Mechanical or Electrical problem - Help

Dear All, during the last few races/enduros on my EXC525 (03) Ive suddenly lost power and ended up not racing. It seems to just suddenly loose power when Im into the first lap, almost as if it gives up when raced hard/warmed up hot.

The first time it happended I checked everything - found no problems and the garage checked the valves, which did need adjusting. So I put it down to that. The second time it happended, just two races later, again I thought it was the valves, the garage had a look, made a very small adjustmet and I thought, oh well, valves again, perhaps they werent spot on the first time. The third race, I didnt even do a lap, lost power, ***ing valves I thought. However, this time I did the valves. Did the next race, same problem, took it to the garage thinking Id not done the adjustment correctly but the garage checked the valves and said they were fine.

So perhaps it was not the valves after all. Could it be somthing electrical, or somthing simple, like a blocked breather tube ? I have stripped it each time and checked everything, but perhaps someone out there with more experience can point me in the right direction.

Im getting desperate and have a race this weekend !

Cheers, Ainsley

How long do you have to let it cool off before it runs right again? It could be as simple as the gas tank pressurizing. Take the gas cap off and pull out the gasket. Remove the check ball. Or maybe the 03's didn't have that? Check it anyway. And if you have any of those one-way, no spill things in your cap vent line, remove it.

Interesting advice, it seems that it dosent take long to run again, but by that time, ive given up being competitive in the race. I will check/remove the vent and ride it to work and back this week and see.

Thanks for your advice.

it wouldnt have been the valves i dont think but is it overheating???? and does it lose so much power that it stops???

Hi, no, its not overheating. Ive got a fan fitted anyway. It just looses power and stalls. Then wont restart. This seems to happen about a lap into the race. Not really too hot, just say 20mins into it.

It then goes again for a short while and repeats, so by then Im too mad and too far behind to want to continue.

I thought it was valves as they got warm then the gaps closed up, but it now seems thats not the case and possilble wasnt the case the last two times !

So, its either electrical, or as the previous post, simply a fuel flow issue. Unless anyone else has a suggestion ??

sounds like a tank vent to me..

I've read in Transworld motocross some of the '03 RFS had faulty spark plug wires, replace the plug, cap and wire, then see how it runs.

Can the black high tech boxes overheat on these???

HI, OK, I will fit a new plug cap and also clean/check the starter button and clean/check the vent hose. Then hopefully I will have no excuse now but to finish the race.

Thanks for your help and advice.

Clogged fuel filter!

Happened with my ol' two smoke 440. It would run fine in the tight stuff, but when I would open it up, it just die after a while. Then of course start up fine. It was a PITA to diagnose.

Oh yes, good point, somthing else to check.


First thing like Jeb said is the tank vent! There are two thinks that can happen hear;

1: Is not let air in to replace the gas so the gas flow slows to a trickle.

A test that is probubly redundant IF the tank vent is OK but easy to do, remove the fuel line at the carb, hang it into a can, turn the petcock on , note the rate of flow and let it run for a few miniutes, then recheck. if it slowed down, then remove the gas cap and see if it speeds back up

2: Is not let air out as the gas expands with engine heat so it developes fuel pressure, sinks the float and floods the engine. This is most likly to happen in challenging slow going with little air movement to cool and small throttle use so the gas expands faster than it burns off.

Take a spare spark plug on your next ride. if it does this again, pop the plug cap off, stick the new plug on the wire cap, lay it on the head, hold the compresion release open and spin the starter. If the spark is blue its OK, if yellow you may have an electrical issue.

The original plug wire & cap are first class componants, however some people pull on the wire to get the cap off the plug and dislodge the wire from the cap! The cap has a wood screw molded in that goes into the center of the wire. While its off hold the wire and screw on the cap a couple turns.

I fixed the bike - thanks for the advice. I also checked the carb breather hoses and two of them were blocked with mud ! The other two I had routed up and into the air box.

The fuel filter was clean, but I did remove the float ball from the tank filler cap and junked the one way hose thing I had fitted to it as well for good measure.

I fixed the bike - thanks for the advice. I also checked the carb breather hoses and two of them were blocked with mud ! The other two I had routed up and into the air box.

The fuel filter was clean, but I did remove the float ball from the tank filler cap and junked the one way hose thing I had fitted to it as well for good measure.

My clogged fuel filter looked clean too! Try blowing air through it.

Did you bleed the coolant? There is a bleeder nut under the hose in the front of the engine. If there is air in the cylinder portion of the cooling system the motor can overheat no matter how cool the rads are.

Could there be water in your float bowl? I've been screwed out of some good races because of this. It doesn't take very much water.

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