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Compression Drop And Newfound Oil Consumption


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Today, the Yamaha XT600 refused to start. It had sat still for 3 weeks, and I would usually have expected stale fuel. But using the kick starter, it felt like the engine had lost just about all compression. Bump starting in 2nd gear easily turned the rear wheel even when not sitting on the bike, instead of locking up like usual.

 

So I checked valve clearances (OK) and decompression mechanism (detached, but made sure it hadn't moved or jammed) and found nothing amiss. Spark was good. Did a compression test and only got 7 kg per square cm (I think - at least it was 7.0 of something). Not good, but enough for an engine to run.

 

Drained the carb, fitted everything back and got the odd puff, at least. Some pushing finally got it going and the engine sounded like normal. Doesn't seem to miss any power, either, but perhaps runs a bit smoother at low rpm than before. No undue noises, either. Only compression and oil consumption suggest something is not right.

 

Oil consumption was zero for the first 2500 km or so after I bought the bike, then about 1500 km ago it suddenly began using oil. A lot. Like about one litre per 1000 km.

 

My personal guess is a broken piston ring that in turn have scored the cylinder. But I'm interested in other theories. So if you have one - or several - please share them.

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  • 3 months later...

Eirik,

 

I had a look into my old YZF600R service manual, and it recommends to do the following: redo the compression test with a squirt of a few drops of oil into the cylinder. If the pressure rises, the piston is worn or damaged. If the pressure does not rise, the rings, valves, cylinder head gasket or piston is defective.

I would probably do the test twice: first without oil (ie redoing the test you just did) and then immediately after with the oils drops in the cylinder.

 

The test and conclusions should be the same, regardless of the (major) differences of the bikes :)

 

/Kai

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Eirik,

 

I had a look into my old YZF600R service manual, and it recommends to do the following: redo the compression test with a squirt of a few drops of oil into the cylinder. If the pressure rises, the piston is worn or damaged. If the pressure does not rise, the rings, valves, cylinder head gasket or piston is defective.

I would probably do the test twice: first without oil (ie redoing the test you just did) and then immediately after with the oils drops in the cylinder.

 

The test and conclusions should be the same, regardless of the (major) differences of the bikes :)

 

/Kai

 

I'll be interested to hear, I've asked Will to take a look at this.

 

CF

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Well, the engine rectified itself after about 800 miles of riding. Oil consumption back to about zero, compression seems about normal again. My best guess is that an inlet valve was sticking every so slightly, causing reduced compression and also suction that would travel back up the stem and suck in oil. What caused the valve to hang open is difficult to say, but probably soot. According to a German XT forum, it's not unusal for valves to stick when the bike has sat for some time.

 

Oh, and the troublesome starting was primarily down to a sticky "choke" piston - the lever moved, presumably by compressing the cable housing, but the piston didn't, preventing fuel enrichening. With the piston freed the engine again became a quick starter.

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Well, the engine rectified itself after about 800 miles of riding. Oil consumption back to about zero, compression seems about normal again. My best guess is that an inlet valve was sticking every so slightly, causing reduced compression and also suction that would travel back up the stem and suck in oil. What caused the valve to hang open is difficult to say, but probably soot. According to a German XT forum, it's not unusal for valves to stick when the bike has sat for some time.

 

Oh, and the troublesome starting was primarily down to a sticky "choke" piston - the lever moved, presumably by compressing the cable housing, but the piston didn't, preventing fuel enrichening. With the piston freed the engine again became a quick starter.

 

If your valves are getting burnt oil then the top end must be getting very hot, it would be an exhaust that would stick first. Marvel mystery oil has been used as an oil additive for this. Seafoam in the oil will work too but I don't like to put it in the oil myself. Synthetic Diesel oil may be your best bet as it has far more detergent than car or bike oil.

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In this case, it's better sorry than safe :D The bike runs silently, nothing of the typical clatter that comes from most XT/SRX-6 units. Even the 5th gear is dead silent, another known weakness. Apart from the episode with the reduced compression (which hardly seemed to influence power once running), the engine has run well over 5000 miles / 8000 km and I fully expect it to go on for several times that. If it had made nasty sounds or used a lot of oil consistently, I would have opened it - I even bought a full gasket set - but not as it is.

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  • 2 weeks later...

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