1
CB350/400 / Compression testing with oil ?
« on: May 24, 2017, 09:39:19 PM »
I was trying to understand if lower compression on cyls 2 and 3 was down to valves or rings.
Measured cold and dry, 150 psi on 2 and 3 v 170 on the outer two.
When hot they all seem to measure around 150.
The other day I did a wet pressure test but was unsure how much (little) oil to add.
Ended up adding 2.5 cc of 10w40 using a syringe. Let it "soak" in for a few minutes then checked the readings again.
Bit of a shock when the readings were all up around 200 psi or more !
But this wasn't due to the improved ring sealing that the oil provided?.. instead I think this was down to the reduced volume in the combustion chamber.
My rough calculation suggests the chamber volume is about 12 cc but the added oil dropped this small volume by about 20% and this raised compression to maybe 12:1 or 13 :1.
So I would think maybe only 1cc or less is needed to be safe !
Even then the volume change is still going to affect the readings.
The oil was extracted from each cylinder using some rubber tubing and a vacuum cleaner.
10 minutes with the engine running before the air was clear.
Measured cold and dry, 150 psi on 2 and 3 v 170 on the outer two.
When hot they all seem to measure around 150.
The other day I did a wet pressure test but was unsure how much (little) oil to add.
Ended up adding 2.5 cc of 10w40 using a syringe. Let it "soak" in for a few minutes then checked the readings again.
Bit of a shock when the readings were all up around 200 psi or more !
But this wasn't due to the improved ring sealing that the oil provided?.. instead I think this was down to the reduced volume in the combustion chamber.
My rough calculation suggests the chamber volume is about 12 cc but the added oil dropped this small volume by about 20% and this raised compression to maybe 12:1 or 13 :1.
So I would think maybe only 1cc or less is needed to be safe !
Even then the volume change is still going to affect the readings.
The oil was extracted from each cylinder using some rubber tubing and a vacuum cleaner.
10 minutes with the engine running before the air was clear.