SOHC.co.uk Forums > CB750
CB750 K1 Valve Timing Question
cbxman:
Hi guys,
I have I have had my CB750 K1 since October 2021 and have been trying to tune it so that it idles and rides nicely….but it won’t. I can get it to idle well ,or pick up on and off the throttle, but not both. I am looking for your help……
I have checked the timing. TDC mark is where it should be.
The advancer had sloppy springs, so I cut the springs, now it advances nicely to full at 2500 rpm without jumping about under the strobe. (As recommended by HondaMan on .net)
Compression 160 psi on all within a pound or so of each other.
Sparks well in excess of 20kV on new TEK coils, resistor caps and non-resistor plugs. (NGK D8EA and ND X24ES-U). Dyna S fitted…..same issues with the points plate.
Carbs stripped numerous times.
It had a Keyster kit installed, so I have re-fitted it with Keihin jets #40 and #120, with the original Keihin needles. The only non-keihin parts are the float valves and the Needle Jets, which had been damaged somehow by a PO.
I checked the float heights out at 26mm, but I thought I would also check the fuel levels with the clear tube method and found that set at 26mm, the fuel level was really low. Reading about this, I have found that the fuel level should be 1-2 mm lower than the bowl rim, so I have adjusted the floats to give me the necessary level. This ended up with the floats being set at 22mm when measured the usual way. I think these floats must have come from another bike as this early K1 should have brass floats and there are two distinctly different pairs of plastic floats in there…ho hum.
I Have changed the slow jets to #35, #38 and the stock #40. Mains #110, #115 and the stock #120. I have tried most combinations of those and with different needle heights. Some just do not work well, but some combinations will work just like mentioned above. Hard work changing needle heights!! I can take carbs out in minutes now!
I had run out of ideas, so I had another think…. maybe I would check the valve timing. Rather than removing the engine for a look see, I would try to measure what the cam is doing….and so ….entered the dark world of timing diagrams….EEK!
The WSM states Intake opens at 5Ëš BTDC and closes at 30Ëš ABDC. Exhaust open at 35Ëš BBDC and closes at 5Ëš ATDC. I understand those, but I wondered how to measure them.
I used a degree wheel and set it up to TDC, but for convenience I used cylinder 4 as it is closer to the degree wheel. I used dial gauges on the tappet screws to measure the closing and opening of valves. The numbers I gathered made no sense to me, so I’m seeking the truth from this august body of SOHCers.
This is a diagram which I thought would help me figure out whether the cam timing is right or not. I’m even more confused now. There must be a procedure to take the book numbers and convert them to reality, but I can’t seem to figure it out.
Of course there are two strokes not shown (compression and power) hence why the valves are so close and out of their natural order, but I think it is simpler to present it this way?
Cheers,
Jerry
K2-K6:
There may be a little information missing from original manual, in that the cam timing specified is listed as "at 1mm lift" in later publication.
It also gives figures for 0 lift in later manual, but those raw cam degrees look nothing like the ones you've stated.
Hopefully that may give you more to go on.
cbxman:
--- Quote from: K2-K6 on April 26, 2022, 09:34:38 PM ---There may be a little information missing from original manual, in that the cam timing specified is listed as "at 1mm lift" in later publication.
It also gives figures for 0 lift in later manual, but those raw cam degrees look nothing like the ones you've stated.
Hopefully that may give you more to go on.
--- End quote ---
Thank you K2-K6 for your quick response. I have a genuine Honda WSM dated 1977 covering up to the F model, and have quickly looked through it,and only found the book figures in the spec section at the front. I'll have another look. Maybe you are referring to a completely different source?
Would 1mm lift make such a difference? Would that be on the cam lobe, or the tappet position?
I must be measuring something really wrong to get such differing values to the book.....
I'm thinking it might be easier to remove the engine and take the cam cover off and have a look.
Cheers,
Jerry
K2-K6:
I've a first iteration 1969 manual, also a last 1978 in which the two sets of information come from, Honda genuine manual.
No mention I can immediately find of where to measure (camshaft are often given timing criteria at a specific lift though as matter of routine) I'm guessing that the original just omitted this detail.
The later manual gives timing for both, and so can be compared directly as follows;- intake opens @1mm lift 5 BTDC & 56 BTDC @ at 0mm lift. Quite a difference and likely to be causing your initial puzzlement.
It follows all the others with dual readings too to check either way (beginning of cam ramp and @1mm) and looks distinctly odd at those numbers if you compare to "normal cam protocol.
cbxman:
--- Quote from: K2-K6 on April 26, 2022, 10:25:39 PM ---I've a first iteration 1969 manual, also a last 1978 in which the two sets of information come from, Honda genuine manual.
No mention I can immediately find of where to measure (camshaft are often given timing criteria at a specific lift though as matter of routine) I'm guessing that the original just omitted this detail.
The later manual gives timing for both, and so can be compared directly as follows;- intake opens @1mm lift 5 BTDC & 56 BTDC @ at 0mm lift. Quite a difference and likely to be causing your initial puzzlement.
It follows all the others with dual readings too to check either way (beginning of cam ramp and @1mm) and looks distinctly odd at those numbers if you compare to "normal cam protocol.
--- End quote ---
Interesting. I’ll take a closer look in the wsm.
The camshaft installation instructions are pretty clear, so unlikely to be timed incorrectly. I don’t suppose that many people look to do this type of check. The tuners will of course be dealing with non standard cams anyway.. the engine is quiet when the idle is set right, so I don’t think it is knackered.
Cheers,
Jerry
I’ll
Navigation
[0] Message Index
[#] Next page
Go to full version