SOHC.co.uk Forums > CB500/550
550K3 cylinder compression
nairb:
Hi, I stopped using my old K3 back in 1995 when it started burning loads of oil and poor performance. Fast forward some 25 years later and I finally decided to restore the bike.
I pulled the top end off and was surprised to find there was no lip on the bores at all. The rings and oil scrapers were well tired. So I thought a very light hone and a new set of rings would be ok. I also put some paraffin down the valve intakes to check for leaks. They were still in good(ish) shape.
So it was all resembled and after much effort to get the carbs to stop leaking it finally was ready for the road. The bike actually starts and runs ok. It could do with the carbs being setup better but it does rev ok and pulls through the gears.
Unfortunately the speed does not match the engine revs. So I thought it might be the clutch (42k) miles and purchased a new cllutch kit and cable from DSS. I fitted these and it made no difference at all.
So next I checked the timing - looks almost spot on. So finally I checked the compression.......
With COLD cylinders and no oil down bores and all plugs out and throttle fully open I tested each in turn.
I found the pressures to all be about 155-165/cylinder. I think the manual says they should be 177psi.
So after some 200 miles the engine has improved a little but the engine still needs to rev too hard. I was expecting about 5200k revs for 60mph.
I have been trying to keep the revs down to start with to run-in the engine after the rebuild.
My question is ...... is 155-165/cylinder too low?? when cold. I should really try the test with a hot engine I guess.
Other than this issue it seems to run ok.
Ta
Nairb
robvangulik:
The amount of revs at any speed is mechanically determined and high or low compression has nothing to do with that.
Seabeowner:
As said compression will make no difference to revs vs speed. The only thing is clutch slip.
With standard final sprocket gearing 17/37 at 5000rpm you should get: 3rd-45mph: 4th-55mph: 5th-63mph.
Different tyres and inaccuracy of gauges will make a difference.
Bryanj:
Thatcompressionis fine and definately not your problem, is your memory offa bit as they do need to be reved through thegears even up to top of red bad
nairb:
--- Quote from: Bryanj on June 13, 2021, 07:17:47 PM ---That compression is fine and definately not your problem, is your memory off a bit as they do need to be reved through the gears even up to top of red bad
--- End quote ---
Well I thought I might add a bit more now its done 450 miles since its rebuild. I think the above statement "memory off a bit" is very true. I have several other bikes. A 500 single, 2x550, a 450 and a xs650. The 550k3 is the only inline 4 I have. But I have done some 42k with it in the past (25+ years) so I did know it quite well...... "did" is the word.
So I thought to start with there MUST be some clutch slip or other problem still with the bike. The engine seemed to be revving far to high. And little acceleration.
But I kept the revs down to start with to try and "run in" the top end. And went about the country lanes. As the miles went by the engine power came up a bit.
The gearing is 17/37 and the engine will pull about 5200k at 60mph. Assuming the gauges are still semi accurate??.
With 450 miles done the engine is running ok. So it seems that I really had forgotten how much these bikes needed to be revved to get up to speed. I still try to change up to a sixth gear. I might try an 18 front sprocket and some stage.
Thanks
Nairb
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