valve clearance do i need to adjust after upgraded piston ?

[adam1471]

yamaha ybr 125 . just upgraded the piston to a 150 kit do i need to adjust valve clearance or any thing else i need to adjust?

[Pete.]

Yes you need to upgrade your driving license.

As far as the engine goes - since the YBR is overhead camshaft it's not likely that the valve clearances have changed but it's such an easy job to do especially with the engine out of the frame it really doesn't make sense to not check them at least.
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132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Ariel Badger]

It is OHC so the clearances will not have changed.
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[adam1471]

its just iv rebuilt it and timing back and all it seems to do is backfire even with the cam chain ofset by a tooth ether way of the mark ? just cant seem to get it to start

[Ariel Badger]

Has your attempts at getting the timing right caused your piston to kiss a valve and bend it?
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[Ichy]

I'd do a compression check first to make sure there are no leaks.

Have you checked the piston position in relation to the the cam timing? Has the piston changed the compression?
Have you tried starting it with something like easy start?
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[Teflon-Mike]

Hmmm....

I like to start with the 'dumb' questions to start with; every time I think "Surely NO-ONE can be that daft!" all too often, I'm proved wrong.. the can, and more.

'upgraded piston to 150cc'..... that would sort of suggest, fitting one that's about 5% bigger in diameter than 'stock'; stock I'd imagine to be about 56mm, so, the 'big-bore' piston would probably be about 58mm-ish... which OUGHT not fit in the standard engine.. the cylinder would have to be made bigger, or replaced with a bigger one, to accept it..

Which begs the 'dumb' question, you DIDN'T just buy a bigger piston, and try sticking it in your engine did you?

'Cos more usually some-one would say, "I have fitted a 'Big-Bore-Kit' or say "I have re-bored the engine to 150cc" rather than 'fitted a bigger piston"

Opens a whole can of worms, like did you fit piston rings? Big-bore piston without rings would possibly fit in a standard bore.. just wouldn't seal very well...or possibly a tad too well, depending on the clearances!

As to the 'symptom' it not starting and back-firing... lots of other stupid stuff would beg investigation long before I started thinking it may be somethg to do with the piston...

Why did youneed/want to fit a new piston? Was the engine buggered before hand? If so, new piston NOT 'fixing' the buggered engine.. WAS the piston the problem?

Have you got a mashed valve? Broken rocker, fucked cam? Was the cylinder head gasket knackered.. THEN.. when you fitted ew piston, you DID use a new head-gasket, didn't you... you DID, PLEASE say you used a new head gasket!

What about exhaust gaskets? Carb gaskets? Did you pinch the carb rubber? IS the carb-clamp actually done up?

Have you fitted the carb slide the right way round? What about the plate in the slide that presses the main-jet needle down.... that is seating, isn't it? Needles not flapping up and down i the jet, or dropped i it, cos the c-clp got pinged?

Were the spark-plug threads stripped in the head at some point? Has it got a new plug? When you put the new piston in, did you use a light lube on the piston rings for first 'start'... has that il burned off yet, fouling the plugs?

The list of dumb questions is very very log and tortuous.. like my posts... and most of them are 'just' common sense... and starting over, at the beginning, and working through the list of easy and obvious, towards the involved and obscure, eliminating possibilities in a logical manner, rather than stabbing wildly in the dark, making guesses and presumptions....

which is all we can do adding to the Brownian motion, random notion generator, as WE don't know the story behind this here engine, or have it in-front of us to poke and prod and swear at.. that be your job!
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[MarJay]

for the benefit of tef tl:dr people, tell us what you ACTUALLY did to the bike, what parts you changed and the process to ensure the cam timing is correct.
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Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[Teflon-Mike]

[Pete.]