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Discussion Starter · #1 ·
Hi everyone, I have a 2017 150 SX which had been sitting for the whole winter, and few days ago when I tried to start it up it wouldn't idle, giving wot from idle would cause it to literally stall. Tried playing with the carb richening it up, leaning it, by the way it has a Lectron carb, but nothing helped. Also replaced the spark plug and reeds just to be sure, and that still didn't help. So, went ahead to Harbor Freight and bought a compression tester, and after doing the test 3 times, the readings were at 70psi every single time. The bike runs great from mid to top range, even with that compression, it just won't idle and is very hard to start moving in 1st gear, have to give a lot of gas and drop the clutch to just start moving.

The bike has 45 hours on it, bought it at around 22 hours, owner claimed he replaced a fresh piston (lesson learned).
I was also confused how could the guy be changing a piston at less than 20 hours on a factory fresh KTM, it's possible but how hard do you have to ride it.
 

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70 psi is all you need to know. If your compression reading is accurate your cylinder head should be removed for an inspection of cylinder. I suspect you will find your answer. More than likely cylinder removal will find your trouble. Not sure if your bike was neglected, but you should never gauge engine life from its hours of usage. I have seen many engines with less than 20 hours destroyed from poor air filter sealing or lack of proper air filter maintenance. If your bikes previous owner replaced piston and neglected air filter it could be cause of premature failure. Another common trick owners will do when the bike is to be sold soon is to install fresh piston into a scored or scratched cylinder to avoid expense of replating cylinder. This restores engine compression for a short time and fails quickly(10-30 hours) indications of this are described exactly as your symptoms and usually hard starting or siezing are first indications.
 
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Discussion Starter · #3 ·
70 psi is all you need to know. If your compression reading is accurate your cylinder head should be removed for an inspection of cylinder. I suspect you will find your answer. More than likely cylinder removal will find your trouble. Not sure if your bike was neglected, but you should never gauge engine life from its hours of usage. I have seen many engines with less than 20 hours destroyed from poor air filter sealing or lack of proper air filter maintenance. If your bikes previous owner replaced piston and neglected air filter it could be cause of premature failure. Another common trick owners will do when the bike is to be sold soon is to install fresh piston into a scored or scratched cylinder to avoid expense of replating cylinder. This restores engine compression for a short time and fails quickly(10-30 hours) indications of this are described exactly as your symptoms and usually hard starting or siezing are first indications.
Will take it apart today and see what's up with it)
 
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