Hi and thanks for looking at my topic, i was riding to work this morning and the bike was riding fine then after around 5 mile it lost most of its power. It was strugling to do 25 then when i got near work it cut out at the lights, when i tried to kick start it i got a pop from the exhaust and i think it seems down on compression. I rebuilt the top end 150 mile ago. All help will be greatly appreiciated.

Did you re-torque the top end after the first 1 or 2 spins ?

More info, could be out of fuel for all we know !!

Yes theres fuel going through it starts for a couple of seconds then dies and it dosnt sound right when its running. I dont have the tool to torqe the nuts up but a guy at work thinks it maybe the head nuts have come loose. They seem tight by hand but i will have to get the bike home to check it proppery.

HI Paul, You can tighten the head down TEMPORARILY by hand just remember to do it in sequence, opposite and across and do each nut up just a little at a time, get it pinched enough to run you home and then do a proper job when your there, DON'T be a gorilla at it you just want to get home on it and just tight enough will work for a short journey, hope you make it ok!

I would agree. Get home and check the compression. If you didnt re tighten the head nuts after you'd ridden it in a bit it could easily be a cylinder head gasket leak. I, personally, would have the head off and put a fresh gasket in and start again. 12-13 quid from yamaha for the peace of mind. Just remember to re tighten this time.

I found it nigh impossible to torque every nut on mine due to various gubbins in the way, so i torqued the ones i could and did the others by hand

Yeah you can't get a torque on all of them. I've found that too. Have to just test the torque wrench on another on the bike and then try and match that feeling with a similar length spanner.

Ok lastnight i got the bike home as my boss lent me a wagon, all nuts/bolts were tight but when i took the exhaust of and operated the kick start and watched the piston it was hissing like air was passing the piston. Is this piston rings? Also it will start for a second then the exhaust pops and it cuts out. : (

hissing is normal paul, try another spark plug

Ok will do after work then il report back. Thanks.

Also is the back firing a sign of anything as it does it as soon as it tries to start and cuts out and it back fires when you try to start it if it dosnt start?

Put a new plug in and it wont even try to start, i took the new plug out after trying and it was soaking wet through and it smelt of petrol???

Are you getting a spark at the plug, cap, lead, coil

Check all the way back and then go for the cdi unit.

If any of your mates has a similar bike swap over the cdi and try that.

Yes theres a good blue spark there. Im starting to think head gasket?

Yip sounds like lack of compression, how does the kickstart feel, ? is it easier than usual , if soft to kick over, strip top end,

broken ring, is my guess,

Ok so ive had it all in bits, piston and ring looked fine so i put gasket seal on the head gasket and put it all back together then took carb off and give it a good clean. It felt better but still wouldnt start so i took the fly wheel cover off to find its the woodruff key : ( how do i get the nut off the crank when the fly wheel spins on the crank? Ps i know im an idiot lol.

Bike switched off.

Put the bike in gear and get someone to sit on the bike with the rear brake on. put a spanner on the nut and give the spanner a sharp crack rather than a push to start it off.

You can get straps and other tools to hold the magneto. I have a pair of mole looking grips that have flat ends on one side - to hold the front sprocket - and studs on the other - to fit into holes in the magneto. Like this LINKY

When the nut is undone you will still need to put the magnet/rotor off. There are specific tools but I prefer (and have) a puller set like this LINKY

Ok my mate is going to bring his impact gun home tomorrow s o will do it with that. Where can i get s woodruff key from? Thanks.

The woodruff key is just a small piece of semi circular metal. I don't know why you want to change yours? It is on the shaft where the nut is you are trying to get off. It they break then the engine tends to run backwards. I would guess a new one is a main dealer purchase.

Earlier you said that the plug was soaked in petrol. I would say that the bike was flooding and if you have a spark and are happy with the compression then I would check the float on the carb and if petrol was getting into the engine when the bike is stationary. I would dry and heat the plug up. with the fuel off, try and run the bike.

The woodruff key has broke the flywheel spins on the crankshaft.