Low Compression Readings. I Think the Rings Are Shot

[Guest]

So, I did a cold compression test on my motor today and these are the readings I got on the cylinders:

1 - 30 psi
2 - 40 psi
3 - 40 psi
4 - 40 psi

Then I added about a teaspoon of motor oil to each cylinder and got the following readings:

1 - 50 psi
2 - 55 psi
3 - 55 psi
4 - 55 psi

All tests were done at WOT and I tested each cylinder 4 - 5 times for consistency. They came out the same each time, so at least I know that my cheap-o Harbor Freight compression gauge reads consistently.

I realize that the motor should be at running temperature, but the 630 chain is toast and I don't trust it and I'm leery about bringing and air cooled motor up to running temps without any airflow.

I also realize that the compression gauge may not be entirely accurate. However, the consistent 15 - 20 psi difference between dry cylinders and oiled cylinders would still seem to point to the rings being trashed.

I'd love to be wrong on this one, so if you guys and gals think I may have missed something, please let me know.

Also, I do have a 530 chain and sprockets on the way from Z1.

[chuck hahn]

Set a house fan on each side of the engine and start it and run it up a bit. give it about 10 minutes just idling and then you can do it on a warmed up engine, and be sure its at wide open throttle

[tkent02]

How long since the bike has been ridden a lot, and did you adjust the valves correctly first?

When they sit a long time the rings tend to get stuck. A lot of miles will fix this.

When the valves are tight the compression is going to be low. Your wet test would indicate rings, but if the valves are not closing there will be no compression. Let's say it was 120 dry and 140 wet, still a difference of twenty, but you wouldn't have a thought about rings.
Tight valves can easily leak this much, even if they haven't yet burnt.

And, how many miles on the bike? Seriously worn rings are not likely unless it's up around 100,000 miles or so. Tight valves and the resulting burnt valves can happen at any mileage.

[almarconi]

I'm surprised with compression numbers that low, the motor would run at all. Put a fan in front of the engine if you are worried about the engine overheating.

[chuck hahn]

Rings that shot would also have a cloud of white smoke behind the bike and the plugs would be covered in oil as well.

[tkent02]

Rings that shot would also have a cloud of white smoke behind the bike and the plugs would be covered in oil as well.

Sometimes, sometimes not.

[chuck hahn]

Usually more often than not

[tkent02]

OK, maybe more often than not on the smoke, maybe occasionally on the oily plugs...

Close enough?

[MadCapsule]

How long since the bike has been ridden a lot, and did you adjust the valves correctly first?

When they sit a long time the rings tend to get stuck. A lot of miles will fix this.

When the valves are tight the compression is going to be low. Your wet test would indicate rings, but if the valves are not closing there will be no compression. Let's say it was 120 dry and 140 wet, still a difference of twenty, but you wouldn't have a thought about rings.
Tight valves can easily leak this much, even if they haven't yet burnt.

And, how many miles on the bike? Seriously worn rings are not likely unless it's up around 100,000 miles or so. Tight valves and the resulting burnt valves can happen at any mileage.

I've been riding it all summer.

The valves were supposedly adjusted while I was having the head replaced (that's another story) by the local mechanic, but I've been doubting their reputability as of late. I suspected the valves as well and ordered the valve tool and a set of feeler gauges from Z1. The feeler gauges were backordered, so the valve tool arrived first. I figured I would practice taking the shims out of the buckets on my old head and noticed that the shims were all missing. The head, which I bought from a member here, came with its own shims, so it's not like the swapped them between heads.

The bike has about 32K on the clock.

I'll check the valve clearances when the feeler gauges arrive, so I'll know more on that front shortly.

Rings that shot would also have a cloud of white smoke behind the bike and the plugs would be covered in oil as well.

Sometimes, sometimes not.

The only time I've gotten white smoke was after burning off the oil I put in the cylinders today. The plugs, on the other hand, look pretty fouled and they were newly replaced at the beginning of the summer. I'll pull them back out again tomorrow and take pics.

Also, after a recent oil change, the oil turned black pretty quickly. By that, I mean that it was black the next day.

I don't think I have a house fan anymore. I'm sure I can find a used one around here for next to nothing though.

[Nessism]

The engine won't run with compression that low. Something must be wrong with your gauge. Sure it's reading psi?

Regarding the engine, sorry for all the trouble. I'd be disappointed if my freshly rebuilt engine had low compression.

Make sure to check and adjust those valves ASAP. Dollars to donuts they have little to no clearance. If the valves/seats were cut then it's almost certain unless your mechanic is an old time Suzuki mechanic with a pile of shims to swap around. FYI, when the valves and seats are cut the valve sinks deeper into the head, thus the need for thinner shims. A good mechanic/machinist will trim the valve stems to shorten them when doing a valve job otherwise you may run out of adjustment room, even when using thin shims.

Good luck

[derwood]

yes, beg borrow or steal a different compression gauge,
I can't see how the engine could run if the compression is that low.
I think you should adjust the valves and ride it before you consider taking it apart.

Oh, what symptoms prompted the compression test?

[MadCapsule]

The engine won't run with compression that low. Something must be wrong with your gauge. Sure it's reading psi?

Regarding the engine, sorry for all the trouble. I'd be disappointed if my freshly rebuilt engine had low compression.

Make sure to check and adjust those valves ASAP. Dollars to donuts they have little to no clearance. If the valves/seats were cut then it's almost certain unless your mechanic is an old time Suzuki mechanic with a pile of shims to swap around. FYI, when the valves and seats are cut the valve sinks deeper into the head, thus the need for thinner shims. A good mechanic/machinist will trim the valve stems to shorten them when doing a valve job otherwise you may run out of adjustment room, even when using thin shims.

Good luck

Yeah, it's frustrating. But I've come this far with the bike, so I figure I might as well this thing through to the end.

The gauge could be way off, for all I know. I'll see if I can test it on Mister Cinders' bike, since it's known to be running well.

If the valves are as tight as you suspect they may be, I think I should probably hold off on the hot compression test until I know for sure what the clearances are.

It seems strange that they would have gone from being in-spec enough to have the motor running strongly at the beginning of summer to this in such a short amount of time. Unless maybe they were already right at the service limit.

[chuck hahn]

READ the post about you HAD HEAD WORK DONE...then there was a post explaining that as they reset the seats the gap changes..The gap will decrease between the shim and the cams. If the gap decreases enough that theres contact with the cam on the BOTTOM of the lobes you get crappy compression because the valves are being held open. And I agree about even doing a cold test untill you have fully verified the valves are all within specs.

[SqDancerLynn1]

You did have the throttle physically open when checking ???

[SqDancerLynn1]

If the valve clearances are Ok? I would check the cam timing ? maybe it has jumped a tooth