Hi all,

Bit of an odd one as it's an intermittant fault. I was writing about my plug fouling, particularly when at low revs. I have since swapped to an 8 (hotter plug).

Only ridden once with the 8, and it was very cold temperatures and damp. The bike ran fine first thing, then had a very bad mis-fire, then ran perfect for the rest of the day. It has done this before but I put it down to the 9 plug being fouled.

I've cleaned the carb several time, and never witnessed any dirt at all, always clean as a whistle so don't think it's that.

I wondered could it have been carb icing?? (No carb de-icer on my bike).

Possibly electrical, but why intermittant, what's causing it and what could it be? Maybe ignition coil? ECU is fairly new.

My latest theory is that maybe the head gasket is leaking into the combustion chamber slightly? The coolant level does drop from time to time, and no obvious signs of leaks, and it hasn't overheated that I know of. This could explain it, my mate aid he had a similar problem on a scooter.

Any advice, experience or theories more than welcome.

I'll keep an eye on how it runs, and if it continues then I'll try and get a better description of symptons.

Thanks.

Shaun.

Anyone at all? Last time I went on it for a couple of hours it was fine.

Sorry Shaun. Can't tell beyond it is not the carb icing. Need to see it and experience it as it is intermitent.

What makes you sure it's not the carb icing?

I wondered if it was the float sticking up or something? I did have the float sticking down a few times and pouring fuel out the overflow. I slightly bent the float tang back and it's been fine since apart from this more recently.

I think if it happens again I'll try and make a video if I can.

Cheers.

The times I have seen a carb icing the slide hasn't stuck but there were (sometimes large) build ups of ice on the outside but particularly the inside of the throat. This ice then randomly broke away in chunks causing the engine to momentarily splutter.

Exclusively observed on hovercraft. Two stroke engines, flat out, carb stuck right out in the air flow with little or no air filter. Never seen it on a bike but I doubt it could get as bad as on a hovercraft in the UK as the carb is so shielded and warmed by the engine, rads and exhaust. And NOBODY holds a bike pinned for 10 minutes at a tme............

Other MX and enduro bikes don't generally ice up in the UK so why would a CRM do it

Fair point, it just seems that co-incidentially with cold/damp conditions it can run poor, seem to remember it doing something similar last winter on and off, and also with me not having a carb heater then this could be a potential reason.

Other than that I thought damp getting in the electrics or something? I sealed them all up extremely well upon build, and it has an expensive NGK waterproof plug cap, with a small cable tie where it meets the HT lead to ensure a good seal.

Intermittant faults are rubbish!!

Have you checked the carb overflow pipe is clear?
Where are the breathers run too?
I have my breathers up on the headstock, well away from water, well till I lay it down in deep water .
Mike

Yeah they all seem fine. The breathers run alongside the overflow and down between the dog bones/shock.... no evidence of entrapment!!

Cheers.

What make and quality of plug are you running?
Is worth popping a brand new cheap plug in immediately it starts to fart about next time?
Mike

Currently NGK B8EGV..... Just swapped from a BR9EG See Topic:

viewtopic.php?f=6&t=8638

Seems to run much better with the B8 apart from the described intermittant mis-fire.

Is there anything I should be looking out for if it happens again to help diagnose it better? My mate said if it's head gasket it will chuck loads of white smoke out..... Not sure if I remember this happening.

Shaun.

I'm a great believer in the cheaper plugs, BR8ES, that I run all the time, and have found them less prone to finicky requirements. They also make a good spare at a couple of £s.
It's going to have to be a question of building up a pattern of behaviour to establish the errant component.
Mike

Yes agreed, if it does persist, I will try to take a video and note where when etc.

as I say it happened when last trail riding, first few hours of the day it was fine, running very clean and well, then developed the mis fire, wasn't running right and wouldn't tick over, though seemed fine on fast roads with throttle pinned back (engine under high load). When in neutral and revved hard it would miss quite badly some of the time. Then later on it the day it just disappeared, ran absolutely perfect for the rest of the day.

I seem to remember it doing this a while bac, and when I returned home it was fine, washed it, started up.... fine, then mis foring started. Then suddenly fine when started up again 5 minutes later.

I will try and keep a log/take a video etc if it persists.

Shaun.

My KDX220 suffered from carb icing in Wales on an enduro school, but it cuts all power, stop bike, ice melts within 20 seconds. Only happens at 5 degrees to zero degrees, below that there is no moisture in the air. Happens at low power settings, same as in piston powered aircraft.

I think you have an ignition problem ,try a new coil.

I did think about a new coil, it's running the original '07 CR250 coil.

Would a coil give an intermittant fault? Or would it just break down and/or pack up all together?

Cheers.

Active Rascal wrote:I did think about a new coil, it's running the original '07 CR250 coil.

Would a coil give an intermittant fault? Or would it just break down and/or pack up all together?

Cheers.

Generally, it's a fault of bad running, or total stoppage, only to burst back into life once the bike has cooled.
It's generally attributed to the insulation melting, thus shorting out the circuit,then cooling and re-insulating the circuit, and alls well till it gets hot again.
So though this partly covers your fault, it's not entirely the same, and I had dismissed it as a probable cause, though of course I could be talking utter carp .
Mike