RM 125 no power at full throttle

[PCMAX]

Yes the spark needs to jump a min of 5mm air gap, but what I'm asking is WHY do you think it needs to be able to jump a 5mm air gap to be considered healthy.

Lozza, The air gap test is starting to make sense particularly after Pancho's comments "Because the resistance across the plug gap increases in accordance with the increased compression pressure as the volumetric efficiency reaches its max." This sound exactly like what's happening as I can rev the crap out of it in neutral but when it's pulling hard in 2nd or 3rd is when it breaks down.
 
How do I set up for this test? Is it just a matter of bending the spark plug tang away from the center elcetrode by 5mm? or do I need to modify a plug somehow?

Ohh almost forgot to ask. If the spark will not jump 5mm gap what is the fix? which component should I replace?

By the way I have checked the timing with a strobe and set TDC with a dial gauge in the plug hole, I rotated the stator the stator to achieve a firing point of about 2mm BTDC at idle.

[Lozza]

2mm static timing sounds like too much, I posted a ebay link to a air gaps tester .
. Could be any or all of the compnents faulty if you don t get a good jump across the gap. Look for min 110vac off the stator . All testing should  be done at the rpm where the  drama is .

[suzuki27]

I had a problem on an RM125D where the LHS crank main seal was leaking slightly due to pitting on the seal fitting surface.Common problem on the old water coolers. It was picked up by someone who noticed an oily residue around the stator and flywheel- blowing past the leaky seal.
Another issue was to do with the airspace/gap between the flywheel and the inner metal of the stator assembly. Something about it being uneven from memory.
Is the keyway/key in good shape and is the rotor tightened properly?

[PCMAX]

Latest update
 
I attempted further trouble shooting today. Replaced the stator coils with an after market set from Ricks Stator in the USA....problem persists.

Put the old carby back on (the other VM32 made no difference) and installed a 240 main jet. I was previously running a 260....problem persists.

Replaced the 5,000 ohm resistor NGK plug cap with a 1,000 ohm one.....problem persists.

When I open the throttle fully with the bike in neutral it presents the normal problem (ie missing and spluttering) but I did notice some squirts of fuel from the carby overflow. Why is the bowl over filling? Is this "fuelling"? How does it happen & why?

Lozza/Pancho---I bought a plug gap tester from a guy in Brissy on ebay "partszoos" 14 days later no part, no response to emails. So unable to test if spark will jump a 5mm gap

[Curly3]

It has probably been mentioned before but are you using the same carb manifold every time?
It may be sucking air.

[PCMAX]

Yeah using the same carb manifold for each carby. Have had a look at it as installed for splits etc. Might pull it off the bike tomeorrow and have a closer look.

[bazza]

still thinking seal

[Lozza]

Just start a PayPal dispute and state in 72 hours I'll start a claim. That gets their attention. Have you made a plug chop and posted a pic of the plug and piston crown?

[PCMAX]

Bazza, I did think it might have been the seal at one stage but I have replaced the right hand crank seal and there is no evidence of the left hand seal leaking. The flywheel casing is completely dry.

[bazza]

PCMAX this is real brain teaser,one day you will laugh about it.
Have you sprayed around carb inlet boot to see if it is sucking air from tiny hole/slit?
Has the bike run ok for a few meetings then started to miss one day? or did you buy it and it was lke this?

[dalesween]

Hi there,my KTM had some different looking type of fuel line when i bought it, it looked more like car type fuel line,any way the first few rides on the bike were not too bad but it wass missing a bit before entering powerband,i did'nt ride the bike much but everytime i did the bike got worse, in the end the bike would hardly run and was flooding real bad so the carby was pulled down,the culprit was the fuel hose had started to delaminate on the inside and some black gooey gunk was around the pin that goes holds the float on and the float was sticking open,after cleaning everything out the problem was solved and the bike runs perfectly now,i know your problem sounds more spark related but hey,anythings woth a try at the moment.

cheers Dale.

[tmman]

don't just spray round inlet for leak do base gasket n crank case gasket had one that was leakn/sukin right below middle of crank

[oldfart]

While it is running spray some aerostart or metho around manifold and gasket areas ....if there is a pick up in revs investigate.

Tmman  beat me too it.

[PCMAX]

Well well well, Bazza said that one day I would laugh at this.......

After several weekends of replacing components and trouble shooting along with a good deal of head shaking and swearing, today I firstly tried a hotter plug B6ES but that didn't make any difference. Reluctantly I pulled the carby & barrell off AGAIN and checked the reeds AGAIN and had a good look at the rubber inlet boot for cracks etc. Found nothing.

Whilst I had my head jammed inbetween the frame backbone and the now exposed piston and crankcase I happened to look rearward at the hole in the airbox. Some of you may know that the early RM models have a (approx) 10cm round plug in the back of the airbox hard to see as it's just in front of the rear shock spring. I have no idea why it's there as it seems to serve no purpose. Anyhow it had become dislodged and was rattling around inside the airbox.

The bike ran fine when not drawing much air but when I hit high revs/full throttle it was being sucked forward restricting or blocking the airflow to the point where the bike ran like sh*t. So after pulling one side off the airbox and resecuring the plug at the back all my problems seem to have been resolved. She goes like a scolded cat again......Yeee Hawww!!!!

Thanks to all of the contributors to this post for your suggestions I've certainly learnt some stuff. Now where's my glass, no make that BOTTLE, of red.

Until the next major drama.....

Peter

[Curly3]

Well done, more often than not it comes down to it being something simple.
Happy riding.