2003 kfx50 randomly stalls
Aug 19, 2020 | 11:31 PM
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Weekend Warrior
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2003 kfx50 randomly stalls
I’ve searched this problem to death and feel like banging my head against a wall. My son has a 2003 KFX 50 that seems to have been nothing but problems since I got it. This is a case where I could have bought a brand new one for what I have into this. Let alone my time invested.
so it started when it was first purchased, I was given a quick tutorial of the ins and outs (not by choice) the guy was just trying to be nice. I asked what oil he ran in the oil injection which I was told but also told that he put some in the tank for good measure. Might I add that the bike was smoking like a pile of wet leaves on fire. I do have a bit of back ground with other 2 strokes so I knew this was not right. I loaded it into my truck when the deal was done and noticed the complete under side dripping with black 2 stroke oil being sprayed out the exhaust.
I get the it home get a manual and start looking into the oil injection system and realize how it works, I did some Research and realize a lot of guys bypass them on lt50/jr50/kfx50 due to the design. So I bypass, put in fresh gas mixed 50:1 and this thing will not start tore apart the carb cleaned it ( didn’t think it was bad ) now starts up after a bunch of pulls. It only seams to run with the choke on. I run it around the yard, I’m 190 lbs after 10 min WOT it keeps stalling and takes like 20 pulls to start but if you let it cool it will start right up. I get told it probably needs a top end.
i pull the top end apart someone was already in there at one point the head was all marked up from a screwdriver or something to pry it off , soot marks on the head gasket, it’s had the restricters removed from the intake and exhaust, 5 thou over-piston and plug was pure white (lean). No scoring or marks
I cleaned the exhaust pipe and restrictor from the oil.
I pulled the flywheel to check for a popped or leaking crank seal ( looks good ) no oil dripping everything nice and clean.
I start looking into rebuild kits and see that aftermarket cylinders don’t have the nipple for the petcock vacuum line at the intake.
I decided to go with oem parts as replacements I installed new factory piston, rings (standard) made sure to take caution installing the rings in the correct order ( see manual for detail) wrist pin and clips wrist pin bearing,cylinder head, cylinder, intake gasket, exhaust gasket, head gasket, base gasket.
at the time I jetted the carb with some of the info I found on this site there was mention to remove the staked jets install a 17.5 pilot and around a 70 main
im also running a 6 heat range plug vs the factory 4. I mixed the gas/oil at 40:1 for the initial tank and it fired right up. Minimal smoke The carb is set to factory specs out of the manual for float high, main metering rod hight and air screw adjustments. Plug now looks good it is a tan colour. I have had some issues with random stalling. Upon researching this some more it looks like there was some miss information in the manual pertaining to idle screw and air mixture screw identification and placement. As mentioned before.
mine was assembled like the line diagram shows flat air screw and Phillips idle screw. I have reversed the screws but still have the same issue , the bike runs like a top. Choke works perfectly starts fist pull every time. When my son rides it hard ( I do have the throttle stop adjusted down to about 40ish % and he comes to a quick stop or shuts the throttle quickly maybe 30 % of the time it will stall. It will fire up with less then half a pull but non the less it still stalls.
sorry for the long winded post I just wanted to paint a clear picture
I know he’s not hitting the kill switch and the rear on
hopefully i can get this repaired
thanks Justin
so it started when it was first purchased, I was given a quick tutorial of the ins and outs (not by choice) the guy was just trying to be nice. I asked what oil he ran in the oil injection which I was told but also told that he put some in the tank for good measure. Might I add that the bike was smoking like a pile of wet leaves on fire. I do have a bit of back ground with other 2 strokes so I knew this was not right. I loaded it into my truck when the deal was done and noticed the complete under side dripping with black 2 stroke oil being sprayed out the exhaust.
I get the it home get a manual and start looking into the oil injection system and realize how it works, I did some Research and realize a lot of guys bypass them on lt50/jr50/kfx50 due to the design. So I bypass, put in fresh gas mixed 50:1 and this thing will not start tore apart the carb cleaned it ( didn’t think it was bad ) now starts up after a bunch of pulls. It only seams to run with the choke on. I run it around the yard, I’m 190 lbs after 10 min WOT it keeps stalling and takes like 20 pulls to start but if you let it cool it will start right up. I get told it probably needs a top end.
i pull the top end apart someone was already in there at one point the head was all marked up from a screwdriver or something to pry it off , soot marks on the head gasket, it’s had the restricters removed from the intake and exhaust, 5 thou over-piston and plug was pure white (lean). No scoring or marks
I cleaned the exhaust pipe and restrictor from the oil.
