Topic: F2 Cam in K4 head in High Comp 849 kit; Valve Clearance question

[banzaibob]

Everything is together and happy in the 849cc Cycle X high compression piston saga.  I decided not to go with a thicker base gasket and instead went with the F2 cam with its greater lift and longer duration.  Everything is torqued down and rotating smoothly.  I have not fired it up yet. 

I am assuming (hoping) that it will probably not have as much of the original problem of low rpm/high load spark knock that it once had.  I'm also hoping that it will have a little more zip in the mid range and top end too.  I have a question that may not even be unique to CB750's though.

Question: By increasing the valve clearance from .002-.003" to something like .004-.005" would that in essence be changing the valve timing to less duration and retarded timing?  I don't want to destroy the stems on my valves first off.  But I guess I was wondering what, if any kind of effect it would have on overall performance? 

[HondaMan]

Everything is together and happy in the 849cc Cycle X high compression piston saga.  I decided not to go with a thicker base gasket and instead went with the F2 cam with its greater lift and longer duration.  Everything is torqued down and rotating smoothly.  I have not fired it up yet. 

I am assuming (hoping) that it will probably not have as much of the original problem of low rpm/high load spark knock that it once had.  I'm also hoping that it will have a little more zip in the mid range and top end too.  I have a question that may not even be unique to CB750's though.

Question: By increasing the valve clearance from .002-.003" to something like .004-.005" would that in essence be changing the valve timing to less duration and retarded timing?  I don't want to destroy the stems on my valves first off.  But I guess I was wondering what, if any kind of effect it would have on overall performance? 

For hi-RPM work (roadracing, midget cars) we would routinely increase the spec to .003" intake and .004" exhaust. Above .005" intake and .007" exhaust, the tops of the stems start mushrooming, and on the K6 and later cams with their steep closing ramps, they cause a dent across the cam face on the closing side of the intake lobes after a while. Technically, it reduces lift by the same amount, but more dynamically, it also raises compression slightly, to have more clearance.

For touring use, I have long (and still) use .003" intake and .004" exhaust clearances. The engines run cooler (hand-to-oiltank ride temperature tests for evaluation) and start better when hot, by just enough to notice.

[banzaibob]

That confirms what I thought.  I think that I first read something like that in one of those Kevin Cameron articles or something.  Anyway, the main issue with the F2 cam was piston/valve clearance.  At no time did it ever exceed .140"(from memory).  Hopefully that number won't change at 8000rpm. 

Thanks for the advice!

[Don R]

 What springs did you use? I used an F2 cam and loved the torque, I did wing it a bit on a missed shift and tagged (bent) a valve. When I repaired it I put in another new valve with aftermarket springs, retainers and a bigger cam. Kenny Harmon F type. The jury is still out on the big cam. Winter came and it still sits.

[banzaibob]

I used the F2 springs with the K4 valves.  I had to use the F2 spring seats also.  The K4 seats were slightly thicker and the F2 springs would bind against them ever so slightly.  The F2 seats fit in the head perfectly. 

Missed shifts are a little scary.  I'll just have to be careful.