backfiring, no power, no speed help! gilera ice 50

i was riding to a party the other night and moped started backfiring and spluttering, it was working okay for the first 10 miles or so then it started spluttering and i lost some power and got the odd backfire, 5 more miles i got into town and went for a mcdonalds not thinking anything of it... jumped back on it and it started but was spluttering but bearing in mind i was a good 15 miles from my house i decided i had to soldier on, it was spluttering and backfiring very often and it just stopped and wouldn't start again got my gran dad to come pick me up in the van, got it home and i was well pissed of... so i set to work in the morning... i got it started with some jump leads and a battery but it was backfiring badly and it wouldn't ride... so here is a list of things i have ruled out... cleaned and drained the carb, tried my spare carb... nothing... checked the spark and it was fine but i double checked with my spare cdi but it was still running the same, tried my other spark plug too, no different... checked my compression (not properly) i put my finger over the hole and turned it over and it blew my finger off... but as i said it starts and normally if the compression is bad on a two stoke it wont fire up at all... since then i have completely removed the engine to take the cylinder head off to take a look at the piston and heres some pictures...




just a cheeky peek at the bike with no engine in...

just a picture of the engine...

right, this was the state of the piston, black as your hat.... got to be about due a new one judging by the piston in my spare engine...


so the plan is do a top end rebuild and hopefully this fixes the problem but im not 100% sure it will... any sugestions? can anybody advise me? also there was no scoring on the barrel so theres nothing wrong with that... thankyou!

 

looks to be running rich. Whats the air filter like?

 

the air filter aint that bad, its a little mucky but ive seen worse... and its the fact thats its been running fine for the 2000 or 3000 miles ive used it thats baffled me!

 

also we can rule the running rich out because i tried my spare carb which has been correctly set up and it didn't fix it.

 

as a cheap option and to rule things out i'd try a NEW plug and check the plug cap n HT lead

 

done that... well i changed the cap and ht lead and put my old plug in which was working fine when it came out... but thanks for the suggestions, if you have any more please go ahead! also i was just thinking, if it has been running rich for ages and its left the piston black then perhaps it wouldn't make a difference if the mixture was turned down? i got some money for putting new gaskets on my mates bike anyways so i think im going to go ahead with the top end rebuild and adjust everything after...

 

quick question... once ive changed the piston ect and got it all back together, do i need to do a run in?

 

if you're honing the barrel then yes but if its just piston and rings then take it easy for 40 miles or so and slowly build up. It'll give the rings a chance to wear to the barrel

 

sorry i dont know what honing is but im just doing the top end bearing, piston and rings.... thanks mate, il be sure to take it steady when i get it built

 

Honing is a process of rotating a set of special stones in a drill press and running it up and down the bores....a bit like a rebore. But honing only removes a VERY minute amount of material...enough to remove existing glaze from the bore and taking off any high spots. It gives the new rings a chance to wear to the rougher new bore sides

 

Have you checked the woodruff key/flywheel on the crank? If it was back firing then ignition timing could be out.

 

thanks Stig! learn something new everyday! and no i havent Marsh, but i wouldn't even know how to check that because ive never done ignitions... how would i check ?