sons yamaha aerox refuses to start.new plug and cap/clean fuel and oil.
seems to be blowing back thru carb
anyone got any ideas?
thanks andy

At risk of blaspheming in my religion by keeping another one of the evil noisy contraptions on the road....

Either the reed valve will be at fault or the pipe that carries the charge to the combustion chamber.

My apologies if you're familiar with 2 strokes but behind the carb is a reed block and reed valve (very thin spring steel) all bolted together.

When the piston moves towards TDC it draws the mixture into the crankcase against the springiness of the reed valve. When moving away from TDC the increased pressure in the crankcase shuts the reed valve and forces the mixture through a little bypass pipe (or sometimes channel cast into the block) and into the combustion chamber.

Have a look at the reed valve and block first to check that the valve isnt loose or distorted and seats ok on the block. If that looks ok, find the the charge pipe / channel.

Years ago on an old outboard motor the fault was that the charge pipe had come loose from the cylinder wall and the mixture was basically venting to air.

Hope that helps

Geoff

thanks,will have a look that

Sounds like you need to take the head off and decoke it, reeds may be broken but doubt it does it feel like there is any compression? strokers pop for a past time but beats a 4 stroke any day...it's all about the noise and smell don't yer know....

Is there a spark? And is it in the right place?
The flywheel could have slipped on the end of the crank and sheared the woodruff key.

If it is sparking, is the plug wet with petrol?

have a strong spark ,it's trying to fire but even spraying petrol thru carb's not working,it seems to blow it back thru carb!

Reed valve, reed valve, reed valve.

One/all of the metal strips won't be seating, dirty, or are bent. Maybe the reed block isn't seating

Reeds are critical, I remember the difference Boysen reeds made on my RD250LC.... spent 6 months in hospital after I'd fitted them though (age 17, L plates, immortality, invicibility vs bus shelter).
Always giggle when I think of a Gamma lump going in the Westfield....

I had an RG500 and a pocket full of spark plugs........