Emergency problem possible blown piston?

I think i have blown a piston? (Told you i would be back) Anyway i was racing some 125 today down a dual carriage way and held it at full throttle probably longer than whats healthy.. anyway it was fine and seemed quite healthy untill about a mile down the road after driving at 40mph for ages i heard a "poof" sound and the engine was running but sounded boggy as hell and i lost drive. I cut the engine and gave it a check over.. I found my HD lead had blown off and my mate who was behind said he saw a huge spark under my bike. I attached the lead back on and tried to start it.. I only have a kick start and i had compression but it just wouldn't turn over? I don't see how it could be the piston but i will take a look tomorrow.. If its not what else could it be?

 

If the HT lead was off, its more than likely the spark under the bike will be the ignition arcing from the cap to the nearest metalic part...it'll jump a fair distance too. As you say tho' it could ALSO have nipped up. Ideally need to do a compression test. It needs to be 100+

 

but would that prevent it from starting when the lead was reconnected? And i just ripped apart the block and the piston is fine, the rings are fine and the barrel is fine.. I was expecting to find either a hole in the piston or a scored barrel but its all fine. What do you think it could be?

 

you cheaked the spark maybe you blew the coil or somthing ?

 

I checked spark.. Held the plug against exhaust and it sparks fine? I'm beginning to get a hate for aerox's :/

 

With CDI ignition they dont like it when you crank the engine with no plug connected to the ht lead and the plug earthed. You may have fukd your CDI. Also sounds like pre-ignition or detonation. With the cylinder off was the crank turning round nicely? Rod not bent?

 

Also check your reed valve and crank seals are still ok.

 

Ahh shit okay yeah the crank is fine.. Rod is true. Okay i will check the seals and reed valves sonn i will get back to you!

 

So if you've tried the plug in the lead against the barrel and theres a good spark, fuel getting through (is the plug wet when its in the barrel and engine cranked), theres good compression, that only leaves the mixture....which as said,... could be seals gone or reeds chewed. Without it being in front of me i'm running out of fresh ideas

Let us know how you get on

 

Reeds are okay but not checked seals yet, Fuels getting through fine, My exhaust gasket has gone so im getting one made tomorrow and then i will rebuild it and try again. Its really stumped me, It sounded like it back fired when i was riding.. I was going along steady on throttle about 40 mph and i just lost power the revs dropped and i head a "foop" sort of pop noise. My mate behind said it was a bang but i think he may be exaggerating and there was millions of fireworks going off at the time lol, Thanks for all the help i will get back to you when i know more

 

Is your bike air cooled or water? So the piston was mint? no cracks broken skirt etc. Plug looked good? No missing electrode or damage. If it wouldnt turn over at all your variator my be jamed. Pop off your cluch cover and check it all out. Check the kick start gear etc.