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Discussion Starter #1
Howdy
just started doing this one day, up till then ran sweetly . Runs for a few minutes then dies. Reeds are fine,
have replaced crank seal, cleaned carby, 1.5 turns out on mixture, does it with or without air filter in. No water inside ignition cover. running 60:1. As per another thread, oil goes milky after 10 mins running..don't think this is related..
Any help would be awesome cause I am all out of solutions, its a 1999 model I think and is in original mint condition.
Thanks
 

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As per another thread, oil goes milky after 10 mins running..don't think this is related..
Why don't you think this is related?
Have you determined what is causing the coolant to mix with the gearbox oil?
Just curious.
A possibility might be a bad head gasket allowing coolant to seep out of the cooling jackets of the cylinder and seep into the combustion chamber therefore fouling the plug in the process.
More common is a failed water pump seal but this should not foul the plug like a bad head gasket would.
Just some ideas.
Augdog, Axzon and DW are the true experts.
They should be along shortly...


Rangerman
93339
 

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Dirt Wizard
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Rangerman is probably pretty right with the wp seal being shot. Most common for getting water/coolant in the gearbox oil.
What's the splooge like coming out the rear?
What does the plug look like after it fouls?
Still a few variables to nail it yet.
The ax and the dog could post up 2 novels with lists of things to check.
Best bet is to post us up some pics of the plugs after they foul.
 

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Sounds likely cooling system could be a good place to start testing. Maybe start by removing cap from COLD radiator and start motor and observe coolant for bubbles when cranking or running. Follow up with cooling system pressure test to isolate coolant leakage. Cylinder compression test could help also.
Report back with results. Good luck
 

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I concur with the postings above.
  • If you have water in the oil (milky oil) it comes from somewhere and should NOT be there... most probable reason is, as stated above, head gasket (an o-ring under the cylinder head) or the water pump seal. If the problem does not go away after you have replaced those two, then the repair costs will go up. So start there and cross your fingers...
  • When did you last change piston or rings? And how many hours has the bike done since last rebuild?
 

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Discussion Starter #6
OK, thanks for input, will drain coolant( only water in there after replacing crank seal) and run it for a couple of minutes and see if plug fouls. Not sure how many hours top end has, lots of compression however, hard to kick over. quite a lot of splooge out of exhaust end and junction. Plug looks wet and oily when it quits. thanks
 

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"Plug looks wet and oily..." indicates either:
  • oil getting in (as you had already guessed as you changed crank seal)
  • too much oil in the oil/fuel mixture (but 60:1 is actually too lean on oil as older KTM's were set up for 40:1)
  • carbi set up too rich (but if bike runs well until the problem comes after a few minutes then i would not think carbi is set up wrong)
  • piston/ring needs replacing
  • fuel level in bowl in carbi too high or needle valve in carbi not closing properly. Here a good vid on how to set fuel level in carbi:
Here a link to your manual: https://www.manualslib.com/manual/904983/Ktm-125-Sx.html?page=37#manual but there is no table for the carbi sadly.

Was the bike running well before? Or did you get the bike with these problems? If the problems were there when you got the bike there is a chance that the earlier owner would have played with the carbi and even changed needle and jets. If that is the case then your best bet is to find out what needle and jets are standard in this bike and start from there. (This would be after you have found the issue with water in the oil)

Have you had the cylinder head off and looked in? Is the barrel in good condition? One old bike we got looked all good, but did run poorly. It turned out that the bore was worn. A re-coating of the bore sorted that.

A long-shot... running 60:1 would mean more fuel than 40:1. Could she be that sensitive that this is enough to give you the issues... it would affect how much fuel goes through the carbi as 40:1 is a thicker mix...
 

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Discussion Starter #8
ok, here is update, drained cooling system and bike run for 3-4 mins no problem, so guessing leaking head gasket. There was tiny bubbles in water when running bike with radiator cap off and abit of foaming too. The engine oil looks really dirt after 10 mon rather than cloudy.

Thanks for all the great advice..
 

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bada bing(y) you are off to a great start. check back as needed for help. good luck
 

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Cool! Those head gaskets (a big O-ring) are cheap. Replace each time you take off head. Keep a few spare ones of the one closest to the bore so you are ready for next time you do work in there. All the best!
 
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