Topic: Advancing My Timeing / Valve adjustments (CB750k8 or 1978 CB750K)

[Knightro]

Hey all, I have yet to find the answer to this question.

I have a cb750k 1978. Stock air box, running (just a touch rich) with 42 slow jet and 120 main. Points are clean and gapped properly. Valves checked/good. This thing purrs like a kitty. BUT... I understand the engine can see some HP/performance improvements when advancing the timing. SO....

Now, the F mark is about 10deg prior to TDC from what I understand. The timing can be advanced up to 30deg according to the manual. Here is the question (Which I'm going to word in a couple different ways to help get my point across):

The valve clearance measurements are to take place at TDC... from what my manual says. With the F being 10deg prior to TDC....

IF I advance my timing to fire say, 15-20deg BTDC(advanced) Where do I test my valve clearances?  Do I set my valves spacing at that same position as recommended in the manual assuming all timing is set as per manual?

If I further the advance to 15-20% BTDC do I now check my valve clearances at 5-10deg BTDC so the same difference in timing is seen between the valve position and the new advanced state of the ignition?

If I missed a forum post (which I could not find) please advise. If I am way off base, please set me straight. Don't want to blow my engine, ruin my valves, or put my bike into a poor performance situation. I am not a mechanic so if I'm lacking obvious knowledge, you will understand why.

Thanks for the time all! Look forward to your replies.

Knightro

[DaveBarbier]

I don't know too much about this but I say set it as the manual says. You have a handful of degrees where the tappet clearance won't change as you rotate the motor. I mean that the rocker is riding on the backside (heel) of the cam lobe so the rocker isn't moving at all. It takes a few degrees to even bring the cam lobes around to where the rocker is moving. So setting the clearance a little before or after TDC won't do anything.

Besides, if you enlarge the tappet clearance you'll just have more noise and wear.

What are your goals? With excessive advance you'll run hotter and knock can occur. How much more power are you planning on getting? Think you'll feel it? 


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[kmb69]

Changing the ignition timing does NOT change where the valves should be adjusted.

Corrected. Sorry, my typo.

[Jimray23]

Setting the spark timing should not effect your valves. Only if you time your cam different. So yes you should check valve clearances where the book says, just like it was factory timed. When you advance the timing with the light, it only changes firing. Be very careful, if u get advanced to far, you will have predetenation and there's a good chance of breaking a con rod or crank shaft. Thats a big risk for a really small reward.

--------------------------------------------------
Jim
1979 kz1000b ltd
    stage 3 cams
    pod air filters
    4-1 open exhaust(no muffler)
    full rewire
    dyna-s ignition
    dyna 3 ohm coils

[754]

Changing the ignition timing does change where the valves should be adjusted.

 The valve are adjusted on the heel of the cam, which I presume should be circular. That area of the cam is lets say in theory, 25 degrees of the circle... So I am of the school of thought, this is where to adjust... Once you came off the cam, all the way to where the cam starts to lift again.
How would you be changing where to position, for checking valve clearance ?

[Keith]

Imagine what is happening inside of your engine; if you overadvance the timing, the air/fuel mixture explodes before the piston reaches the top of its travel, in effect trying to force the piston back down in its bore. The result is detonation, pinging, engine damage, overheating. Valve adjustment is based on the mechanical relationship between the crankshaft and the camshaft. Altering your ignition timing has no effect on how you adjust the valves.

[kmb69]

Changing the ignition timing does change where the valves should be adjusted.

 The valve are adjusted on the heel of the cam, which I presume should be circular. That area of the cam is lets say in theory, 25 degrees of the circle... So I am of the school of thought, this is where to adjust... Once you came off the cam, all the way to where the cam starts to lift again.
How would you be changing where to position, for checking valve clearance ?

Frank, Corrected my post above. You know I know better. Thanks.

[Knightro]

Ok folks. Thanks for the replies.

After further research I realize the 'performance benefits' regarding timing are mostly seen in adjusting cam timing, not as much ignition timing? (getting my 'timing' confused)

If I adjust Cam Timing, then I probably have to pay closer attention to when / where the valves are opening/being measured for clearances and also pay closer attention to the ignition timing?  But if I am just adjusting the ignition timing a little, the performance gains will not be very noticeable and valves/cam adjustments don't change.

So, am I right to assume 'ignition timing' adjustments are meant to 'tune' the engine to get to function/fire properly more than to realize performance benefits?

In other words, unless I'm tweaking the cam, I should set ignition timing as per manual?

[flybox1]

In other words, unless I'm tweaking the cam, I should set ignition timing as per manual?

Correct, but all of this is a lot of work for minimal, if any, gains 
Might not even feel it on a tired 40yo bike.  Butt dynos usually dont pick up such small gains. 

Tune it properly and enjoy it.

[PeWe]

Setting the point gap with a dwell meter is my recommendation (feeler gauge is PITA). Same setting for both points  too. (8cyl) 23 degrees (23-24.5) It will increase by wear.
F align with mark at 1000-1200 rpm and full advance after 2500 rpm. Sloppy springs will cause earlier advance. Too low idle can cause low oil pressure, flickering oil lamp.
Easier to drive in the city with higher idle

[Keith]

I would forget about cam timing, best left to an expert.

[PeWe]

Cam timing demands a cam sprocket with oval holes so it can be adjusted. Original sprocket has just 2 holes, no adjustment possible except the correct position at 'T' 1-4 TDC
Hondaman has explanations about advancing original cam a few degrees.

An aftermarket tune cam has much longer duration (time in degrees when valves is open until it close) and higher lift. More duration will end up in more overlap which will reduce compression and other pistons with higher CR is needed......hotter cam and pistons will work mutch better with a ported head too.....
The domino effect has started...

I forgot.... when power increase a lot the rear tire will disappear really quick and getting flat with bad road handling....new tire more often

[Knightro]

Thanks all,

That pretty much answers the question.

I'm sure there are more posts on how to's and why not's and where for art thou cam issues.... but to be or not to be ignition timing vs cam timing vs valve adjustments, I think this one is done "thumbs up emoji"

Thanks again all!

[DaveBarbier]

Useless information of the day. Wherefore means why. Juliet was sad and asking herself why Romeo had the name Montague that her family hated. Many people think she was calling asking where he was.


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[Jimray23]

As for timing the cam for more power, leave it to the pros as said earlier. You add 2 degrees to the exhaust timing and still don't have enough of a return to worry about unless using a turbo or blower. The risk greatly out ways the reward for a street bike.

--------------------------------------------------
Jim
1979 kz1000b ltd
    stage 3 cams
    pod air filters
    4-1 open exhaust(no muffler)
    full rewire
    dyna-s ignition
    dyna 3 ohm coils

[Jimray23]

And for a sohc you cant advance exhaust without advancing intake or getting a special ground cam.

--------------------------------------------------
Jim
1979 kz1000b ltd
    stage 3 cams
    pod air filters
    4-1 open exhaust(no muffler)
    full rewire
    dyna-s ignition
    dyna 3 ohm coils