Looks like its top end rebuild time!

[pjm204]

Well the GS400 I bought about a month ago seems to need a rebuild. I just compression tested it and one cylinder was 145 and the other was 120. I put a bit of oil into the "bad" cylinder and the compression rose to about 160 or so. The one that is 120 blows a good amount of white smoke until the bike warms up. What parts should I get before I start tearing it down? I suspect I might want to do new valve seals, piston rings, head gasket.... what else should I get? Thanks for the input guys.

Also, it has a knock that sounds like it is coming from the intake end of the left cylinder up top. Could this be a bent valve?

[ryonker]

I am not sure, but your numbers don't seem bad. What does your manual say? I think the specs on mine are like 128 or something. I have heard that anything over 100 is still ok and one of the GSers down under said that they don't rebuild theirs till they get below 70 psi. What are your valve clearances?

Rick

[chef1366]

If your valve was bent you would get closer to 0 compression.
Did it sit awhile before you fired it up?

[SqDancerLynn1]

Start by adjusting the valves, I would not jump into a rebuild until the motor has run at least 1/2-1 hours. You have the right idea, but you need to take it apart & check the pistons / cylinders before ordering parts.

[rustybronco]

Well the GS400 I bought about a month ago seems to need a rebuild. I just compression tested it and one cylinder was 145 and the other was 120. I put a bit of oil into the "bad" cylinder and the compression rose to about 160 or so. The one that is 120 blows a good amount of white smoke until the bike warms up.

Also, it has a knock that sounds like it is coming from the intake end of the left cylinder up top. Could this be a bent valve?

Which cylinder has 145 and which 120 psi ? (left/right)
white smoke is steam (water vapor on an air cooled engine), blue is oil burning, black tint is a rich fuel mixture. look at it again carefully.
what did the cylinder with 145 psi do when you did the "wet (added oil)" test?

most importantly... what are your valve clearances?
edit---(sorry sdl1 I was typing when you posted)

[pjm204]

From what I know the bike sat for several years before I got it. I bought it from a guy who fixed up motorcycles so I have no idea what he did before I got it.

I adjusted the valves when I got it, changed the oil and then rode it about 20 miles before changing the oil again. I've ridden it about 30 miles. It smokes heavily upon startup for a few minutes until warm.

The left side is the one with lower compression, this is also where the sound seems to come from and is the one that smokes heavily. The left side is also where the vacuum line goes into, and my vacuum line doesn't seem to be super tight.

I gapped the points but I am sort of clueless when it comes to timing, it seems to be missfiring a little bit.


I set the air/fuel screws to 1 1/4 turns out. There was no air filter when I got it so I have some uni filter foam oiled in the box, no frame though. Plugs seem to be relatively black, left one is sometimes wet looking.

[chef1366]

Wet with oil or gas?

[pjm204]

Not sure, I'll have to go give them a smell. If the vacuum line is loose could this be causing it to draw fuel in?

Either way, the compression test points to needing new rings.

[psyguy]

i would make sure the carbs are properly adjusted, then ride it hard for a while, then re-check the compression

but if you're very keen to take it apart, sure, it wont hurt anything

[pjm204]

could riding it suddenly change the compression? I'm just a little weary to ride it much with how much smoke comes out.

[chef1366]

The compression numbers are good. The one that is lower is that the one that the vacuum line for the petcock hooks into? If you have a bad petcock the fuel will be sucked into that cylinder and wash out the cylinder wall. Change the petcock and ride it. The rings will seat which will raise the compression. If that's where the trouble lies.

[ryonker]

Maybe a senior member can help me (and you) with this, but I don't understand how your numbers point to needing new rings? Is it the differences in numbers between cylinders?

To quote my Clymers; "When interpreting the results (of the compression test), actual readings are not as important as differences between readings. Cylinder compression should be 128 to 171 psi. A reading below 100 psi indicates an engine overhaul is due. Maximum differences between cylinders is 28 psi." (p. 48). This is for my 79 GS 750.

[psyguy]

keep an eye on the oil level, but other than that you're good to go
when the compression readings consistently show values below a 100psi (with the valves in spec), yes, time to start thinking about a rebuild

[psyguy]

could riding it suddenly change the compression?

you may have a stuck ring or two, from the bike sitting a long time
riding it may help loosen the ring so it can seat properly

[Nessism]

Maybe a senior member can help me (and you) with this, but I don't understand how your numbers point to needing new rings? Is it the differences in numbers between cylinders?

It's common to perform the compression test first with dry cylinders and then after oil is added (couple of oz.). If the compression increases after the second test, it indicates worn rings since the oil will help seal things up. If the compression does not increase with oil in the cylinder, the valves are leaking.