Finally got around to taking off the cam towers. Looks like a helicoil was to blame. When I bolted down the exhaust side of the cam tower it started to pull the helicoil out. Problem identified, now I have to fix it. I can put another helicoil in but screw it further down the hole and hope fit doesn't pull out, or.....................
Ace:
This is the Achilles' Heel of the 750, on reassembly. That long 6mm bolt on the cam towers should only be bolted to 7 ft-lbs, no more, ESPECIALLY on the bikes before the K3. This item is only covered in the original K0 manual (like mine), and never appeared after that in any manual I've seen: I have a post about it that Steve D. put into the Thoughts of Hondaman collection.
You can set that helicoil in deeper or install a new one: if you do a new one, use the 1/4" long version for more bite. When tightening the bolt, also spin the rocker shaft: stop tightening if the shaft stops spinning. (I'm presuming the little 5mm bolts are not being used to lock the shaft in place, here: you don't want to use those, if you have them...) If, at the non-spin point, you have not reached 7 ft-lbs, then pull the bolt back out and apply some BLUE Loctite on the threads and reassemble, again stopping early if the shaft wants to stop turning.
The story of these rocker shafts: in the K3 engines (first appeared in late K2, actually) Honda installed these little 5mm bolts, two per shaft, to stop the shaft from rattling a little. This was due to an effort on their part to quiet down the bikes mechanically: the whole country at that time was riding the 750 with Vetter Windjammer fairings (by the tens of thousands) and the 750 was the Premier Tourer machine. But, the top end noises get reflected back into your face with those fairings, and Honda wanted a quieter image.
When they locked down this little rattle, they also made small oiling grooves on the shafts that were supposed to collect the splash oil through the rocker's oiling holes to get oil down to the bottom of the rocker, where the pressure is highest. Trouble: the holes on the rockers are not accurately placed. Many, many (most, really) ended up with very little oil on those spots, and the shafts and rockers wore out in as little as 10,000 miles.
At first, we used to replace the rockers and shafts and NOT install the little 5mm bolts. This change alone puts between 1/2 and 1 HP more on the ground. Later, I came up with the following performance enhancements, which I just did on a K3 (Hondaman Special #1) last month and photo-documented: open up the rocker oiling holes (sideways if necessary) to "reach" the oiling grooves on the shafts, and grind the sharp edges of those shafts' grooves, if needed (and try installing them in different orientations), so as to get the groove to show up in the rocker's oiling hole when the shaft is pushed to both the left and right extremes of its travel against the single center bolt. This change almost completely stopped all the tiny clicking noises by using oil as the cushion, and it will add even more HP to the ground on that bike.
This fixup presumes you have the K3 & later style of shafts: it can be applied to the current Honda replacement parts for a K0-K2 as well, because they have changed those parts to NOT rotate by changing the center V-groove to a side-slot instead: this stops rotation again. I am making my own, now, from Thomson rod (12mm), grinding in the center 360 degree groove. Maybe by Winter I'll have some for sale for those who want them, but my bike gets the first set.
