Odd compression issue... any ideas?

[Mjolnir2k]

I have a 1983 XS400 that seems to be having an odd compression issue.

I just tested the compression and the right side has 125psi, while the left side barely registers (at about 25psi).

The thing is, it is running seemingly fine. I am getting combustion from both sides and it will run with the spark off the "good side", so long as I keep it revved up a bit.

Is there some weird thing about the make of the bike that would account for this odd compression issue?

Any thoughts on what the culprit could be? Is it ok to keep running like this?

Thanks in advance.

 

[drewpy]

I think you will find its not running right.

I had a cylinder go down in early ownership and only noticed I had a problem when it wouldn't tick-over.

 

[Drewcifer]

If you can get a manual and make sure your valves are correctly adjusted you might improve the timing on the cylinder losing compression.

Check your points, and put your thumb over the hole and press as hard as you can. If starting the bike pushes your thumb off the hole, the cylinder will still probably combust.

But it sounds like a head gasket leak on that side. Is there more oil on that side on the fins than the other side?

 

[ThePsuedoMonkey]

I would not recommend continuing to run it, even your good cylinder is low. Put a spoonful of oil into the cylinders and repeat the compression test; if it is much better than the cause is most likely the cylinder rings. Otherwise you are looking at valves, gaskets, or wear to the cylinder/piston. Any way you cut it, you are looking at a top end tear-down.

 

[Mjolnir2k]

I would not recommend continuing to run it, even your good cylinder is low. Put a spoonful of oil into the cylinders and repeat the compression test; if it is much better than the cause is most likely the cylinder rings. Otherwise you are looking at valves, gaskets, or wear to the cylinder/piston. Any way you cut it, you are looking at a top end tear-down.

We did the oil test and got the "good" side up a few extra psi (140 or so) and the "bad" side came up to about 35 psi.

This all sounds very ominous though. Is a "top end tear down" a big deal / expensive undertaking?

I was hoping to have an inexpensive motorcycle and so far I have already replaced the chain and sprockets. Not sure how much more I can drop into this.

Any thoughts on expense for this repair?

Thanks again for all the info!!!!

 

[ThePsuedoMonkey]

140 to 170 is where is should be, so new rings may be a good investment, open her up to make sure though. To be honest a top end tear down is not that difficult, but you will need a fair amount of patience to do it right. Just grab the manual, bag and tag the parts as you go, and take pictures. I was able to (with great effort and patience) remove the head cover without pulling the engine, so it is possible that you can service the top end with the engine still in the frame. I uploaded all of the ones from my rebuild to my profile here, so that may be worth a look just for reference. As far as the expense goes, you are looking at increments of about $50 or so for each thing you need to replace. With any luck, you might get away with just the gasket kit.

 

[drewpy]

might even have a holed valve or piston

 

[Mjolnir2k]

might even have a holed valve or piston

Yeah, I don't even know what that means (though I am guessing it's not "good")..lol.

Updates

ok, so here's some other details from some additional testing:

1. It's not backfiring through the carb

2. There is some leakdown coming out the exhaust

3. A little air is coming out of the intake

4. Both valves are not closing at the same time

The thinking now is that I may be losing compression through the exhaust...

 

[drewpy]

need to do the valve gaps, if that doesn't help you need to take the head off as it sounds like the valves are leaking!

 

[Mjolnir2k]

need to do the valve gaps, if that doesn't help you need to take the head off as it sounds like the valves are leaking!

sounds good, we'll give that a try!

 

[bentwrench]

Yes - check your valve clearances. If you're off, find a friendly neighbourhood bike mechanic and swap and trade until you're within tolerance. My mech charges me $5 a shim for a trade, because he eventually needs to replace some popular sizes.

After that, the head gasket is time consuming but cheap to replace. Rings for the pistons are the same (at that point, you'll need a full gasket set, but that's one of the cheapest ways to get the head gasket anyway.)

It will only cost you big money if you get someone else to do it.

 

[FUBAR]

valve seals and the rings might be gummed up and stuck out of position a bit, same thing i went through, except i only has 25 psi in each side