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CB750 K1 Valve Timing Question

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K2-K6:
If it's the K1 from Farnham, I was looking at that and got stuck, literally, when we couldn't turn the airscrew to adjust idle mixture.

Sounded one on the nicest 750 I've heard for a while when running, appears to be in very good condition mechanically.

Thinking further about cam timing, think convention has in imperial given timing at 40thou lift (39.37mm) as recognized flow is considered from that opening to understand cam performance. 

Nigel.

Lobo:
This is all too technical for me, but having recently had to buy a ‘new’ OHC for my K2, have become aware that there were many Honda tweaks to the lobe heights and timings over the years, and not necessarily accompanied by a part # change.

Whether or not this is relevant I don’t know….

K2-K6:
As long as you're using the numbers with the lift set as they are intended, then it'll serve well to establish if cam is correctly timed.

The reason for the difference is that the @ 1mm numbers describe in reality what performance (in terms of rpm, powerband etc) the cam is going to give, with the absolute (start and finish of approach ramps) showing details of peripheral geometry more related to how much acceleration load the valve gear can realistically sustain, plus cylinder leak down (below that 1mm there's not much absolute flow) outright compression, cylinder efficiency directed toward reducing compression resistance etc.

For example, a lighter valve train can sustain shorter ramps as it's less likely to give valve float (valves bouncing in excess of cam lobe peak) and more a feature/advantage of 4 valve heads and engines like Ducati desmodromic actuators in using shorter more staccato geometry.

cbxman:
Nigel,

Really helpful. Thank you.

So if I set the valve lash to 1mm and then measure from the valve stem as before, that should set the threshold I think you are describing.

You're right about the bike it was Andrew's. The stiff airscrews were due to the ill fitting Keyster kit which I think enlarged the threads.  I have added some small 'O' rings to the airscrews as the original screws are quite sloppy in the carb.It does seem to have evened up the action of the airscrews but that wasn't going to fix it.

I'm pretty sure that the engine has been removed for some work, as there are witness marks on the frame, hence wondering whether the cam was put in a tooth out or something.

I have looked in the WSM and there are variations in the valve timing, but not by much.
K1, 2, 3, 4 has Inlet opens 5Ëš BTDC, closes 30Ëš ABDC, exhaust opens 35Ëš BBDC and closes 5Ëš ATDC.
K5   supplement does not state the timing
F    has as above order 5Ëš, 35Ëš, 35Ëš, 5Ëš
K6   supplement does not state the timing
K7  has as above order  0Ëš, 40Ëš,40Ëš, 0Ëš
F2  has as above order  5Ëš, 40Ëš, 40Ëš,5Ëš
K8  has as above order  0Ëš, 40Ëš,40Ëš, 0Ëš
F3  has as above order  5Ëš, 40Ëš, 40Ëš,5Ëš

The 'tuners' here would probably make some sense of it.

All to do with emissions I suppose, but they keep changing their minds!

Cheers,
Jerry

K2-K6:
"So if I set the valve lash to 1mm and then measure from the valve stem as before, that should set the threshold I think you are describing."

Yes, I see that as correct as the dial guage will pick up immediately the 1mm gap is closed to give you the book spec stated. Hopefully you'll see a conformance (or at least valid statement) of if it's right or wrong without having to remove the motor. 

The timings usually vary for torque point in rev range generally.  The F2/F3 (same motor) have raised red line, bigger valves, stronger valve springs, shorter final drive gearing than the others.

Nigel.

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