I'm wanting to really keep my project stock, which includes the existing paint. Does trying to keep original paint in place make the most sense or should I be considering a full re-paint on the tank, ears, headlight bucket and side panels? The original paint is not perfect, with slight surface rust and areas that have worn down to bare metal in places. I like the patina of the original paint as all the marks remind of something in the bikes past, but I don't want any of the surface rust and bare spots to progress into more issues later on.
Thoughts?
Also, a question related to the original throttle tubes ... were all original throttle tubes made of metal like mine? As I look online for parts, all I see are plastic tubes.
From your underside picture of the tank(bubbling paint and cancer rust flaking), there might be rust inside the tank at the bottom forward of the petcock from gas with water in it, (water in gas settles under the gas and rusts tanks) and that is probably why the paint is bubbling and rusting there. This will need addressing or you'll be leaking gas there fairly soon. Close inspection inside the tank down low is necessary. Use a good strong flashlight or better a bore scope to determine. If bad, there are people who have cut out the bad area, which usually runs into the folded edge under the trim chrome strips. (carefully bend the retainer tabs back to remove the chrome strip for further inspection. Is the outer paint bubbling anywhere under the strips?).
There was a post in the past year where the tank looked great and he had a dent massaged out but he failed to inspect the lower inside for rust and ended up finding bubbling paint with pinhole leaks under the paint that looked so good.
I have seen posts on repairs having been done to save the tank when the paint is as good looking as yours is(top picture shows minimal sun fading or scratches, no bubbles in the paint?). Have you de-rusted the inside of the tank yet?(5 gallons of Evaporust or Electrolosys method)
I'm not a fan of tank lining as many of the problems with failed liners are from someone not getting All the rust out first before putting the liquid liner in and also the chemicals added to the gas today attacking the liners and clogging the carbs later on. Do your full homework before you choose to go down that road......
The underside of the tank needs addressing. I would suggest after removing the chrome strips and inspection, is to sand off the loose rust areas to bare metal to see if the metal is solid. Pin holes will require tank repair of cut out bad areas and new metal welded in.
Evaporust has a gel that you can spray on if the metal is good, use a brush to coat the rust fully then cover with plastic wrap to slow evaporation. Once the time is up you rinse it off with water and dry the surface with paper towels or rags and then a heat gun on low(be careful not to blister the outer tank paint!). Then prime the underside of the tank followed by paint to protect it from further rust. Honda didn't put a lot of paint under the tanks....
As to the throttle tube, apparently the early 750s had metal and probably in 1972 k2 they changed to plastic (the plastic was being made better around that period of time than early days of plastic, as formulations evolved). I was a parts manager in the mid 70s and kind of remember the change period of the tubes.
If the barrels of the cables hold well in your tube, you don't necessarily have to change to the plastic. If there is a lot of slop in holding the barrels, you can add solder using a torch to build the hole wear back up if needed, shape flat with a file, then re-drill the hole. Or change to a new plastic tube for riding but keep the metal tube to stay with the bike for a 100% restoration. K1 models will be gaining higher values soon enough for 100% original parts complete bikes.
