OK, I have done the tensioning procedure as set out in the Helms manual provided by qtiger.
The instructions are slightly unclear on one point. They say, "Rotate the crankshaft 5-6 revolutions to set the belt," but they don't say if the tensioner bolt should be tight or loose while doing so.
However, since later on in the sequence they say to back the tensioner bolt out a half-turn, I assume that the bolt should be tight during the crank-turning step.
Anyway -- with tensioner bolt still tight, I pulled the valve cover and the plugs, got a socket on the crank pulley bolt, rotated the engine 6 full turns CCW, brought it to TDC1, backed off the tensioner bolt 1/2 turn, rotated the crank CCW to produce 3 teeth past TDC1 on the exhaust cam marks, stopped, tightened up the tensioner bolt, reinstalled plugs and valve cover, started engine.
Instant rattle. Gar.
So let's assume for the moment that I am doing the tensioning procedure properly but the belt may still be too loose. What could be the cause? I can think of three possible sources:
(a) New belt may be so far out of spec on length that the tensioning mechanism can't get it properly tight. The new belt *was* noticeably longer, as mentioned.
(b) The tensioner (which is also new, a fresh-box GMB unit) might be boogered up somehow. It felt fine and it was visually identical to the old unit. I did check to see if the tensioner spring was on tight. It seems to be.
(c) The water pump, also new GMB, might be the wrong unit. I do recall that I did a close visual comparo between that and the original pump. No difference to my eye.
So how loose is the belt? Let me run this by some more experienced eyes. I took some photos that illustrate the amount of slack in the belt system. Looking in between the cam sprockets, the belt varies from visibly tight to visibly loose at various points in the rotation of the crank. I have shot tight, loose, and the amount of deflection that a thumb can easily produce at both the tight and loose points. Sorry, these aren't 56K friendly.
http://home.pacbell.net/rteasdal/belt_at_taut_point.jpg
http://home.pacbell.net/rteasdal/def...taut_point.jpg
http://home.pacbell.net/rteasdal/bel...lack_point.jpg
http://home.pacbell.net/rteasdal/def...lack_point.jpg
This is a fairly large amount of variation, but I don't know how much is too much on a Honda engine. It seems reasonable that too much variation between taut and slack could be whipping the distributor drive and causing the rattle.