swa

Member
Jun 1, 2005
15
0
I have tried everything on this 99 KTM 250exc. From changing jets to
(MJ 178*IJ 48* Needle is n85c), cleaning the build-up on the piston, adjusting the stator, premium gas with racing fuel w/40:1 mix, using ice anti freeze and Ngk br8ecm spark plug. I am stumped. Is there anything else that I can check? Thanks for any help, Tom
 

ellandoh

dismount art student
~SPONSOR~
Mi. Trail Riders
Aug 29, 2004
2,958
0
extra base gasket ??? dont know if thats a remedy or not but makes sense :think:
have you owned the bike since new?? mods done to head??
 

swa

Member
Jun 1, 2005
15
0
Dont know about the resistor type plug I will look into that.

I had the bike since May. The previus owner had a new piston and nickel cylinder in o4.
 

ellandoh

dismount art student
~SPONSOR~
Mi. Trail Riders
Aug 29, 2004
2,958
0
i would double check that the piston is stock and no other work done, also renikasil the cyl ?check to make sure the reason it was redone is not reappearing :blah: just guesses :blah: ...............extra base gasket im not positive on but ive seen it as a suggestion to relieve knocking in other posts. yet i would wait for further more knowledgable help in that dept. or start a thread on base gasket :cool:
 

elf

Member
Jun 7, 2003
695
0
Ktm makes a large variety of base gaskets. All are a different thickness. Maybe the bike was put together with the wrong one.
You can check to see if yours is ok by removing the cylinder head. Then put your piston at top dead center and check the X dimention. which is the distance between the top of the piston and top of the cylinder.
Check your owners manual to see where this should be.
 

Matt Fisher

Member
Apr 17, 2002
136
0
Over on KTMtalk I've read about KTM's having detonation like sounds that weren't detonation, but are actually from unburnt gas exploding inside the pipe. It usually gets worse with more richer mixtures, btw.
 

swa

Member
Jun 1, 2005
15
0
Thanks for the info. Would the wrong size base gasket make the engine pre-detonate?

I will try leaner jets and fuel, see if that helps. I will also check the distance between the cylinder and the piston. Thanks for all the help.
 

elf

Member
Jun 7, 2003
695
0
Maybe you can explain when its making the noise? Under load? climbing hills? when you let off the gas after a wide open run? Does it make the noise when you rev it with the clutch in?


Pre-detonate........ The noise it makes before it explodes :ohmy:
 

Vince Stokes

Member
May 30, 2002
64
0
I have heard this called preignision or detenation not predetenation,its more commonly called pinging or pinking,
sounds like a can of marbles being shook.Can be caused by too
high compression for the fuel being used or carbon build up,high
compression in a well used motor or baddly considered engine mods eg changes to the combustion chamber,or the timing is out
or the jetting is too lean.First thing to do is to do a compression test,this will rule out too high a compression,at a guess it should be arround 150psi to maybe 180 or so.Have a look at the plug,does it look white or worst case are there deposits, spots of stuff,BITS OF MELTED PISTON,on it,
 

pace

Member
Nov 21, 2003
479
0
Just FYI, pre-ignition and detonation are two distinct conditions. When people talk about pinging here they are generally referring to detonation, which is an uncontrolled/spontaneous burn of the charge AFTER spark ignition.
 

Agie rider 76

Member
Apr 9, 2005
26
0
different sized base gaskets have an affect on the compression ratio. if you have a too thin base gasket, then compression has been increased. if you are running a low octane fuel, then your bike will detonate. check you have the stock/recommended gasket size.
 
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