Bad Compression? How to fix?

[CafeRookie]

I checked compression on a 1981 GS550T. After cranking it over these are my numbers. I took an average of 3 tests. The engine was cold. I used a compression gauge from Harbor Freight.

1: 70 psi
2: 70 psi
3: 60 psi
4: 80 psi

Is this motor broken? Is there any way to fix bad compression?

Thanks!

[pdqford]

Not a whole lot to go on. Is this a freshly rebuilt engine? Or does it have 75k on the clock?
Was the throttle wide open?
Where is the valve lash?

[Nessism]

Agree on checking/adjusting the valves. If you have valves with no clearance, then there is a very good chance that there is carbon on the sealing surfaces holding the valves open. Adjust now, run the bike some good hard miles, and then check the clearances again since they may close up further.

[rphillips]

Uhh, seems suspicious all cyl. being that low & that equal. Normally will be one or two quite a bit lower than others. Yours seem to be appx. 1/2 of what they should be. Does your eng. start? how does it run? To fix, need to know what the problem is, Valves would be 1st thing to check but first I'd confirm your readings are accurate. Check with another gauge.

[Brendan W]

What they said and are you sure the valve timing is correct.
Have you done both wet and dry tests?
No improvement on wet would be a strong pointer to valves.

[Gorminrider]

The throttle need be cranked WIDE OPEN when doing compression tests. If it's closed, all readings will be low.

[CafeRookie]

Thanks for the insight.

The engine has never been rebuilt. There are 9K miles on the engine. The Engine starts and runs strong, but I'm dealing with some carb issues so it idols super high right now.

What is a 'wet test'?

I checked valve clearances recently and if I remember correctly they were all within spec. I will check the valve clearances again to be sure.

[zed1015]

Thanks for the insight.

The engine has never been rebuilt. There are 9K miles on the engine. The Engine starts and runs strong, but I'm dealing with some carb issues so it idols super high right now.

What is a 'wet test'?

I checked valve clearances recently and if I remember correctly they were all within spec. I will check the valve clearances again to be sure.

With your quoted PSI figures any engine would barely struggle to start, if at all so i suggest your compression test method if flawed.
If the motor runs strong i wouldn't be concerned with comp figures as long as the difference between the highest and lowest cylinder isn't much more than 15psi once the figures are confirmed using the correct method of testing..
As regards to the high idle i would be looking at air leaks,split intake boots etc..

[Brendan W]

The wet test adds some oil through the spark plug holes.
If the rings are worn this will seal them up to an extent and give higher compression readings.

[bwringer]

Over the years, we've found that it is far, FAR more common to have problems getting a good reliable compression test than to actually have bad compression. The best way to test is with the carbs removed.

If it starts and runs, look elsewhere to solve your problems. Also, your readings were similar enough that it does not necessarily point to a problem. There are also a lot of inexpensive compression testers out there with serious accuracy and reliability issues.

I have seen a GS550 with over 100,000 miles that was actually worn out and had bad compression. (Some of you may long-timers may remember the owner, "Jimcor".) The bores were so worn that he was starting to have trouble getting it started when cold at around 80-90,000 miles, but he was DETERMINED to reach 100,000 before opening the top end, which he did achieve. Once it was warmed up a little, he could generally start it without too many issues throughout the day, and it ran well enough, although it was certainly down on power.

Anyway, it was pretty striking how fast the starter would spin the engine with such leaky rings. What I'm saying is that the signs of actual low compression are pretty blatant, and compression has to get very bad indeed to cause problems.

And the unscientific "thumb" test is usually sufficient to tell whether there's enough compression to run; block the spark plug hole with an experienced thumb, crank the engine, and feel for a strong burst of air that pushes your thumb off.

I have also been a witness to a freshly assembled 550 engine where the rings were left off of one piston by accident. This caused all sorts of weird problems, and there was little to no compression in that cylinder. This was quite obvious with the thumb test.

Finally, even a very healthy air-cooled engine tested correctly with a reliable gauge can have very different compression readings when cold than when warmed up, and with the carbs removed vs. simply held open. And with CV carbs, you can't really get a reliable reading with the carbs installed at all. Overall, compression testing is very approximate, with lots of sources of errors, and while the results can be very useful, they should also be regarded with a large slice of skepticism. Never condemn an engine or waste a set of gaskets and rings based on compression readings alone.

[Rob S.]

Just to add...if an engine is running well, why bother to even check compression? You don't need to know.

[CafeRookie]

I took off the carb and replaced the intake boots. The new readings were still low. They are as follows:
1 - 70
2 - 60
3 - 60
4 - 70

I think I was able to fasten the gauge better on 1 & 4 which resulted in a 10psi higher reading. I'll try to track down a different compression tester to see if it works better. Hopefully, it does! There doesn't seem to be any cracks in the engine so I'm not sure why it would still have low compression if I'm doing the tests correctly. My plan going forward is to keep the compression numbers in mind, but not to dwell on them. As many of you have stated, why worry about it if the bike's motor runs smooth? My thought too. I'm just going to keep doing my thing and address it if it becomes a problem. Thanks for all your help.

[mattd1205]

I took off the carb and replaced the intake boots. The new readings were still low. They are as follows:
1 - 70
2 - 60
3 - 60
4 - 70

I think I was able to fasten the gauge better on 1 & 4 which resulted in a 10psi higher reading. I'll try to track down a different compression tester to see if it works better. Hopefully, it does! There doesn't seem to be any cracks in the engine so I'm not sure why it would still have low compression if I'm doing the tests correctly. My plan going forward is to keep the compression numbers in mind, but not to dwell on them. As many of you have stated, why worry about it if the bike's motor runs smooth? My thought too. I'm just going to keep doing my thing and address it if it becomes a problem. Thanks for all your help.

You can rent compression testers for free at autozone and oreilly ($40 deposit that you get back when you return them).

[zed1015]

The new readings were still low. They are as follows:
1 - 70
2 - 60
3 - 60
4 - 70

As already mentioned.
If the engine starts easy and runs strong as you say then compression isn't your problem.
Generally you need 90 psi for succesfull combustion to occur.
With your figures the bike would barely run, if at all.
You are chasing a fault that doesn't exist.
If there's no other issues then just ride the bike.

[GS1150Pilot]

Why are you so focused on a compression test? You've got the carbs off, so get them fully sorted, make sure the intake boots are in good shape and sealing, that the valves are correctly adjusted-and ride that thing.