Yet another cam timing Q

OK sorry guys but this is frustrating the heck out of me.  I'm working on a buddy's 2005 WR250F.  It had a lot of top end noise so I adjusted the valves (one of them was way loose???  Why would a valve loosen? OK never mind, its in spec now and that's not my real question here).  It was also hard to start which I figured is due to dirty pilot jet etc (this bike sits a lot more than it runs).  Anyways, valve clearances are all in spec now and when I set the timing I can not get the dots to line up as they should.  I also went through the carb and cleaned it all out, set the AP timing.  I put it together with the timing as close as I could get it and it runs like crap - starts right up on the choke now but won't idle, backfires - runs "ok" if you're revving it but won't run 'clean' until you rev it to maybe 2500 rpm - anything less and it wants to die.   I'm not a newbie when it comes to working on bikes so I would think the carb is ok but I'll likely pull it and blow it all out and check passages again.  And no, I didn't look at the cam marks prior to pulling the cams to change valve shims - dumbass move I know.

Attached a pic of the cam timing - I've read enough on here to see that it is timed to be "WR" timing (13 links between marks on the cams) and not YZ timing (12 links).  I have no idea if this is a YZ or WR exhaust cam - my buddy bought the bike used.  No idea how much time is on the bike.  I went ahead and ordered a new cam chain in case this is stretched, but for now, my questions:

1: Anyone know how to ID the cam to know if its YZ or WR?

2: Does this cam timing appear correct?  The flywheel mark is spot on with the notch in the inspection hole when this pic was taken.  See how the intake cam looks like its a little retarded?  I chalk that up to cam chain stretch but what do you guys think?  



WR250f cams.jpg

which valve was loose ?? center intake maybe ?? and how loose was it ?? maybe it is a bent valve.. i have had a number of these bikes with cam chain tensioner failure which can be serious for the valve health -- timing mark alignment is probably due to chain stretch which can "back up" the intake cam when you install the tensoiner .. run a leak down test to check for valve leakage and go from there

I want to say it was the right side intake valve, but might have been the center (I'm old and forget stuff 😀 ). It was about 10 thousandths loose.  I'll try and find my leak down tester and see if that's a problem. See the weird thing is it more or less ran ok before I took it apart to check the valves. A lot of top end noise but didn't run nearly as bad as it does now. And since I cleaned the carb it starts right up on choke, which if it had a bent valve I doubt it would do.  I'll check though, good info, thanks.

I'm guessing that your valves are now cupped and need replacing, and yes cam chains can stretch. I don't now if this has happened to Yamahas but the first Kawasaki KX250f would spin their sprockets on the cams and throw the timing way off. 


How old is the bike... what year?

You would need to google the specs or see if a manual has what you're looking for.

You have the correct number of chain pins between the marks on top.  But you need to check the small cam sprocket on the crankshaft.  A worn cam chain will wear that sprocket.  Pull the left side covers and flywheel.  Inspect the small sprocket for shape of the teeth.  Sharp pointed teeth there indicate wear.  They should be the same shape (profile) as the teeth on the cam sprockets.  Note, a chain will jump time if its worn and take out your valves.  If you find that sprocket worn, your only path to correct it is to replace the crank or that crank half if you can find someone willing to rebuild that crank.  Yamaha made the sprocket as part of the crank half, and that's the only fix.  Honda splined their crank and made it a separate part.

That cam chain is stretched and one tooth out of time. 

I had a 450 with similar issues when I bought it, after a lot of head scratching it turned out the pressed on sprocket had turned on the camshaft so even though the marks lined up the timing was wrong.

After 03/04 (e-start motors) it doesn't matter if it's WR or YZ cam, the timing marks are the same. YZ vs WR timing by rotation the exhaust cam one tooth was pre-autodecompression.  If you attempt to rotate the WR cam the autodecompression will not work properly. 

Edited by tcmII

What you look for on the WR cam timing is the intake dot at the 3:00 o'clock position and the exhaust cam dot at the 9:00 o'clock position.  Those should be where pictured at the joint line on the head.  This is according to the manual.  Profs to TCMII, he is correct.

Thanks for the responses guys.

BoxcarWilly - what in the picture makes you say its out of time?  I get the stretched cam chain part but the timing seems as close as its going to get with this cam chain.

I couldn't find my leakdown gauges so I went thru the carb again (all looked fine) and put it back together.  I had to turn the fuel screw out to like 3.5 turns and up the idle and it sort of seems ok now, but still not "right".  My pal is riding it this weekend so I really wanted to just get it together for him, and I'll tell him that he should take it to a local shop (we have a really good local KTM shop who has great mechanics and they work on all brands) and have them pull the head and check everything out.  If it were my bike I'd do it myself but I have about 6 other bikes of my own that need work so I'm not going to get into this one any further.  It does seem odd that Yamaha wouldn't key the crank sprocket to the crank so it can be replaced.

Thanks again

Edited by bbrownsberger

" I had to turn the fuel screw out to like 3.5 turns and up the idle and it sort of seems ok now, but still not "right""

If you're going to 3.5 turns out for the fuel screw, your pilot jet is partially clogged up.  You need to clean it with a small guitar string or replace it.  If you ride with it over 3 turns out, you run the risk of losing it.  Ask me how I know that....... just saying, save yourself some trouble.

The pilot jet sure didn't seem dirty to me - I soaked it in the good carb cleaner that's illegal in California even!  And sprayed and air blasted it.  I've cleaned a lot of pilot jets - it seems ok.  When my friend hopefully takes it to the shop for the top end, they also check jetting and will set him straight.  

I did always wonder how far out you can turn the fuel screw before the spring pressure no longer holds them in.  Good to know!

OK, then lets look at another lean condition cause.... around the Hot Start.  Pull the Hot Start out, check the rubber o-ring for cuts, and the bottom seal as well.  Make sure its not hanging up and when you release the lever its snapping down solid.  Check the area where the spring is at the plunger and its not full of dirt, and the screw that the cable goes thru at the carb is fully seated.  The stock one is plastic, and I've seen a lot of them stripped out, not fully seated/sealing up.  I've also seen the cable adjusted too tight not allowing the plunger to fully seat/seal; and I've see it where there was no slack in the cable, so when you turn the bars, the hot start opens up.  They make an aluminum accessory replacement for the plastic item.  Beyond that, if your pilot jet is not a 40 or 42, assuming you are at or below 3,000 ft altitude, you could need a different pilot jet, 1 size richer.

OK good info Ron!  The hot start plastic threaded piece does seem like a Mickey Mouse design - its screwed in fine but seems like its only held in with maybe 2 threads.  But, I didn't see anything wrong.  Its not dirty around the plunger , but I didn't check the seals, so I'll do that.

It has a 40 pilot jet in it, and I'm basically at sea level and its cold right now (~40F), so it might need a 42 at this altitude/temp anyways.

Thanks again on the advice - I'll check the hot start and weld it shut if I find anything wrong :)


"I'll check the hot start ..........and weld it shut if I find anything wrong"


I once spent 4 hours cleaning one out that somebody JB welded.  Get the correct parts, or don't do anything to it.  But DONT JBWeld it.!!!!!!!

Edited by ronbuell

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