The older posts about 7+ degrees advance isn't very practical with the combination of today's gasolines and the K7/8 cam, which opens intakes at 0 degrees BTC. It is useful in cammed earlier bikes (like roadrace level cams) but in street use would be pretty hard on the starter, at the least. It also requires a much slower advance rate (i.e., heavier springs or significantly trimmed stock springs in the spark advancer) so that it doesn't spit back out the carbs around 2000-3000 RPM or so. As one example: my own K2 has springs trimmed back to reach full advance at 3800 RPM currently (1 full turn on one side and 1-1/2 turns from the other, I think - would have to go look at my last notes, might be 1-1/2 from both by now) because I seldom run below 3200 RPM, even in town. This is due more to the roundtop carbs than anything else: the K8 has more accurate carbs, if leaner-burning to meet smog rules of the 1970s era.
When you set up the carbs, pay close attention to whether the air breather hose(s) exists that vent the bowls: on the K7/8 bikes they are often missing because of age making the rubber hard, and they split off at the little elbow where they join the carb (make new ones!). Some have 1 per carb set, some have 2. Make sure those hose(s) route to a quiet-air place (higher pressure there), like up under the seat, if you change the airbox: Honda's original design used the large airbox to create the quieter air behind it for this vent, and it worked excellently with the Vetter touring fairings or roadrace fairings that enclose the whole carb area. But out in the open, not quite as much: the PD carbs are just real sensitive to bowl pressures altering the depth-of-fuel inside the bowls when running faster than 40 MPH. Some folks have (wisely) opened up these vent holes a little bit: a max size of 0.035" would be my gut feel for that operation.
The K8 carbs run lean, on purpose, for smog reasons. They can be enrichened about 5% to make up for that: if they are #105 now then going to #110 is about right (genuine Keihin numbers, not much more. Modern fuels burn MUCH slower than those of old, which quickly becomes a problem. (!)
Hondaman,
The 77-78k camshaft has the same part number as the 76F. Part #14101-392-000.
Same for the camshaft sprockets. Part #14321-392-000. The 76F’s supplement is showing 5* before and 35* cam timing. Compared to 78k’s 0* & 40*. I don’t see how it gets 5* retarded in the Ks’ with the same cam and sprocket as the 76F..
Perhaps a typo…?
Yeah, Honda's 'kaizen' updated part system at work - it was set up so that any later-made parts would still work in an earlier-made bike (it almost does, but not quite).
The cam sprockets all provide identical cam timing if a new sprocket is just bolted to any cam, but the ones that showed up in late K5 and F-onward bikes have the lightening holes in them. The aftermarket ones often have slots milled for the timing holes for playing that game.
The F0/1 bikes I've measured (roughly) but putting a 0.001" feeler under the #4 intake rocker and slowly turning the engine toward its TDC have showed the gage trapped before 0 degrees TDC (I consider this to be Honda's 5 BTDC version). In the F2/3 and K7/8 the TDC goes past the (points-plate's) window before the feeler gets trapped, indicating a bit of cam chain stretch combined with the later-opening intake. This has long been my 'quick test' of which cam they have.
But...I have come across K6 bikes of BOTH stripes, meaning some had the earlier cams and others had the later ones, most interesting...
Along with this, I have seen a (virgin) K7 with a cam that acts like the early ones with this 'test' (more than one, now that I think about it) and at least one F1 (which was also an 836cc forged-piston engine) with the intake that opened ATC, and upon entering the engine found it to be a Honda cam (maybe someone swapped it for an F2/3-K7/8? Dunno..). The cams look so similar when sitting side-by-side that I always have to put it in an engine to figure out which one it is (yep, I have a half-assembled engine I use for this!). The only OEM cams I can ID by looking at them directly is the K4/5 versions (one is now in my engine) that came from the famous 50 MPG gas mileage ad campaign: they have steep closing ramps for the intakes, so steep that they often show no wear at all on the backside of the lobe. They open intakes just like the early cams, at 5 degrees BTC.
The 'anti-smog' cams for these engines have the later-closing exhaust valve (40 ABDC) to increase scavenge a little bit: I've seen these in California and Washington (State) bike engines before, all were 1976 or later builds. They all had PD carbs, too. But, at least one of those had a 'normal' intake of +5 BTDC, too (in fact, I think that engine is the abandoned black-painted 836cc rebuild that is sitting in my garage since 2014).
I should also publish this 'disclaimer' about the numbers: while Honda called them 5 degrees BTDC, the K2-5 engines (and others, similarly) were often just 2 or 3 degrees BTDC in real-life measurement, even in brand-new bikes - such was the production tolerance at the time. I actually measured new bikes coming out of the crates myself when the "K" number changed - I'm not all that rabid about it nowadays.
