bike runs with spark lead removed....?

aleks82 · Post by **aleks82** » 23 Jan 2007, 01:26

its an mkI 250
recent rebuild
ok so i start it up and after it runs for a bit the rev's jump up and the bike idles on medium rev's

so i flick the kill switch and the bike still runs
so i kill the fuel and remove fuel lead all together from tap to carby
but still runs coz of fuel in bowl
so i disconnect the spark plug cap from the spark plug itself and the bike still keeps running until there is no fuel alltogether!?!?!? (40 - 60 seconds)

how the hell is the fuel still igniting?
any ideas?

its done this twice and on the second time one of water hoses(the one running from radiator to the carb) popped off and squirted all the water out, luckily the bike just ran out of fuel at this point so no seizure

Post by **Jonorow** » 23 Jan 2007, 09:09

I think the technical term is dieseling, as the revs are so high the plug is heating up and igniting the fuel under prressure, the only way to stop it is to let it run out of fuel, or put it in gear and hanging on to the brakes dump the clutch ( not the best option). just flick the kill switch and leave it, it will die after a few seconds. Jon

aleks82 · Post by **aleks82** » 23 Jan 2007, 13:42

ahh ok
but the thing is this is a re-occuring thing
any idea what could be causing it?
i was thinking maybe mixture is to lean and causing it to overheat ?
or maybe water isnt flowing through engine?

the thermostat has been removed so the engine should be cold
(temp sensor and all other gauges have been removed so cant telll if its overheating but engine gets real hot afterwards)

this happens after like 5 minutes of sitting there idling with the occasional rev..

Banshee · Post by **Banshee** » 23 Jan 2007, 14:49

Jono's spot on with the dieseling.

You have a air leak somewhere causing it to run lean, make sure all your manifold gaskets etc are ok and check fuel lines for ar leaks as only a small amount of air can affect this.

Only way to stop engine is by stalling, cuuting off fuel or air supplies.

d5aul · Post by **d5aul** » 23 Jan 2007, 18:13

I have heard that this can sometimes be caused due to carbon build up within the combustion chamber, a small partical of carbon can get very hot & start glowing red causing an ignition source to the fuel entering the engine.

mikey11000 · Post by **mikey11000** » 23 Jan 2007, 18:59

Just wondering what plug you have in? also i agree with previous posts, my first thought would be air leak on a 2 stroke ,check rubbers.also a carbon build up can glow and get hot or ignition timing but that should not be an issue.My moneys on air leak and running hot,check plug has right heat setting sometimes it can cause them to run hot .

mikey11000 · Post by **mikey11000** » 23 Jan 2007, 19:02

Forgot to say i had the same prob on an air cooled 2stroke and to shut it off fast just cover up the exhaust

aleks82 · Post by **aleks82** » 23 Jan 2007, 22:08

plug is a ngk BR8ES
there wouldnt be a carbon build up coz its a fresh rebuild, hasnt even done 100m yet so all clean inside

just checked the spark to, its quite white/grey so looks like its running real lean

ill start the search of an air leak

thanks for advice !

Post by **madmanreevo** » 23 Jan 2007, 22:11

umm sounds like mikey on the ball