Just picked this up : 1974 Kawasaki Bighorn 350...

Molasses and water mix and soak/fill the tank

Just get this. Problem solved.

https://www.kbs-coatings.com/cycle-tank-sealer-kit.html

I wouldn't try sealing a dirty/rusty tank until it was cleaned. Even then, I wouldn't seal it unless it was leaking and unable to be welded.

Try using mineral spirits and drip a length of chain (not motorcycle drive chain but something you'd use to lock your fence with) into the tank and slosh it around. Keep changing out the dirty chemical. I heard Evaporust works good too, but I've gotten by with the chain method more than once. Actually was able to remove a bad seal job with chain and MEK from a 79 KD100 tank.

My Dad had a 75 F9, would love to find another one someday.

Best way to clean a tank is to use some sort of media inside to remove all of the loose stuff. Something with rough edges or multi-faced media works best. I personally like steel grit blasting media available at places like Northern Tool or Harbor Freight. Also use some mineral spirits to help lift the media. You can run the media through a paint strainer after each cycle to save for re-use. In a pinch, I have used misc. hardware like nuts, bolts, gun pellets, etc.

This process takes a long time. It typically takes me hours to completely clean a tank to where I am sure it is usable. You will never get it back to 100% new, but the important thing is that you do it long enough to remove anything that could be knocked loose. I highly recommend running a cheap inline fuel filter for the first several tank fulls.

In the past, I have had ideas to rig up some sort of vibratory plate using a palm sander. I think you could construct some sort of wood box to hold the tank and then attach the vibratory sander to the wood which would then let you just fill it with media and vibrate for hours on end. I just never got around to trying it. Then again, a regular vibratory sander may not be violent enough to do the job.

I do not recommend cream seal kits unless your tank is severely (i mean really bad shape) rusted. Cream seal kits are more headaches in the long run. I have cleaned many tanks w/o cream sealers and never had an issue as long as you spend the required time up front to do a good job.

Those apparently make surprisingly good streetbikes as production class vintage racers and runabouts. I was fascinated by what Harry Klemm does with them at Basic F9 Bighorn restoration/performance methods/issues by Klemm and F9 and F8 mods. If I had a solid F9, I'd be shipping the motor, swingarm and front end off to him, or selling the bike to someone who would. The AMA used to allow that engine in the 250 roadracing class when it was new, and Kawasaki U.S. had a factory team on custom chassis that made life more interesting for the ubiquitous Yamaha twins. I expect that a Klemm-built street version is about as quick/fast as a 250 or even 300 Ninja, but a pretty cool/stealthy alternative way to get there, especially if it can do it on pump gas with the stock exhaust pipe. Also a potentially rad dualsport bike with DOT knobbies, though Klemm does not like oil injection because it's not "smart" enough to handle certain throttle situations.

Ancient riders from the Houston area might remember Robby Holbert riding a modified 350 Bighorn for A.J. Foyt's Kawasaki dealership back in 1971, before the F11/F12 MX bikes got here. Holbert ran up front on anything but possibly did fall off that Bighorn more often than his CZs or Huskys. Not a good offroad chassis.

The frames Kawasaki used for the Bighorn road racer were production A1R 250cc twin or H1R 500cc triple frames.

My older brother bought an A1R framed Bighorn ,with the F81M lower end, 350cc top end from a local dealer here about 20 years ago.

It hung in the rafters of the shop after being crashed at Daytona in the Sportsman races around '71-72. A sprocket bolt had backed out, jammed against the swingarm, and crashed the rider at about 90 mph.

When it was purchased , my brother brought it home, cleaned the carb, fresh fuel, and it lit right off. It had a huge Mikuni hanging off the side, at least a 40mm. A noted two stroke Kawasaki tuner (Woody Kyle) mentioned he had only seen a few that had the cases machined to accept that big of carb.

He stated Irv Kanemoto did most of the ones he saw with that much carb on them.

Sadly, the motor was removed from the chassis, and traded off for Kawasaki Greenstreak 100cc parts, which never came through after the bike was sent out west. The engine remained on the shelf, until about 5 years ago, when it was sold off .

It was a cool bike, crashed up or not.