Plagiarized right off the SOHC/4 Owner's Group mailing list, I've attempted to construct a mini FAQ about the SOHC/4 bikes, though I only [attempt to] answer the 650-related ones.

This list is the brains-children of Darrell F, Dieringer and Neel Vasavada.

I took my bike to the dealer/shop and its still not fixed...
Bummer. Happens a lot, though. Best to remain calm, bring a list of post- and pre- shop problems, an older man [preferably with a bit of wrenching experience] an a good attitude back to the shop. Be cool, 'cos nobody wants to help out an asshole.
In a word (or rather, number) 836.
:) A rather popular overbore displacement for the 750s. Unfortunately, the SOHC/4 CB650 wasn't popularly raced, so there's no overbore kit for these bikes. At least, none that I know of.
Does anyone have some tank badges they want to sell?
J.C. Whitless
Now, now, you get what you pay for, and you pay for what you get. Though sometimes, the J.C. Whitney catalog has some good products. MCN did a review on a motorcyle ramp that was extremely favorable. The precision measuring gauges I bought (a digital micrometer and telescope gauges) are of rather high quality. Can't say much for the cheaper two-dial carb-synchornizerm, though. Not too pleased with it. I'll buy mercury, next time.
Stupid f*&#in'oil filter bolt!
Yeah, tell me about it. It's supposed to be a 17mm cap-screw, but age will tend to shrink the actual working size (read stripped head).
How to fabricate your own heated vest/EFI/12v - 3000W 110V inverter/rectifier in 35 easy steps.
For sale - 1969 sand cast k0: $43,500 firm, Saskatchewan.
What's the deal on the sandcast motors?
Wheelies/waving... Sorry, that's rec.moto.
Or wreck.moto as it's affectionately (and appropriately) called. rec.motorcycles is one-size-fits-none motorcyling Usenet newsgroup.
Is $3100 too much for a 82 650 with a fairing made from a garbage can?
Probably. My 80 CB650C cost US$1050 from a dealer. Inspected and all. YMMV.
My CB750 leaks oil from around the head...
Really? Remove the engine. The SOHC/4's are a bit notorious for leeking top-end gaskets. Haven't heard any complaints about the bottom-end, though.
I have an old tank that's a little rusty. Does this Kreem Stuff work?
Yes, just follow the directions.
My main fuse keeps blowing...
I was at my local dealer and they wanted $2,325.32 for a clutch cable...
Parts will be expensive, since the bikes are all rather old. Fortunately, many are interchangable.
Splitfire sucks.
Yes, they do. No appreciable gain from using them. Stick to NGK or Nippondenso. Folklore says Jap plugs for Jap motors, German plugs for German motors, American plugs for American motors. Maybe that's a crock, maybe not. Hmmm.
Does anyone have opinions about DYNA S electronic ignition?
Yes. And they vary. OEM-replacement Dyna's seem to be nearly trouble-free, on installation, performance, and lifetime.
I need to store my bike this winter...
Where can I get NOS exhaust?
From dealers and scrapyards. OEM-replacements can be found at Motorcycles Accessories Warehouse.
My cam chain makes this awful noise...
Mine, too. I think it's supposed to do that.
I went riding yesterday and when I stopped there was gas flowing out of my carbs...
Pick your bike up off the ground, and the leak will stop. :)
Looking for original 750F sidecovers...
Forget it.