Honda-SOHC
SOHC.co.uk Forums => CB350/400 => Topic started by: Clem2112 on May 24, 2017, 09:39:19 PM
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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.
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Oops, guess you're heading up towards a diesel there! Looks roughly right, your calculations, the chamber volumes are quite small.
I'd use a runnier? sort of oil like 3 in 1 or similar as the 10/40 is quite sticky.
You just need to get it to run round the compression ring to improve the seal really.
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Yes, afterwards I read that others suggested a thinner oil to do the test !
As you say, only enough oil to cover the top ring is needed .... maybe a couple of drops ?
Those pistons are quite small !
Peering into each plug hole, I could see that the honing marks are still clearly visible, which makes me wonder if the bores and rings have fully "bedded" even though the rebuilt engine has done several hundred miles .
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If you are getting the same readings across all four when hot which is when you should do your compression test every thing is fine, and you have nothing to worry about, keep using it and don't be afraid to open it up.
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That's the plan now !
It's been a slow process getting the engine running well enough for long enough !
Good sparks and carb setup now. :)
New rubber ready to go on. Front brake is still poor but MoT'd ok !
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The front brake may be poor by todays standards but for it's time it was good, we just have to consider this when out riding.
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I tend to stay well back from everything that's in front of me. ;)
The disc may benefit from a light cut to help the pads bed in though.
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On the compression readings....
I use the tester without the flexi hose and just have the solid tube with the tapered bung as a seal.
So there is additional volume in the spark plug hole (c. 2cc ?) which should reduce the readings to about 150 by my rough calcs.