2003 KX125 Blowing up

[Rich Rohrich]

Originally posted by bclapham
ps. back on topic, can we assume (sorry thats the second assumption i have made today)!;) that since the head stays are aluminium, that they arent relied upon to take any heat away from the head?

Bruce - That seems like a pretty safe assumption. :thumb:

Sorry I didn't get to your PM earlier, not much time the last few days. I would have just sent you to Eric anyway . I'm way too dense to remember all those interchange combinations. :eek:

 

[marcusgunby]

Rich im looking at the plug now but i need a magnifing glass to see better.One thing i do see is a few(3) white spots(near the electrode). where the colour has been broken through to reveal the porcelain.

quote
How far back on the ground strap does the timing "burn" extend?

Im not sure about the question?you need to learn me some of these terms? as the ground strap is gone im not sure what you mean??


The thing im confused about is i have ran engines to the point of bad detonation for a whole meeting and not had the spark plug break.The piston/head was totally ruined but the plug was intact.

Also the other failures seem to be on the std BR9EIX plug where the electrode nib breaks off-would this be down to detonation? i ask because the engines where this has happened have no signs of detonation in terms of piston errosion.

 

I have shimmed up the engine mounts as these had big gaps on them.Im wondering if the head stays are very stiff it will cause problems.The cr and KTMs are very long.Im wondering if the rm has short/stiff head stays.

 

[Rich Rohrich]

Marcus - I was just speculating and from everything you have posted so far I'm less inclined to think detonation is the root cause. But I would take a close look at the various motor mounting points as a possible source of virbration or weird loads being fed into the head stay and causing a problem.

I'm going to break off a few ground straps tonight and heat up a few plugs just to see if I can find anything interesting.

 

[marcusgunby]

I wish i could read plugs better to give better answers.I just hate not knowing if it will do this again at the weekend.The kxs over here are experiencing some weird problems
eg change a jet(say the main) and the bike just plain refuses to start-you can hear the engine doesnt want to fire(try leaning it over to fill the float bowl).Kick kick for a few mins and all of a sudden it starts up and chugs for a few seconds before sounding normal.
A good few coils/stators and one cdi has been reported faulty.Plus the rash of std plug failures(as many as 3 in one day) and i think the kx has a demon inside.

 

[Rich Rohrich]

Originally posted by marcusgunby
and i think the kx has a demon inside.

... plus it's GREEN :scream:

 

[Eddy V]

I've had the same problem as Marcus with my stock Iridium plug and didn't see any sings of detonation at that moment.We changed to a denso iwt27 plug and at a head check 14 day's later,we clearly saw detonation marks .The piston crown was spotted at the outer 2mm of the whole perimeter.I really don't know whats cuasing that detonation because the compression (valves open and closed) is rather lower than the other 125's.Mine squish is 1mm.Everyone here in Belgium has to use pump fuel/Avgas at 50/50 to avoid pinging.This plug is till now ok.
Another question.My bike has after a good blueprinting job less pk than some stockers.The man from the dyno looked at my cylinder and said that this one was the kx with the biggest champer on the port windows of all the kx 2003 he has seen.Can this be a reason for the bad results.We have also enlarged the boost port at the transition from case to cilinder(the rectangular window at the base gasket)Can this move the centre of the fresh incoming mixture to far to the exhaust port so that there will be to much loss of mixture?

 

[steve125]

EddieV After your plug failure the first time you said you saw no signs of detonation. But 2 weeks later you checked and had signs of detonation. What if any changes were made to the bike from the first failure besides a new plug?......... compression,porting spec,pipe,fuel, any changes here? 

 

[marcusgunby]

Eddy i got some specs for you on the head
1.1mm gap
6mm wide squish
3 degrees more angle than piston crown
same volume as you are running.
This is the most aggressive head profile we can use and not get detonation.This is for using super unleaded 98 octane.So with a mix of av gas you will have a nice safety margin.

 

[holeshot]

I just picked up the R6918B-8 plug intended for the RM125 ($16 at the Suzi dealer). This plug has a "different" design grounding strap - the bottom of the plug has a beveled edge and the strap leaves at an angle toward the center of the plug (instead of up and 90 degrees over). The strap stops short, barely reaching the center of the electrode. The result is a straighter and shorter strap that should be much more resistant to metal fatigue due to "harmonics". The electrode looks a bit more stubby as well. Cheap insurance, but the '03 KX125 also needs more than just pump gas (the RM125 as well).

The detonation may not be audible if occurs high the RPM range in lieu of a low RPM ping (of which there is none on my KX).

 

[marcusgunby]

Well i ended up using a Nippon Denso plug-i will report back how it works.However i am uisng a 99 KX piston and head shape with lower compression so its not going to really prove much unless it fails again.

 

[bigred455]

Any reports in the US of the same findings on the 125? I am starting to think it is fuel related over the pond their.

 

[Mxracer695]

R6918B8 so this plug can be run in the 03 kx125's and is like the same as a br9eix just way better quality and less prone to failure . What about the evx plugs but they stop selling them so it doesnt matter. Another thing could it just be the eix plugs that are poor quality becasue I had one in my old 00cr125 but I usually ran the evx but I couldbnt find it anymore which tom morgan built the motor and I was at the world minis last year and he says if you wanna help your bike take that plug out of there and he had a evx on hand and threw it in for me it changed throtle respnse a lil bit and on another note when i first threw the eix plug in it threw off my jetting a lil bit too and when I had br9eg in there it didnt throught it off that really at all . but at the minis it felt like my bike ran better after the eix was out and the evx was in not lights and day difference but you could tell a lil bit and jetting was better as well and all I used to run in that bike was c-12 at 40:1 motul 800

 

[bclapham]

Originally posted by bigred455
Any reports in the US of the same findings on the 125? I am starting to think it is fuel related over the pond their.

i think its a world wide phenomenon!