Hey folks,

So I have a relatively new to me XT 600 that's pretty old but only has about 7000 miles on it. It runs well, starts easy, etc. but it is using a fair amount of oil. I'd guess about a half-quart every three hundred miles. Most of my riding is on the dirt, so it's certainly a hard three hundred miles. I can see substantial smoke coming out of the exhaust for the first five minutes of riding.

I don't mind the prospect of a new piston and rings, but I don't want to invest the time and money if that's not the issue. Is it more likely that the valves or valve seals are at issue; and, if so, how do I go about diagnosing those?

Any advice would be appreciated.

Remove the exhaust and use a endoscope, you can see if oil comes down from oil seals. You can buy cheap ones from ebay and attach to computer, i would also do a compression test.

So big an amount is almost certantly piston and rings, but do the oil stem valves aswell, while youre inthere.

A clear sign of worn oil stem seals is a lot of smoke on cold startups.

with such low milleage bike must have stood still for VERY long periods, so the rings could be simply gummed up in their grooves....
you could lift the barrel and check, if you are lucky, just free them and bolt it back together (with new valve seals too while you are there ), if piston is indeed worn, rebore to next oversize.

There are no half jobs when it comes to fixing smoking engines.

Don't be lazy. Pull off the barrel. It's not much more work at all.

You do it properly or you usually end up doing it twice.

First step is to pull the motor apart to inspect and measure the bore. You're unlikely to have a set of bore gauges in your tool box so ask a local engine place to measure the bore for you. It's not difficult and should cost very little. The sizes are in your manual. You can also measure the the cylinder skirt for the stock size.

If your bore is within limit then have the bore honed and fit a new set of rings.

If your bore is worn then you're going to have to have the bore over-sized and then fit over-size piston and rings. OR, you can fit a new cylinder liner and use standard size piston/rings. Sometimes it can be cheaper.

Then re-build your head with new valve stem seals. Don't use cheap Ebay stem seals. Use genuine or Viton.

If you don't have the tools and experience to do this then maybe look for some professional advice.

I've had a lot of work from people who've tried doing this DIY and messed it up.

Good luck.

__________________
Did some trips.
Rode some bikes.
Fix them for a living.
Can't say anymore.

As a rule of thumb, if an engine smokes on power it's bores/rings. If it smokes on over-run or puts out a big puff of smoke after being on over-run it's valves guides/seals.

Which of these you suspect depends on how the engine has been used in the past. If a previous owner liked buzzing it or didn't use a good air filter or change the oil often enough he could have put a lot of wear on the bore, in which case Ted's advice is good. If you pull the top off you can do rings/bore/guide seals all at the same time. However that said as I recall the big Yamaha singles did tend to use a bit of oil, so if you're using it hard that consumption may be normal.

Thanks for the advice, everyone. I'm hoping to spend some time properly diagnosing this thing.

I figured I'd add a little bit of info based on a recent trip.

First, when I first got the bike at the beginning of the year, I checked the compression and it seemed within spec. That's part of the reason this high oil consumption confuses me.

This last weekend I took the bike on a fairly long trip. Day 1 was about three hours on the highway; day 2 was about 10 hours piddling on county roads. On day 1, on the highway doing about 75 mph, the bike burnt through a quart and a half of oil (I checked it frequently and kept it topped off). On day 2, I used less than a quarter of a quart.

The second day, I rode at fairly low RPMs (2k-3k most of the time) while obviously at highway speeds it runs around 4-5k. I wouldn't expect such a dramatic difference in oil consumption if the sole culprit were the rings, but I don't know enough. This may be going out on a limb, but I wondered if there might be an issue with the oil pump system (pump itself or check valve) that's bringing too much oil in at higher RPMs, sumping, and burning it off. With relatively low miles but old age, I wonder if the check valve has deteriorated. Obviously the previous owner could have abused it and worn out the rings/bore, but I want to rule out other possibilities, too.

I hope to measure my cylinder and check the rings, but I want to know that an engine rebuild will fix the problem before I put much money into it.

You are sure its using oil, not just wrong readings when measure? Only way to be sure is drain oil and measure whats left, mine show min when refilled with 2,8l on warm engine. Startet it up again and let it idle for 30sec and its full on measure stick.

I measure it the same way following recommendations I've seen online. I check it after it's been running for at least 20 minutes; I stop, let it idle for a half a minute, and then check it right away.

Since your highest oil consumption is at higher RPM and high load, it certainly points to a piston/rings/bore problem. If you were to repair that, of course you would renew valve seals at the same time.
One thing you should first rule out, is a possible blocked engine breather.

Bob

__________________
Remember that Assumption is the mother of all f**k-ups.

What colour do you have on the spark plug?

It wouldn't surprise me in the least to have some oil consumption when going 75 at higher rpms for a day , I've heard of that from all singles . The piston speed is high enough that more oil getting by the rings is normal . The usage should go down once running at low rpms and the amount used at high rpms varies quite a bit . Probably one of the #1 reason for cooked cam journal in these big single from lack of oil comes from a day of high rpm running , catches people off guard.


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