Topic: CB750 cam timing: Different valve lash OK to achieve closer result on all 4?

[PeWe]

I'll taking out my engine to do some job. One important issue is cam timing, more advance on my CX-7 cam.
Inlet valve closing as first prio. 55* ABDC @ 0,004" (1mm) lift. (before 56*). 

I think I have rich idle issues due to late IN closing, reversed flow thru carbs at low rpm. I stop for red light, waiting for green 

I saw that the results were different when I timed the cam. I measured all 4 IN, EX.

OK to check the timing with actual valve lash and adjust more lash on some valves to make all 4 closer to each other?
Cyl 1, 3,4 IN need more lash. Cyl 1,2 EX little bit more too.

I'll test with lash in the range of 0.0004- 0.0006 (0,10-0,15mm) to make the numbers below closer to each other. Maybe more.
I'll replace the complete cam towers with axes and rocker arms, found a good used set, probably better than my old. The numbers might change then.

@0,004 Lift (1,0mm)  0 lash
Cyl 1.   29/60             56,5/25,5
Cyl 2.   27/58,5      56,5/26,5
Cyl 3.   27,5/60      56/25
Cyl 4.   27,5/60      55,5/25

I know, more lash will cause lower lift and duration, cooling down the hot cam :-)

Maybe this is how the pros doing it?

[TurboD]

I would'nt be afraid to adjust the lash, I consider the stock settings a general guideline to start with. Every engine combination is different and it never hurts to fine tune to suit your needs.

 

[Don R]

 I read different rockers vary slightly in their ratio. I wonder if you could achieve better results by swapping a few sets around?

[Jerry Rxman Griffin aka MuthaF'er]

From what I'm reading you want to adjust lashes differently on individual cylinders? From what I'm thinking is you could achieve the same lifts perhaps but wouldn't that affect the opening and closing degrees?

[PeWe]

From what I'm reading you want to adjust lashes differently on individual cylinders? From what I'm thinking is you could achieve the same lifts perhaps but wouldn't that affect the opening and closing degrees?

My idea was to achieve more even opening/closing which is possible in the theory when reading my cam timing result. Lift will be slightly different then....
I have another set of cam towers, rocker arms with axes that all look much better that I'll put in and time cam again, advance it 3-4 degrees.

[Old Scrambler]

I'm watching this with great interest.................particularly since I have the same cam.  My concern is the valve to piston as well as valve to valve clearance on overlap at the end of the exhaust stroke. My pistons are 12.5 to 1 at 1mm over stock with an F2 head.  A few thou will be skimmed from the head which will favor higher compression. Optional base-gasket and head-gasket thickness is ready for maximum compression if the clearances are OK.

My cam tower selection is ample.............taken from 3 low-mileage motors. Agree that not all rockers were created equal  

[PeWe]

12.5:1 pistons.. designed for an F2 Head or K ? 
K 12.5:1 pistons in an F2 head will be lower CR. Maybe 10.5 something?
F2 head has bigger valves on both IN and EX with different angles too If I have understood things correct.

Valve to piston is easy to check at max lift, tighten the adjuster screws until it stop to verify the clearance.
2mm clearance will be OK IF I have read and understood all threads here correct

I have F2 inlet valve in a K head.
I'll replace the cam towers and aim cam timing to close IN at 55-56 degrees @0,004 Lift (1,0mm)  0 lash

[Old Scrambler]

My pistons are for a K motor..........and fully agree with the lower CR in F2 head...........until you consider the 1mm overbore, the 3-4 thou" skimmed from the head and cylinder (total 6-8) and the thinner gaskets.

Your F2 intake valve is about 1.3mm shorter than the K-valve and requires different collets and keepers.

[PeWe]

Yes I've noticed that the valves are shorter when adjusting the lash. The F2 valves were installed by our local tuning company back in the days when I let them port the head.

I have a set of CB900 DOHC con rods to be used in a future project. They should be possible to use as direct bolt on if 1,5mm extra lenght is possible without head/valve contact and too high compression. I suppose a long duration cam will be one way. Extra base gasket on the list when testing too. The old Henry Abe 900 pistons I have look not that high, 1,5mm closer the head might be fine.

[Old Scrambler]

The CB900 DOHC rods are also in my box for next-year's build..................then I think I'll kick-back and enjoy my riding days rather than sweat in the shop. The F2 head has chambers that are about 1.1mm deeper at the valve and 1.9mm deeper at the dome-center between the valves.

[PeWe]

I look forward to have my bike in shape and enjoy it. It will be fun to have another side project to build on without the stress to finalize it for the riding season.
- I'll cut the frame coming weekend, install a frame mod kit http://forums.sohc4.net/index.php?topic=62832.200
- Open head cover and check the timing again with same equipment and procedure I'll use later after replacement of cam towers....
- Remove everything on the head and replace HD studs, hopefully possible. I snapped one when tighten last year, replaced only the snapped one. I'll replace all the other (except the short ones) just to be sure that they will not snap later on when driving and get a nut in the engine.
- Install other cam towers with rocker arms and axes. look better than my old. Maybe different timing result.
- Time the cam once again, but this time advance it, prioritize IN valve closing at 56 degrees.
I'll take photos of all steps so I can check later on if I used proper rubbers under the towers, all 4 small o-rings etc...

[Old Scrambler]

So all of that should eliminate the temptation to set individual valve-adjuster clearances. In a perfect world the cam to rocker to valve-stem contact will all be in-sync with the same amount of lash.  I have compensated for 'dimples' on the valve stems by reducing the lash.

[PeWe]

Maybe differences in valve openings is not a problem. I thought it could be like carbs out of synch.

[70CB750]

The valve lash has secondary effects that are not visible at the first glance.  Like bigger lash on exhaust side lets the valve sit on the head longer - leads to transfer of more heat from the valve and thats a good thing.

I don't think you can get it done within possible valve lash interval anyway.

[PeWe]

............ Like bigger lash on exhaust side lets the valve sit on the head longer - leads to transfer of more heat from the valve and thats a good thing.
.............

Thanks 70CB750, I always forget why the lash is so different between cams... It must be the cooling factor..I remember now that I have read about it but forgot it.
Some cams have as much as 0,15-0,2mm lash on EX