I ran across this old thread while searching for other 510-related info and have decided to add my 2 cents on the air-cooled vs water-cooled power debate.
Back in 1986, my father purchased a new 510TX. The next year, I purchased a used 510TC and the memory of riding them back-to-back still lingers.
The air cooled bike seemed to have more engine response at the crack of the throttle but then revved fairly slowly after that regardless of throttle input. The sound of the fins was substantial but the 2-into-1 Supertrap Dad had on it was as well. When compared to his previous bike, a 1979 Honda XR500R, the TX seemed like a pure racing machine with lighter weight, more throttle response, and way more horsepower.
The water cooled bike was a big improvement and just seemed "tighter" overall, probably due to the lack of cooling fin noise. When I first purchased the bike, it had the stock twin FC mufflers on it which seemed to be a significant restriction compared to the 2-into-1 Supertrap. But, even so, my bike was still much quicker than Dad's. It didn't quite have the same initial response but more than made up for it in mid-range and top end. I would argue that it didn't have the additional 10 horses (or reindeer) mentioned above, but I'll bet it had at least 5 more. Then I installed the Supertrap and it got even better. The power came on earlier and pulled longer with more mid range and top end. I would agree that the rotating assembly was lighter on the L/C bike because it definitely revved quicker when just blipping the throttle.
Years later, after moving on from the TC (which I wish I had never sold because it was one of the best bikes I have ever owned), I ended up a starving college student with no motorcycle. Dad took pity on me and gave me the TX since he had moved on to a Honda XR650R and the Husky was just sitting around. I rode the TX mercilessly and it never missed a beat. Eventually it seemed to finally get tired so I took it to an old school Husky Dealer in Reno to have him look it over. We decided to retrofit a newer piston from a Husky 410 L/C bike. He said that this piston would have tighter piston-to-cylinder sizing so as to better fill the tired old cylinder and the diameter and compression height matched those of the original. I seem to recall him also doing some valve work. Maybe it was new valve springs and valves with guides and seals. In any case, after some machining of the piston to fit the width of the little end of the rod, he got it all together and I spent a few more years beating on it. I can say, with much certainty, that the piston change resulted in the '86TX running almost as good as the '87TC.
Eventually, the TX ended up in the hands of my cousin-in-law, Alex, who continued to ride it until, tragically, losing a battle with leukemia just a few weeks ago. My hope is to end up with the bike so I may, in his memory, treat it to a nice restoration.
So, there are my 2 cents.
510 should account for the new four bangers, just not as responsive (=good!)
