LOW compression for no reason at all!

I don't believe it, today has to be one of the worst motorcycle maintenance days i've had in ages! First it started with the blue wire mod, I havn't ridden the bike in 3weeks and thought i'd give it a go. At anyrate it was something that should of only taken 10mins. I was cutting the tape, with a craft knife, that seals the pulled out wire and slipped with the knife, that needed 2 stiches. 2hrs later I got back from the A&E.

I put the bike back together and went to fire her up with no luck whatsoever. I also noticed that it was real hard to find that spot just below TDC using the compression release. Anyway turns out the bike was real low on compression, I could kick it through its stroke with my foot like you'd kick an 80. By now im starting to get worried as one of the best trailrides of the year is tomorrow and i've been looking forward to this ride for weeks. I hadn't started the bike in 3 weeks and found out that the oil runs off the cylinder causing a poor ring/cylinder seal. So I kicked It through and through but no real luck. I kicked like mad for almost 3/4 of an hour when it finally fired. Relief at last.

However after taking it for a ride up and down the street for 15mins I noticed that it was still low on compression and that it was also backfiring on deceleration much worse than normal. It's slowly starting to get its compression back (I cant start it now without the compression release like I could this morning, 03 cam pfft) but I can still kick it through if I try hard enough. My 160lb is easily enough to send it through the stroke and im pretty sure it wasn't enough before.

The rings were done by the last owner in August 2003 when I purchased the bike, i've ridden nearly every weekend since then and probably clocked up around 130hrs of moderate riding. I think I might of had a stuck intake valve that freed up but why is it still gaining more compression and power with every ride? Sould I ride tomorrow? I don't want to risk anything bad happening like bending a valve. I don't know why this happened all of a sudden the day before the best ride of the year condering 3 weeks ago it was fine.

So does anyone have ideas why it's still slightly low on compression?

Thanks in advance.

after I rebuilt my gearbox on my WR I also had a compression issue. Had fine compression 1st start, stopped it, then it had minimal compression. It appears that I dislodged some carbon and it was partially holding a valve open. I mixed up a 100:1 2 stroke mix and idled it on that for about 10 min. Problem solved. At a later date when I had the head off I had the same thing happen. But I cant see why yours lost compression when you didnt disturb the head. You can try the 2 stroke trick, it helps free up sticky valves. To find out for sure where it's losing compression, do a leakdown test. These engines run very light pressure on the springs due to the light weight of the valves, doesnt take much to hold one open.


Do a leakdown test after checking the valves! If you have a tight valve, you will lose compression.

Hey thanks for the help, I'll give the 2 stroke trick a go this morning. Valves should be in spec as I shimmed them about 10hrs ago :)

Hey thanks for the help, I'll give the 2 stroke trick a go this morning. Valves should be in spec as I shimmed them about 10hrs ago :)

should, but that was 10hours ago!

Just curious, is the cam timing correct, or is the chain one or two teeth off possibly.

Oh well, the bike seems to have fixed itself. Came back from the ride and noticed that it was gaining more power as the day went through and now its back to normal. I think it was a stuck valve that has freed itself up. Im gonna pull the head off this weekend and have a look to see if anything happened :)

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