Miss fire/piston change??

Hi there. I really hope you techy guys can help out here!!

.I have a 2005 WR450F.. 6500kms.. i got the bike at 5300kms..

Runs o.k in the mid to high range but, at lower revs or constant lower throttle the bike seems to miss or have a flat spot! Like a hestitation..

The bike also Sometimes back fires from high revs/high speed to shutting the throttle off or gearing down from high speed!!!

The other SMALL problem is the bike doesnt want to start with the electric start once the engine is warm, but kicks easy when warm..

I have had the bike to 2 bike shops(not dealer) and one ripped me off, tweaked the carby and saying was an intermitant ignition barrel problem, got this checked by an auto electrician and barrel found to be fine($250AU bill):tears

2nd shop put new jets in the carby(not sure which ones) , Still have the problem...$120AU..

Finally took it the the yamaha dealer in frustration to get the valves checked... $140AU later the valves are FINE and within spec,

they say that i have 50PSI dry pressure in the piston and 70PSI wet..whatever that means. They say should have 100PSI^dunno

i dont truely trust the dealer tho..

Dealer suggests a rebuild AND a new battery..

Just thinking of a new battery first for the starting issue on the electric start , but what to do for the missing/flat spot issue??

Apparently im looking at around $1000-$1100 for a new piston rings, this doesnt include if i need valve seals or valves for that matter..^frt

There is NO smoke EVER(except it did when i got it back from the dealer after valve check, but soon settled down)

Looking at other posts it seems i shouldnt be ready for a rebuild yet surely??

I have done heaps of water cooled 2 strokers before but the 4 stroke looks alot more complicated and my yamaha workshop manual isnt very informative compared to say a Haynes manual..

I would feel confident doing the piston change if i had better information on how to do it!!!

Any advice on the next step and yours suggestions would be appriciated..

I can get a wiseco piston/rings/pin, on ebay from USA around $180 AU..

Just carnt stretch to $1000 right now...^mad

Do dealers tell the truth??LOL

well the deal must be charging u a fair bit because just before i bought my new wr450 i asked about the cost of new piston and rings and they said usualy $600 and thats in aus

Try disconnecting your TPS, bike was not made to run at "constant lower throttle" and that will change the ignition timing to be more friendly for that mode of operation. If that helps you may want to leave it that way or re-adjust the TPS per the manual.

A little backfire from hard engine braking is normal, if its bad you may be a touch lean on the pilot circuit or your ACV is hosed, or exhaust leak.

With regard to starting, Make sure your pilot circuit is adjusted right (search and you will find the procedure), and try the Blue/White wire mod for the e-start it makes a huge difference on my bike. I could start my bike warm pushing the kicker with my hand and it had trouble with the e-start, until the blue/white wire mod. Also a battery in good shape and full charge is key.

Not sure you can do a proper compression test with the auto decomp on the exhuast valve so I don't think that means much for your top end.

Some good points here and i will try the blue /white wire mod...

i have decided to do the piston change myself.I cannot justify $1000 for a bike shop to do it.... i have done quite a few

2 strokers in my time but never a 4 stroke...

I have ordered my Wiseco 13.5.1 comp, piston/rings, timing chain and valve stem seals from ebay in USA all for $325 all up inc delivery which seems a good price...

I have a service manual, but is not as informal as say a haynes manual so any tips and tricks or how to links on piston/cam chain/stem seals change would be adventagous..

My valve clearences were done last week by Yamaha and are all within spec, does this mean all i have to do is fit the piston and i shouldnt have to take the cams off AGAIN to check the valve clearences? Do you just bolt the cams on after the piston has been fitted and check the clearences at this point?

Any Further advice to tackle this job guys? thanks..Gav..

I have received the new 13.5.1 Wiseco piston kit, top end gasket kit and valve stem seals and cam chain also..

i have looked in the manual at replacement of top end, but it doesnt show how to change the cam chain..

any step by step guide with pics would be really helpful if anyone has tackled a full top end rebuild before?

This will be my first time on a 4 stroke. used to doing 2 strokes in the past..


The manual does show how to replace the cam chain so you may want to look again. The factory manual is kind of weird where things are and how they are labeled but it's a common wear thing and is covered.

Best advice I can give is follow the manual and check everything as you go. A 4 stroke is different than a 2 stroke as far as timing elements and topend stuff but is largely the same thing.

And to answer your earlier question, yes, you will need to recheck you valves as it ALL is taken apart, but more than likely they will still be in spec if it was done correctly.


Oh yeah, SEARCH THIS SITE. Cam chain change has been covered many times which is why you really didn't receive many responses.


i have got to the point of taking off the cam cover..

. When turning the crank once the mark is alighned through the view point it feels that the PISTON is JUST starting to rise up the barrel,doesnt FEEL AT TDC..

this certainly is worrying(you can feel the piston drop obviously when turning the crank after turning several times.) The dealer checked the valves and were in spec , but said there was inly 50PSI compression.. maybe thats why it doesnt feel TDC once the cam marks alighn:confused:..

What is The best way keeping crank alighned once the flywheel is off??

Dont want to put the cams back together throwing something out..

got the head off the bike.next is to take out the piston and cam chain...


The spark plug was more black than a tan colour, may have been a bit rich..

There is Subtantial build up on the valves and piston head...

There is no scratches on the barrel, hardly see any minor scratches either, etill ahs a nice mirror finish..You have to look REAL close the see any fine scratches.. you can see a slight lines near the op of the head where the rings have been sat..





Not sure wether i should touch the barrel or if i should send in the head to the shop to change the valve stem seals..

At this stage im not sure how far to go with the head???

Edited by chiefster1976

How did things turn out? I was just told basically the same thing for my 2006.

new cam chain in... a bike shop said basically if you cannot see any carbon build up when you look through both the exhaust and inlet side of the cylinder (bottom of the valves are clean from carbon build up) then you DONT need vlave stem seals...

I couldnt se any so called cross hatching on the cylinder..more of a mirrored finish, no real marks really just a few up an down... Decided to go with a light hone from a bike shop just for safety really, instead of the scotch brite pad jobby..

There is no way i am putting the piston and rings in DRY/ without a liberal amount of oil..just doesnt seem to make sense!!! After all the service manual says use a LIBERAL amount of oil when fitting the piston/rings..

As suggested by a few people now ill use a mineral oil on start up and a few KMS to seat the rings then go for fully synthetic...

P.S.. I RANG Yamaha shop...Both the cylinder and BASE gasket are to be applied DRY with NO sealant..:thumbsup:o

Ill post more pics as i go along... ;-)

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