No Compression? Burnt Valve?

I've just reassembled my Cams after reshimming and during the process of aligning the flywheel up to TDC by turning with a socket on the nut, i noticed that on the compression stroke it was very easy to turn, although there was a bit of compression it was still easy enough to rotate the crank with one hand while with the other holding a torch on the alignment marks to check for TDC. I could also hear a slight hissing sound which sounded like it was coming from the exhaust side of the head, indicating a stuck valve that wasn't seated correctly. it was possible to go through the complete cycle, compression/release/exhaust etc by hand, never once having to relieve any pressure via the de-comp lever? Now i know this shouldn't be, as it would be nigh on impossible to do this by hand if the combustion chamber was sealed properly? so, have i a sticking exhaust valve or is it more likely to be that the compression gases are escaping down past the piston rings? How costly is this to fix, i know it's a "head-off" job to check the valves but what about repair costs, is it going to be expensive do you think or is it a matter of re-lapping the offending valve? the cams are aligned correctly, facing away from each other and the marks are at 9.12.3 and all valves are closed and clear of the buckets, also the de-comp lever is seated correctly and not contacting the far exhaust bucket with a little play in the lever. Could this be a burnt valve from having too tight a clearance between the cams and buckets, prior to my re-shimming them? I thought in my last few rides that it felt like it had lost power but then i'd presumed it was my "being used to it"......

Also, another thing i noticed while feeling the pistons movement up/down the bore while turning the crank was that it felt very easy to turn (as mentioned ealier) and would tend to "jump" past TDC, just a fraction and would always be out of alignment which would require going round again to try to line up the marks? is this an indication that i have a lot of play in the needle roller bearing or is it much worse than that, big end bearings or worn crank? It doesn't show signs of burning oil so i'm presuming the big-ends ok, but without wanting to completely strip the engine down i'm not 100% sure i am correct that it is?

How hard is a sticking valve to fix?

before i decided to re-shim, the bike ran good and started pretty easy and the jetting seemed ok if not a little rich? if the black sooty deposits on the rear fender are anything to go by. The plug also seems to be good too, not oily or black? It's just this ease of pushing it through the compression cycle that is worrying me, it shouldn't be THAT easy should it?

thanks for any help guys.

I would suggest that you check to see if the alignment shim that fits between the cam bearing and cam holder to align the cam with the cam chain, hasn't slipped out and got jammed between the cam and a valve bucket.

That would hold a valve open and cause a loss of compression.

I have heard of it happening.

Before you tear into the engine, try pouring a teaspoon or 2 of engine oil into the cylinder through the sparkplug hole. Spin it over a few times to coat the cylinder with oil and then pop the sparkplug back in and check for compression. If your rings are bad this'll temporarily increase the compression and you'll know it's not a bad valve.

It may also be that the oil washed off the cylinder walls. A few times I have had little or no compression when I tried to start my bike. I was sure it was a bent or burnt valve. A shot of oil in the cylinder brought the compression back and it started right away. Check for the simple stuff first.

yeh, i was gonna do the oil in the plug method tomorrow to see if there's an increase in compression? but i'm about 95% sure it a sticking/burnt valve as i'm pretty sure the hissing sound i'm hearing when on the compression stroke is the gases escaping exhaust side? coz if i put my ear to the manifold it sounds like it's exhaust gases, if you understand what i mean? sounds like it's in the pipe, but then again if it was being forced down passed the rings that too would make a hissing sound? hmmm? guess i'll find out tomorra when i do the oil trick? how hard is it to fit new pistons in these? Costly i bet?

Also, i checked and double checked the valves/clearances and they are now ok and within the tolerances, and made sure the cam clips were inplace too, so it's not one of these that's sticking a valve....

come to think of it? I'm pretty sure it was relatively easy on my foot when kicking it over before i even touched the valve train? So that may end the "chamber wall dry" theory, as i rode it everyday, and it did seem easy to turn over even when warm? Hmmm? looking towards either a sticky valve from riding with improper valve clearances (too tight) or my piston has had one thrash too many?

