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Low Compression Readings. I Think the Rings Are Shot

  • Thread starter Thread starter MadCapsule
  • Start date Start date
M

MadCapsule

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.
 
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
 
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'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.
 
Rings that shot would also have a cloud of white smoke behind the bike and the plugs would be covered in oil as well.
 
OK, maybe more often than not on the smoke, maybe occasionally on the oily plugs...

Close enough?
 
tkent02 said:
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.
Click to expand...

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.

chuck hahn said:
Rings that shot would also have a cloud of white smoke behind the bike and the plugs would be covered in oil as well.
Click to expand...

tkent02 said:
Sometimes, sometimes not.
Click to expand...

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.
 
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
 
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?
 
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Nessism said:
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
Click to expand...

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.
 
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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.
 
If the valve clearances are Ok? I would check the cam timing ? maybe it has jumped a tooth
 
FYI, you don't need any fancy metric feeler gauges to check the valves, you just need a set that goes down to .0015". If that feeler doesn't go, you need a thinner shim. You also can use the tie wrap method to remove the old shims (check Basscliff's website for info). Also, get Steve's spread sheet and get ready to take an inventory of your existing shims and clearance. First thing to do is take inventory, then put in an order with Z1 for new shims. Oh, and never turn over the engine with any shims removed. You will have to pull the shims one by one after measuring the clearance, then record the shim thickness, then replace that shim back into the engine before moving to the next shim.

I wouldn't bother trying another compression gauge until the valves are verified. It could very well be that the valves were never adjusted and now they are all tight.
 
SqDancerLynn1 said:
You did have the throttle physically open when checking ???
Click to expand...

Yep. I was holding the throttle open while doing the test. Also, just out of curiosity, I did a couple tests with the throttle closed and didn't notice too much of a difference. Could that be an indication of anything?

SqDancerLynn1 said:
If the valve clearances are Ok? I would check the cam timing ? maybe it has jumped a tooth
Click to expand...

I'll check the cam timing while I've got the cover off.

Nessism said:
FYI, you don't need any fancy metric feeler gauges to check the valves, you just need a set that goes down to .0015". If that feeler doesn't go, you need a thinner shim. You also can use the tie wrap method to remove the old shims (check Basscliff's website for info). Also, get Steve's spread sheet and get ready to take an inventory of your existing shims and clearance. First thing to do is take inventory, then put in an order with Z1 for new shims. Oh, and never turn over the engine with any shims removed. You will have to pull the shims one by one after measuring the clearance, then record the shim thickness, then replace that shim back into the engine before moving to the next shim.

I wouldn't bother trying another compression gauge until the valves are verified. It could very well be that the valves were never adjusted and now they are all tight.
Click to expand...

Well, I already have a 25 pc. metric feeler gauge set on the way from Z1. Probably overkill.

"Oh, and never turn over the engine with any shims removed." I'll commit that to memory! Thanks for the tip.
 
Are you in the city or 'burbs? Who did the head? Just wondering right now...
Curt
 
bellucci said:
Are you in the city or 'burbs? Who did the head? Just wondering right now...
Curt
Click to expand...

I'm in the city.

Motorcycle Mania installed the head. They've got a really good reputation and are one of the only places around here that will work on vintage Japanese bikes.

I'm not ready to fault them just yet. Not until I get the valve cover off and see what's what with the valve clearances.

The feeler gauges came in today, but it started raining after I got off work and I don't have a garage. I'll have to wait till it clears up.
 
It's critical to always torque the cylinder head and check the valve clearances after a brief break in period, 600 miles or less, after performing engine work.
 
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