I've got a KTM 125sx 2002 I've done a top end rebuild piston etc and put a new coil and plug on it and the bike won't start with the kicker, only with a bump start took the plug out and its got a weak

Lots of engines make characteristic noises and air-cooled 2-strokes have lots of characteristic noises even when new and even when there is nothing wrong.

If you have just spent a parcel of money on it your first port of call should be to the person or dealer who carried out the work. Having a top and bottom overhaul means a world of different things to different people. If you don't know what has been done and what needed to be done you will be somewhat in the dark and at a disadvantage with perhaps unrealistic expectations.

Perhaps the engine hasn't had as much of an overhaul as you thought. A full bottom end overhaul is a good deal of cash and lots of work that would make it as good as a new engine again. Maybe the dealer decided for reasons of economy to refit some of the old parts instead of replacing everything.
I suggest you read your invoice and then go and ask some leading questions.

unfortunately the most cmmn cause for a bike not starting ff the kick start but starting with a bump is being too low on compression,although to you foot it may feel like enough it does need to be nearly 100% as high as it should be to kickstart.the easiest way to check the condition of the piston is to take the exhaust off and look up the port in the barrel ,it is usually the exhaust side of the piston that gets damaged as it gets hotter.good luck.Adam

Thank you for contacting FixYa with your inquiry.
KTM recommends .7 Liters Motorex Top Speed 4T15W50 for the 125SX.
Let us know if we can be of further service.


Kind regards,
TF

Not unless the engine is smoking, or knocking or something. It depends on what kind of maintenance has been done on the engine. Evidently, the maintenance has been done pretty well. I've torn down Evolution model engines with more miles than this on them and found the bottom ends to be in very good condition not needing a rebuild. Your top end may need a refreshing. It may not even need boring. I'd check the piston to cylinder wall clearance before I bored it. It may just need a new set of piston rings and a valve job. Don't go jumping in rebuilding your engine just because you think it might need it. Ride it until it quits. It won't cost you any more.

Good Luck
Steve

Thank you for contacting FixYa with your inquiry. I have been going over some of the older help requests that have been archived and
trying to provide solutions for those that didn't get answered. Hope
this information helps.

Try these:

KTM Owners Manual

Start here

http://www.carlsalter.com/motorcycle-manuals.asp

http://www.ktmusa.com/download.php?dataID=8101G9_OM_EN.pdf

KTM Service Manual

http://www.ktmcyclehutt.com/ktm-parts/ktm-parts-search.php?ktm-motorcycle-parts=Repair-Manuals&year=2007&ktm-model=sxf-450&fitment=bigbikes-all

KTM Repair Manuals Order Site

http://www.repairmanual.com/KTM.html

http://www.tryvod.com/search/ktm-2008-450-xcf-owners-manual-download.html

http://www.pdf-manual.com/user-guide-instruction-owner-manual-ktm-450-exc-2008-17793.html

http://www.pdf-manual.com/telechargement2.php?id=17793

http://www.mydrive.ch/browse#/

Manuals General:

http://www.dansmc.com/shopmanual2.htm

Other Manuals Sites:

http://www.fixya.com/motorcycles/ktm

I'd bet you 10 to 1 it's not the coil. It sounds like it is starving for fuel and needs a carburetor adjustment. When you rebuilt the top end, did you put a bigger jug and piston? If so, your problem most definitely is the carburetor needs to be re-jetted.

What caused you to rebuild it? Did it seize, or quit running? Was it losing power before the rebuild?

That's where my money is but if you still believe it is the coil, you can check it with a spark checker. This is a handy little tool you can find at AutoZone, Harbor Freight, etc. It's a one piece tool that connects between the plug and the coil. The plug stays in but you remove the wire and put the checker on, one side connects to the plug, the other side you connect the plug wire. There is a light in the middle that will flicker when you crank the engine if it has a good coil.

Put the checker on and start the bike and see if you are losing spark...if the light stops flickering at top end, then you do have a coil issue...but like I said before, having seen this problem hundreds of times, my money will be on re-jetting the carburetor.

Try fitting a new spark plug.