right up. It idles great doesn't need choke. When I rev it however about half throttle it it stumbles and if I roll more it'll eventually start revving. Same when I took it for a ride. When pull the choke out it runs better the stumble is gone. I run it with the airbox cover it runs a little better.

I'm thinking this means I'm running lean in the middle circuit (needle). So I move the needle one clip down which raises the needle to richen the mid throttle up. I fired it up and it still kinda runs the same.

Any one know what these conditions mean and what I could do to get it running well? I'm thinking it could be the main also. I have the next main size up. I'm not a carb tuning genius =/ It sounds like its running lean because with choke it runs better and by putting airbox cover on it restricts the airflow making it richer right?
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Old 04-24-2019, 09:28 AM   #15
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Thanks for the roundup with the final jetting. I have a KLX that may eventually get a similar upgrade.
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