SOHC.co.uk Forums > CB750
Compression numbers.
MrDavo:
Factory manual says 175psi on a new one.
Whatever happens you know that #1 is well and truly up the pictures, the cause should be obvious once you're in there. Four doesn't seem too clever, if it were only normal wear and tear you'd expect them to be similar if low.
onethumb:
Update. Right took the head off and the good news is there is no damage on the top of the pistons and the bores look good. Soaked the top of the pistons with a carb cleaner and wd40 mix for 2hrs and lost none.
Stripped the head but left the valves and springs in placed tipped it over filled the chamber with wd40 and lost it all though the inlet valve in no1 in about 5 seconds, lost most of it out of no4 in about 1 minute though both valves. Now I know I've got issues with no1 and 4 but I think I might get all of them redone.
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Trigger:
Number 1&4 will have pitted valve faces and seats or bent valves. If pitted only, this is due to the tappets being too tight.
onethumb:
Thanks trigger and mrdavo for the input. I'll leave it to the engineers to give me more info on what to do with the reconditioning.
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onethumb:
Well trigger you were right. So the head went to the shop and he found pitting on number 1 along with a bent valve and two of the exhaust guides were gone. So it's been machined, new guides a new valve and the seats and valves re cut. Looks amazing.
Now the issue is: I've built the engine back and have done a test before putting it back in the frame and the numbers are weird. So before I was getting 1-4 40-150-150-125 now I'm getting 1-4 110-110-115-125. No plugs no carbs and a squirt of thin oil in the bores.
I did notice that the new head gasket that was from DS sat a little funny when on but flattened with the head in place. Could this be the issue? I'm getting a bit stressed over this. Help please.
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