SOHC.co.uk Forums > CB500/550

Low compression but the bike runs fine?

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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):

--- Quote from: alexdecker on September 09, 2021, 09:11:07 PM ---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 :(

--- End quote ---

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...

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