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CB750F3

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royhall:
It's a while ago now, but I think that information came from Hondaman on the US site.

Had a look and here it is  http://forums.sohc4.net/index.php?topic=10540.0  It was a Hondaman thread but sixth reply down by "OldFart" has the details. Makes for very interesting reading about the Delta changes.

neilg:

--- Quote from: K2-K6 on October 02, 2017, 07:17:30 PM ---Piki, the F3 I don't think you'd tell it apart from a F2 as they're essentially their same bike just moved the name along one notch,  did the same with the K7 to K8 as well. Think USA only.

Roy,  I know it's recorded some places ( I've found it mentioned in contemporary road tests)  that the F2 used different valve angle but I can't find anything official that says that,  don't know if anything exists.
I've seen the two head's apart but never next to each other to make a comparison directly. The F2 has bigger fins as far as I know but it just doesn't seem to me that the valve angle changed,  after all its only 3mm on exhaust and 2mm on inlet.  I'm not certain of it but certainly intrigued if they did change it.
Also,  more time ago than I want to remember,  a friend ran what I thought was K2 cam gear on an F2 head to replace a damaged set, or at least that's how I recall it.  I don't know how true that is but would obviously not be possible if the valve were moved.
They also seem to be in the same place inside the tappet adjust caps with the rocker covers being the same part it seems.
It don't know if anyone else on here has for example the rocker part numbers to compare perhaps.

--- End quote ---

I believe the valve angle would have had to be wider due to the deeper combustion chamber in order for the valves to fit the shape of it. Also this would gain full advantage of the cross flow effect. In a way it shows that the original CB750 SOHC design was spot on and later attempts to "improve" stretched it to the point where DOHC were the only way forward. 

royhall:
Rushed updates to keep the 750 current against newer competition. Not enough testing time initially as the 'delta' part later fixed the problem.

K2-K6:
I guess it's interesting for those working on them as to what may fit considering how few parts may be left.

I understand the reasons for moving valves but just from an straight update engineering point of view it just doesn't look like the valve springs moved or have any space to move from original architecture. As before,  I'm not certain at all what happened it just looks like they didn't move.

As you say Roy,  they must have already been developing the next engine,  the twin cam 750/900 while they did a last iteration of the SOHC motor for its last year.
You can see the thinking in the endurance bike they first ran in 1976 I think. Called the RCB, it was a 750 SOHC bottom end with a narrower angle 16valve twin cam head on it. They even did one with a gear primary drive modded into those cases as it developed. Then there racer switched to the 900 dohc road bike design as a complete update.

Interestingly the road test I've got at the time puts the F1 as faster in time at 13.2 secs over the 1/4 mile as opposed to the F2 at 13.5,  they speculate it's because the F2 gained over 25lb in weight. So all that development and risk to go slower.

neilg:
The location of springs and the top of the valve stems would have stayed the same the slight change in angle would affect the position of the valve head within the deeper combustion chamber, moving the head slightly higher to allow the bigger valves room to overlap. Hence the exhaust valve is slightly more in a horizontal position, (too slight to see by eye I would imagine). This in effect threw out the geometry so the stem is pushing to one side in the guide and so the excessive wear. Honda's original cure was to do away with a couple of oil drains which I imagined they hoped would cure the issue.     

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