Low compression but the bike runs fine?

[alexdecker]

Hey

Just finished a tune-up of my CB550F2, and carb synced it in the end. My new compression tester just came in the mail today, so wanted to try it out. I am surprised that I get really low compression on all four cylinders though! Between 60-70 on all of them. I tested the bike while it was warm, and had choke and throttle fully open.

I have a clip of the bike idling (warm, as I took it right after the compression test), and it idles well and runs well, and not rough at all. So why the low compression??

https://youtu.be/3XW8BiG1dDk

Any help or advice is greatly appreciated.

- Alex

[alexdecker]

No smoke at all. And the motor is running fine at low and high speeds…

[Nurse Julie]

With all 4 reading low but very close to each other and the bike is idling / running fine, I would suspect the accuracy of the guages.

[alexdecker]

It's a no name brand I think. You can see the tester here: https://www.vidaxl.dk/e/kompression-tester-til-benzinmotorer/8718475816249.html?gclid=Cj0KCQjw4eaJBhDMARIsANhrQAC0pyvjfPzjCYf-QU30mMlAD0I0k3xOdMCAamOyiaTZqdA8_jaue1kaAh_hEALw_wcB

I do have an oil leak on each side of the head (the oil galley because of too small O-rings), but that shouldnt affect the results that much, should it? I will try the oil drop trick. Though I could only warm the engine up and get the same results.

Also, do all spark plugs have to be off at once?

I just set the valves, so that should not be it…

[McCabe-Thiele (Ted)]

When I checked mine on my 400/4 last year (unrestored)  after it's first run for 12 years mine were all around  the 80psi mark that was with all the plugs out and throttle fully open with no air cleaner in place. Hot start was good but a swine from cold so hence the strip down.

If it goes okay & starts easily I would not worry about the readings as the gauge is probably not very accurate. One of the disadvantages of checking them is you are likely to be disappointed.

[Laverda Dave]

Are you using the rubber bung type attachment or a screw in type? I've used the rubber bung type years ago and it gave a low reading, I borrowed a screw in type with an o-ring and the reading was much higher. I think you have to really push hard on the rubber bung type to make an effective seal.

[alexdecker]

I used the screw-in type with a O-ring seal.

Problem is, this is my first bike, so not really sure how its supposed to ride with proper compression. I took it out today, and had it going 150 km/h with more juice to spare, so it doesn't really seem like it has lost any energy. Maybe I should return the compression tester

[McCabe-Thiele (Ted)]

I used the screw-in type with a O-ring seal.

Problem is, this is my first bike, so not really sure how its supposed to ride with proper compression. I took it out today, and had it going 150 km/h with more juice to spare, so it doesn't really seem like it has lost any energy. Maybe I should return the compression tester

My late Father who did his apprenticeship at RR/Bentley in Crewe during the war years always told me that unless you had a problem with an engine do not check the compressions as you will usually be disappointed. In the 1960's when he was teaching me to strip down engines it struck me as a strange thing to tell me - nowadays I see he was right. If they are all pretty much even and you do not have a load of smoke my view is that if it starts easily from cold then it's okay. Fwiw avoid getting fixated on the numbers.

[Seabeowner]

They are small cylinders, 137cc and presumably compress into 13.7cc if it's about 10:1. If the check valve is at the far (gauge) end of the hose the volume in the hose is added to the 13.7 could be the source of some loss, but not anywhere near that much.
But if you are not burning oil and speed seems ok, why worry.

[SumpMagnet]

At 150km/h ...you are only maybe 20Km/h away from Hondas claimed top speed....and you weren;t flat out. Considering it's going pretty well for a 40+ year old 550, not smoking, sstarting and idling nicely....well... I would put away the gauges.

I would guess the desired compression is going to be around 170psi or some such....so 80psi would be pretty low, and I would expect to be seeing symptoms of poor running. Reluctant warm/hot starting, sluggish performance....something.

I would really want to stick those gauges on another engine and see if you got similar low readings...

[heli_madken]

I bought exactly the same set of gauges a couple of years ago and tried them on my freshly restored GL1000 and got 80 PSI on each cylinder (should be around 150 > 160 PSI). Bike starts on the button from cold and pulls like a train, I dont bother using them anymore.

[deltarider]

My late Father who did his apprenticeship at RR/Bentley in Crewe during the war years always told me that unless you had a problem with an engine do not check the compressions as you will usually be disappointed. In the 1960's when he was teaching me to strip down engines it struck me as a strange thing to tell me - nowadays I see he was right. If they are all pretty much even and you do not have a load of smoke my view is that if it starts easily from cold then it's okay. Fwiw avoid getting fixated on the numbers.

This ^
Over the years I have done a few attempts to do a compression test and failed each time. I have given up, as I don't have a problem.

[alexdecker]

Okay okay okay … I will leave it be. For now at least 

[Sesman]

Alex. Did you pick up the mirror dimensions?