Compression - How low is too low?

[TheRealFatShady]

Just curious what everyone believes is too low of compression. Is 105 too low and needing rings, or is that enough? Was thinking it should be at 120+.
GS1100G

[posplayr]

check service limit which is forgiving of what is acceptable.

[Nessism]

It "should" be over 125 psi but the low service limit is generally considered 100 psi.

The way the engine is tested makes a difference (should be hot engine and throttle held wide open), plus engines that have been sitting for a number of years typically have low compression untill they get out on the road and are thrashed for awhile. Details matter. Oh, and if the valves are not adjusted regularly they can burn which causes low compression. I believe this is a common reason for GS deaths.

[almarconi]

If the 105 is consistent across all cylinders the engine should run ok. If there is no oil loss, smoke..etc I would run the bike. How many miles are on the engine?

The compression could be down if the engine has been sitting along time. Sometimes after running the bike the compression numbers will improve. I don't think that compression alone should be the only factor when deciding if re ringing is needed.

[spyug]

After freeing up my seized engine and trying to get it going I found my "cold' pressure was 90psi or less across the board. From all my reading and all the input from folks the thinking was this while low should still be enough to fire. Once I got the carbs to co-operate I found they were correct and it did run. After several hours of running off and on ( just at idle) I tested again and got 180+psi.

If you read the famous Dan's On-line Motorcycle Course, he says that he will not work on any bike with 100 psi or less as you just can't get it starting and running right. Compression is just too low.

From what I've learned, I'd say 105psi is lower than recommended but if it still starts and runs easily enough use it until it doesn't go anymore. Mind you, if you are looking for a project and can afford it this is a good reason to rip into it now as its not going to get better and can only get worse.

Let us know what you end up doing.

Cheers,
Spyug

[Nessism]

After freeing up my seized engine and trying to get it going I found my "cold' pressure was 90psi or less across the board. From all my reading and all the input from folks the thinking was this while low should still be enough to fire. Once I got the carbs to co-operate I found they were correct and it did run. After several hours of running off and on ( just at idle) I tested again and got 180+psi.



Cheers,
Spyug

180 psi is amazing for an engine with used rings and valves that were not even lapped. You got lucky big time.

BTW, I wouldn't run the bike at idle any more than absolutely necessary. It doesn't do the engine much if any good other than maybe heat it up so you can retorque the cylinder head.

[gggGary]

As noted technique makes a big difference on readings, generally consistency across all cylinders is more important than any special number. Then it depends on what you want out of the bike. If it starts easy and isn't trailing an oil cloud, keep riding to work on it. If you want "performance" then you have an excuse to open it up and fix. After years of getting low compression numbers from my tester (even after changing to a high quality pressure gauge) I finally took it apart and lubed all the joints and Schrader valves. Miraculously all the engines I have checked since have better compression. Leak down tests are considered to be more definitive and reliable. I bought a leak down tester but still have not "figured out" it's proper use. Maybe this gs1100e I just started on will be the test mule for leak down testing proficiency.

[nisom512]

I had a 81 gs550t and it had 808010090 it was terrible but ran fine and started right up all the time. but I knew that was bad so I and I suggest this for you i put marvel mystery oil in each of the cyclanders over night so it would run down and swell up the rings a little. and it did each cylandar went up 20psi after that.

Jake

[Nessism]

I had a 81 gs550t and it had 808010090 it was terrible but ran fine and started right up all the time. but I knew that was bad so I and I suggest this for you i put marvel mystery oil in each of the cyclanders over night so it would run down and swell up the rings a little. and it did each cylandar went up 20psi after that.

Jake

Did you adjust the valves? If so, that engine is going to need work soon.

[Ponderosa]

Compression...

I have gotten bikes started with as little as thirty pounds, try kicking a old Beaser to achieve a high compression reading! I had a old Norton once that had thirty eight pounds on both cylinders, it fired and after running for a half an hour, it had 125# per, and held on leakdown, which before firing had a leakdown time of twelve seconds, and high crankcase blowby. It smoked for ten, fifteen seconds after firing, and then settled down clean.. It had been sitting for ten years! Of course both of the above bikes had to be pushed down the street a few times! UGH...

Leakdown testers are a true diagnostic of where the compression is going, by the rings, which will show in crankcase pressure, by the valves, which you can hear, and how quickly you lose your pressure.. Again, warming the engine, putting cylinder on TDC, and applying air pressure, then shutting down pressure source and watching, timing, and listening for leakdown.. I know there will be some here to add to my explanation, always is, this is the basic princible...

[Dave8338]

If any engine is at or near the 100psi mark, I prefer to give the benefit of the doubt. Run them, see what they do, and then make decisions.

As Ed stated, I hope that isn't with the adjusted valves. Even if that is the case, keeping up on the oil changes and valve adjustments, will keep you on the bike for a looooooooong time.