Was planning on maybe doing a full stripdown and check over the winter months anyway, looks like it's no longer an option now....... :banghead:

thanks guys for your input, i'll let yer know tomorrow about the oil-trick...

Also, i forgot to mention........

have any of you ever heard your yammy fart? No seriuosly! today i could've sworn i heard it break wind while i was getting the cams aligned at TDC? not from the exhaust pipe, but from inside the engine (to those who haven't read the full post? the engine was off at the time) i thought i was hearing things!

Could it be air escaping down the barrel and making the farting noise through the oil? :banghead:

We all know the bicycle-pump trick now don't we, finger over the end when there's compression? was my favourite trick while at school :applause:

what year is your bike?? Does it have an autodecompression cam? If so, what u r describing is normal

sorry, i should've mentioned this ealier.... It's a 2002 WR426cc and it doesn't have the auto decompression just manual and standard Camshafts.

I have heard of this happening with bikes that don't stop at TDC like the WR does, the valves are left open and condensation forms on them or the valve seats and rust spots appear very quickly (look at the brake rotors on a car they rust over-night) which will not allow the valves to shut/seal properly thus losing compression, that's why it's important to plug the pipe on bikes that don't stop at TDC.

this exact thing happend to my 2002 yz 426. 1st make shure theres plenty of oil to seal the cylinder walls. then make shure that the cams are in PERFECT alignment. i tore back into mine and cams were a tooth or so off. thats is what was causing mine to do it. then after that kick it over about 10 times after all of that it should regain regular compression.

I did try it with a bit of oil in the spark plug hole but there was still not much compression? some, but not enough to have to release the pressure via the de-comp lever, and when i listen very closely i can definetely hear a hissing sound internally, but can't determine whether it's a leaking valve or blow-back on the piston? I made sure both cams are clear of the buckets when on the compression stroke and the cams are aligned, although the exhaust is YZ timed (12 cogs). Cam chain and guides seem ok too.

I have since stripped down the engine today to check the piston/bore, and have found more dilemnas? see here:

Tomorrow i will buy a valve spring compressor and check the valves thoroughly. I tried to do a leak test on them by spraying some brake/clutch cleaner in the ports to see if there was any signs of seepage but couldn't see anything leaking through? Also, i noticed if i put my hand over the inlet port, the vacuum created when the crank was on it's downstroke was very strong and sealed and retained the vacuum (it was trying real hard to devour my hand!) :lol: so maybe i was wrong in thinking it was the piston that was weak? Could it be the head gasket, but then again there is no signs of that in the water?

It does feel kinda "sloppy" too when just coming up to TDC? it'll sort of snatch past it and then you have to go round again. That is what made me think it was the Gudgeon pin (not needle roller bearing that i originally thought? Found out today, they DON'T have one!) :banghead: and on testing the cranks free-play? it's ok and within the specs in the manual for the side lash but if i waggle the very top (piston removed) it does waggle a bit and makes a slight knocking noise like the big end bearings gone, but without one of them specialist tools like indicated in the manual i can't get an accurate tolerance measure to compare to?

Hopefully tomorrow when i can see the valve faces i'll be more enlightened as to where it's losing the combustion? :applause:

thanx for the input guys anyway.

this exact thing happend to my 2002 yz 426. 1st make shure theres plenty of oil to seal the cylinder walls. then make shure that the cams are in PERFECT alignment. i tore back into mine and cams were a tooth or so off. thats is what was causing mine to do it. then after that kick it over about 10 times after all of that it should regain regular compression.
trust me just because it looks clear dosent mean its not pushing down on the exhaust valve. it has to be in EXACT timing it cant even be a centimeter off on ANYTHING. i didnt think so either till my neibhor prooved me wrong. also like it says in the owners manual kick it over a couple of times. i have no idea why this works :banghead: but mine was doing the exact same thing, the hissing , no compression etc and it fixed mine

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