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CB750 K1 Valve Timing Question
cbxman:
Hi Folks,
I have set a lash of 1mm on the intake and exhaust valves. I used a dial gauge, but I am aware that there may be deviation due to the arc of the rocker and the end of the tappet screw's flatness (gauge's nose is not long enough to reach the spring top). So I expect there to be some slight mis-measurement. The ramps can be quite sharp so there maybe a tiny a discrepancy in setting lash might give a significant alteration of the measured open time. This is what I think may be happening in this chart.
The opening ramp and closing ramps are likely different rates so the variation from stock maybe due to the lash again.
I may run another test, but with 0.75mm lash and see what that looks like.
What do you think?
Cheers,
Jerry
K2-K6:
As I understand it the 1mm is at valve rather than anywhere else, so direct onto valve spring cap.
The reason it's usual to work there is that the valve is not deemed to flow enough volume to influence peak power characteristics until it passes that point. It's much more common on USA V8 when they routinely choose camshaft timing to consider it this way too as they have the option of changing rocker ratios in addition.
The ramps do vary, but for different reasons. This is used to soften or shorten the abruptness of the whole valve train to stay within durability limits on road engines (often compromised on race for pure power) also it can affect efficiency as follows;- the K7 to F2 shift the absolute degree for exhaust more the the 1mm setting, this can either promote flow starting at the margins or delay it to reduce the active compression ratio and make the motor softer with less pumping loss (bleeds just a little of the absolute compression away and reduces running resistance) to help with economy.
That last bit about bleeding out compression is what they do in "Atkinson" cycle engines, leaving the intake valve open too long rejects some of the intake mixture back out the inlet to lower the final compression at low speeds. Toyota do this on the Prius to get soft starting, quiet running, increased efficiency when used in combination with electric torque.
cbxman:
Nigel,
Thank you for your continuing insight into this.
I have run some readings with the lash set to 1.25mm.
I will try to bracket these readings with one at 0.75mm to see if I can find a trend.
Cheers,
Jerry
cbxman:
Nigel,
Well. I have learned a few things!!. Measuring valve timings requires very accurate Lash adjustment. Setting the lash to exactly 1mm took quite some time.
Testing Cylinder #4 at the start was fortuitous, because if I had started with cylinder #1, I would have found that the timing was spot on and started scratching my head again and looking elsewhere. However after checking cylinder #3 and finding that was spot on too, I can now say that cylinder #4 definitely has a problem.
I have decided to remove the engine and have a look.
I expect it will be expensive, whatever I find... :-[
Thanks for your help
Cheers,
Jerry
K2-K6:
It'll be interesting to hear what you find, strange indeed to get a difference there.
Speculative, but one of the two prominent owner induced problems with a 750 (final drive chain break to damage cases, oil supply to head area) may have given cause for someone in the past to "repair" it. Could only guess at the cam being refinished if damaged to alter just one set of lobes.
I saw it running with the two previous owners (UKpete, Andrew) and don't think it's been worked on in the UK, perhaps much earlier in it's life previous to import.
Hopefully your investigation will get to the bottom of it.
Nigel.
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