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Menifee, CA
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765th
Falcon
8/16/2019 11:19am
8/16/2019 11:19am
Edited Date/Time
6/5/2022 1:01am
EDIT: I'll check the advice below to make sure the piston clears the exhaust port. Right now I'm having bigger issues, though: the darn thing won't start. I have the new top end in it, the spark plug is new, I've cleaned the carburetor (it didn't need it, but I cleaned it anyway,) the reeds look good, there is no blockage in the exhaust nor in the airbox... no go. It doesn't help that the kickstarter on this thing only has about a 45-degree angle of operation when you kick it. I don't feel like I can get a good bit of momentum into the crank.
Anyway, is there a secret trick to getting an SX50 to start?
Anyway, is there a secret trick to getting an SX50 to start?
The Shop
Trying to start a KTM SX50 with a brand-new top end. It has fresh gas, a new spark plug, reeds and carb are clean and in good condition, but the thing won't fire up. No rag in the airbox, no blockage in the pipe. Help!
I actually tore it all back down today to make sure I didn't do anything wrong in the top end and I noticed some excess oil on the crank and in the bottom part of the cylinder. It could just be premix but I'm wondering if there's a main seal problem. Would that keep the thing from starting though??
I'm pretty much confident in the spark too, but I understand the stators on these things go out regularly and sometimes the voltage isn't enough to overcome the pressure created by compression when the cylinders are all buttoned up. I took a look at the ignition side and there's some fairly significant rust. What do you think? Bad stator? Is there a way to test this one? I don't think I know anybody with a part I can swap onto this bike to try it out. I'd hate to spend $100 and have it not be the issue.
Help! I'm getting tired of kicking this thing! I think I put a tenth or two on the hour meter just by kicking!
I have a 09 SX50 and it was a nightmare to start until I found this out. Now it starts within two kicks.
here is the link
I've even tried starting fluid down the carb throat and nothing. Nada, zilch. It's like trying to kickstart a toaster; there's nothing at all happening.
Pull the carb, shoot the starter fluid directly into the intake. It should at least pop when kicked over in this manner.
Is it making at least 100psi cranking pressure on a gauge ? Low compression will also hinder the amount of "pull" of the fuel charge .
What was the clearance on the piston to bore ? It could still be "loose" ,even with a new piston /ring. Low cranking pressure makes for no or hard starting. I bought a complete KTM cylinder, piston kit for about $210 for an old JR I rescued way back.
If the cylinder has a lot of wear, they used to offer A-D pistons. When I bought my kit, my dealer said they only offered C pistons , I believe, which is also what the kit came with. The small bikes are very sensitive to the vacuum signal, or amount of "pull" of the fuel charge.
I would also clean the flywheel of rust.
Paw Paw
Project Racer, I'll try the "no carb" starter fluid idea if the stator turns out to be good. I just changed the top end myself; it's a "B" piston in an "A" cylinder and it seems to fit properly with no rocking at all. You may be on to something with the air leak idea, though. I did notice some oil on the cases below the piston when I re-inspected it. I figured that wouldn't cause it not to start, but if it's a big leak, maybe you're right about the vacuum pressure on the upstroke.
I have seen this bike run before, so I know it's just one small thing.
PawPaw, I'll clean that flywheel off and see what happens.
Pit Row
The stator tests within normal range for resistance (510 Ohms.) Like I said, it's giving me a solid spark outside the cylinder with the plug grounded to the studs. I cleaned off the rust just to be sure, but I'm pretty sure we've eliminated that as a culprit.
The plug looks dry to me, but I don't know how long I should be kicking it to see. Is 10 kicks enough? 100? I've completely gone through the carb, the floats are set correctly, the fuel shutoff valve is operating, and there is plenty of fuel present in the bowl when I remove the carb. The reeds seem to open and close easily.
So, to recap, I have compression, spark, and fuel (I THINK,) and still no combustion. What the actual F? This thing is going to kill me.
If it is making at least 100-110 pounds of cranking pressure, and the crank seals are holding pressure it should run. 100-110 would be considered low IMO, but it should a least give a hit.
I'll do that and if the compression looks good we'll be that much closer.
I pulled my hair out fixing 50sx!! As soon as my daughter was tall enough I put her on a 65 and had no issues since. Good luck
-Stator is dry as a bone, with no sign of any oil or exhaust residue
-Dry plug
-Heavy oil on the crank (too much to be combustion residue, I feel.) Same color as the the trans oil.
-Brand-new top end rules out compression (mostly - still have to put it on a gauge, but I'm confident this is not it)
-Good spark and stator tests within range
All these point toward fuel as the limiting factor. However, I tried the old "saran wrap on the carb throat" trick to see if the reeds were operating when the kickstarter was engaged and sure enough I'm getting some suction. Arrrrrrgh! I may just replace the clutch-side seal anyway to see if that's it. After all, saran wrap isn't hard to move and maybe there's just not sufficient vacuum going on under the piston.
Can that seal be removed and replaced without splitting the cases?
Paw Paw
I'm going to do the clutch side crank seal first and see if that's the issue. If not, I'll drop in the stator side too and see. After that, I'm done with this one!
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