I pulled the flywheel to check for a popped or leaking crank seal ( looks good ) no oil dripping everything nice and clean.
I start looking into rebuild kits and see that aftermarket cylinders don’t have the nipple for the petcock vacuum line at the intake.
I decided to go with oem parts as replacements I installed new factory piston, rings (standard) made sure to take caution installing the rings in the correct order ( see manual for detail) wrist pin and clips wrist pin bearing,cylinder head, cylinder, intake gasket, exhaust gasket, head gasket, base gasket.
at the time I jetted the carb with some of the info I found on this site there was mention to remove the staked jets install a 17.5 pilot and around a 70 main
im also running a 6 heat range plug vs the factory 4. I mixed the gas/oil at 40:1 for the initial tank and it fired right up. Minimal smoke The carb is set to factory specs out of the manual for float high, main metering rod hight and air screw adjustments. Plug now looks good it is a tan colour. I have had some issues with random stalling. Upon researching this some more it looks like there was some miss information in the manual pertaining to idle screw and air mixture screw identification and placement. As mentioned before.
My son's 2002 LT-A50 (2 stroke) was purchased used and I have no idea if the two prior owners ever cleaned or rebuilt the carb. I tore it down and discovered that - according to the manual - the pilot air screw and the throttle stop screw were exchanged. The machine runs perfectly, yet according to the descriptions and every single photo in the service manual the screws were reversed. One has a slotted head while the other has a cross head, one is longer than the other, one has a flat end while the other comes more to a point, etc. They're different in most ways except the threading.
I also discovered the float was sitting too low. The manual calls for 0.96-0.98 inches, but it was sitting at about half that according to my calipers.
During the rebuilt I put things back the way they should be: Each screw was put where it belonged, the float was adjusted properly, etc. The carb was remarkably clean (I use Seafoam, which probably helps) and was spotless when I put it back together.
Once the carb was reinstalled, the engine fired right up... but idled really high. I had to back out the throttle stop screw a LOT to get it to calm down. Throttle performance was weird, too. For example, if I revved up the engine for a while and then released the throttle, the engine would continue to run at the same high RPM's for 10-15 seconds before it started to slowly rev down.
After fiddling for a long time with screw settings, partially covering the air intake (due to the colder, denser air), etc. I couldn't get the engine to behave. As a last resort, I swapped the two screws back to their original positions. PRESTO! Things started working properly again! WTF?!?
I checked and rechecked the manual to confirm that the screws were, and now are again, backwards. The manual is clear and consistent, and they are backwards. Yet the engine runs properly, idles great, has great throttle response, everything. It's the weirdest thing I've seen in a while.
If you have an LT-A50, do me a favor and checked your carb. The screws are visible through the footwell opening that yields access to the choke lever. The manual shows that the front screw, which is the pilot air screw, has a slotted head while the rear screw, which is the throttle stop screw, has a Phillips head. Yet on my machine, the screws are the opposite. What does yours have?
I also discovered the float was sitting too low. The manual calls for 0.96-0.98 inches, but it was sitting at about half that according to my calipers.
During the rebuilt I put things back the way they should be: Each screw was put where it belonged, the float was adjusted properly, etc. The carb was remarkably clean (I use Seafoam, which probably helps) and was spotless when I put it back together.
Once the carb was reinstalled, the engine fired right up... but idled really high. I had to back out the throttle stop screw a LOT to get it to calm down. Throttle performance was weird, too. For example, if I revved up the engine for a while and then released the throttle, the engine would continue to run at the same high RPM's for 10-15 seconds before it started to slowly rev down.
After fiddling for a long time with screw settings, partially covering the air intake (due to the colder, denser air), etc. I couldn't get the engine to behave. As a last resort, I swapped the two screws back to their original positions. PRESTO! Things started working properly again! WTF?!?
I checked and rechecked the manual to confirm that the screws were, and now are again, backwards. The manual is clear and consistent, and they are backwards. Yet the engine runs properly, idles great, has great throttle response, everything. It's the weirdest thing I've seen in a while.
If you have an LT-A50, do me a favor and checked your carb. The screws are visible through the footwell opening that yields access to the choke lever. The manual shows that the front screw, which is the pilot air screw, has a slotted head while the rear screw, which is the throttle stop screw, has a Phillips head. Yet on my machine, the screws are the opposite. What does yours have?
sorry for the long winded post I just wanted to paint a clear picture
I know he’s not hitting the kill switch and the rear on
hopefully i can get this repaired
thanks Justin
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