2003 KX125 3rd gear powerband bog and increased RPMs when pulling in clutch

tldelaney
Posts
23
Joined
6/29/2018
Location
San Antonio, TX US
8/29/2019 6:40pm Edited Date/Time 9/8/2019 11:09am
Hi guys,

Just purchased a 2003 KX125. Made sure to change the top end and bits pieces that were missing with the bike.

I got it started last week and initially had idling issues where it wouldn't idle without cracking the throttle slightly. I figured it was because I was using the incorrect diameter overflow bowl hose which was resolved once I used the original.

My first issue has me questioning a couple things and would like your guys's feedback. Currently when starting the bike and running it through the gears, I will get instant bog when I hit the powerband in third. This issues resolves once i release the throttle back to zero and don't enter the powerband. Sometimes I can run through the gears and powerbands and it runs great but its very inconsistent. One possibility I've heard is a clogged powervalve (KIPS). I have made to sure go through the arm actuator and exhaust to clean all the carbon and goop out of it. This did not solve the issue. I also completely disassembled the carb, installed a new rebuild kit along with A LOT of compressed air and carb cleaner to make sure everything was clean (Stock jetting in San Antonio, TX)

The second issue is once running the bike, it will pick up about 200-600 RPM when I pull in the clutch lever. I wasn't having this issue initially, but after running a lap I noticed increased idle and pulling in the clutch made it worse. I can pretty much cruise the bike with 0 throttle and no clutch engagement after a few minutes. of note, It does not do this when the bike is idling. That leads me to think of the possibility of an air leak. My thoughts jump straight to the Air intake carb holder rubber boot. It is in pretty rough shape with a bunch of cracks in it. Attached are some picture.

If it is the carb boot, I am clueless on where I can purchase a new one because it seems every online store no longer makes them. I understand that a Leak down test will probably be my next best bet but currently dont have the tools to do so. Can anyone be of assistance?




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Matt Fisher
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4/1/2008
Location
Visalia, CA US
Fantasy
1084th
8/29/2019 8:12pm
If you want new, try a Rad Valve from Boyesen. RAD-11J fits the 03 KX125 for about $180.
Paul_Pitzonka
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693
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5/1/2018
Location
Grand Terrace, CA US
8/30/2019 12:57am
A QD way to find an air leak is to take a can of brake cleaner, stating fluid, or carb cleaner, and spray around the intake boot to see if the rpms increase, if they do you’ve located a leak ( you can also do this around the case mating seam)... second, odds are crank seals aren’t in the best condition, very overlooked on used two strokes very common source of an air leak... as far as a new intake if it’s obsolete you’re going to have to do what’s suggested above... eBay, or an aftermarket setup.
1
tldelaney
Posts
23
Joined
6/29/2018
Location
San Antonio, TX US
8/30/2019 7:52am
A QD way to find an air leak is to take a can of brake cleaner, stating fluid, or carb cleaner, and spray around the intake...
A QD way to find an air leak is to take a can of brake cleaner, stating fluid, or carb cleaner, and spray around the intake boot to see if the rpms increase, if they do you’ve located a leak ( you can also do this around the case mating seam)... second, odds are crank seals aren’t in the best condition, very overlooked on used two strokes very common source of an air leak... as far as a new intake if it’s obsolete you’re going to have to do what’s suggested above... eBay, or an aftermarket setup.
Thanks, I’ll check it out.

The Shop

AJ565
Posts
2075
Joined
3/12/2012
Location
San Antonio, TX US
8/30/2019 8:57am
Until you get that intake boot replaced don't change anything jetting wise. Everything you have going on could be cause by that leaking air.
funkj62
Posts
6
Joined
9/3/2019
Location
Monett, MO US
9/4/2019 8:46pm
I suspect a air leak. Plug your exhaust and intake opening and do a leak down test. See if it will hold 6 lbs of air/pressure. You can find the leaks with a spray bottle with water/soap.
Paw Paw 271
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4/3/2013
Location
Benton, LA US
9/5/2019 11:15am Edited Date/Time 9/5/2019 11:17am
If you can't find a replacement boot...Go to Harbor Freight and purchase a can of "tool dip"..It comes in different colors and Black. Use the Black as it will not look as trashy. Use it to put ( use a brush) a coating or 2 on the out side of the boot and let it dry for 24 hours. It will seal the leaks until you can get a new boot.

Paw Paw
tldelaney
Posts
23
Joined
6/29/2018
Location
San Antonio, TX US
9/8/2019 9:16am
Hey guys, sorry for the long wait. So I did a leak down test with a homemade tester. Surprised because guess what. It held 4 PSI for 30 minutes and then I pulled it. I just hit the track today and it was running great until about the 3rd lap and then it started jogging down sometimes as I hit the power band and was slightly revving when the clutch was pulled in. I’m happy there is no leak but still need to find out why it’s behaving the way it is.
funkj62
Posts
6
Joined
9/3/2019
Location
Monett, MO US
9/8/2019 10:58am
What carb is on it and has it been bored/taper bored?
funkj62
Posts
6
Joined
9/3/2019
Location
Monett, MO US
9/8/2019 11:09am
My guess is pull mag side cover remove flywheel and replace crank seal.im betting problem goes away, cheap easy to do and good insrance.

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