Hi Steve, I like long rides. 60 miles is my coffee run. Lunch run 150-200. A good all day ride 300-350 miles. 300 mile day on my '73 TR7RV, no problem.
Cross the USA is about 3000 miles. That's a long way on a bike seat. You want to do many practice rides to condition your body & prove your bike.
2 questions. First for comfort I find the T140 type bike to be a little better. To my butt the seat is more comfortable & the high US type bars tend to be easier on my back. I don't use windshield, but that really reduces fatigue. I find wearing ear plugs really reduces fatigue also. Cuts wind & bike noise. I like the cheap rollup soft foam type. The seat is a killer for many on long rides. There are various seat pads available that seem to really help especially if your padding is not enough. They just strap on over the normal seat.
Regarding reliability all the unit motors are similar in construction so no huge advantage. Even preunit motor is similar in concept & from what I observe on club rides is about the same it reliability.
As Hawaiian Tiger says it's the build quality that counts. His best bike was made of used parts. Good used parts are good & work good. However, again the build quality is everything. Build quality is not just installing new parts, but how the parts are installed. This goes from end to end on the whole bike. Some new parts are junk. Every part must be examined on its own merit used or new.
If I may I'll state some observations on reliability. First, will this be a supported ride with a parts truck that has spares, oil, & can haul your bike in case of break down. If yes that's easy.
Remember parts will often need to be ordered when you're on the road. You'll have to carry all your tools.
If no you are 100% on your own. I've traveled much in this land & even today many (most) remote areas & many canyons have no cell phone service. Fuel stops can be a good 100+ miles apart. Then they may be closed or have a power failure & can't pump gas. I recently ran into that, then a 50 mile detour with no gas. I was sweating it, but was ok.
First thing I see is parts falling off bike on the road. Various Fasteners
came loose. Lock nuts & Loctite can pretty much prevent that as well as proper tightening. No telling what might fall off.
Next is electrical issues that cause not spark. Almost always looking at bike the wiring is substandard. Frayed/cracked insulation, loose connections. Connector nuts coming loose like on points, coil, battery
connections. Many times fuse holder problems. Very often bike works ok in dry, but in rain or wet fog bike starts running poorly. Again poor connections & when wet get too bad to work. Also water in points housing is often found. Short circuits where wire chafes through insulation & blows fuse. That is fairly common. Most of this is 100% avoidable by examining every last inch of wire & all the connectors on entire machine.
Broken cables is seen. Most often the clutch cable end pulls off unexpectedly. This had been my bikes only failing. Pulled end off of 4 cables. Barnett cable has been only one to last with the swaged steel ends. I still am gun shy, so carry a spare.
Overall carb issues are few. Most I see are carb flooding out tickler hole continuously. Almost always removing needle & flowing fuel flushes out the dirt & all is well. Examining bowl often shows tiny rust particles. Tank often has rust. A real fuel filter, not just the screens can reduce this a great deal. Of course a clean tank is a must.
Regarding carb drains, the later Concentric
bowl with drain is what I'd call a must. It's almost impossible to ride across country & not encounter rain. While I don't see water actually entering carb while riding I've seen it get wet parked in heavy rain. The drain allows one to drain out any water & loose dirt. I've ridden in rain many times & many miles. Seen lots of water in carb, again can't say how it got there. Maybe even through gas cap hole? My tank had light rust & caused me carb problems.
The large gas tank on some of the Tigers is very helpful when fuel stations are hard to come by. To my ears I think the wider tank deflects some engine clatter noise also.
The later trans cover with the oil filler plug allows very easy access for clutch cable.
The 1970 & later primary breather makes the oil level in primary self leveling so you never have to worry about primary oil level. Not a big deal, but saves some time & hassle.
Oil filter is a good idea on the long haul. Personally I'd ride 3000 miles across country without oil change using filter & synthetic oil.
Electronic ignition or points is an issue only you can decide. Well serviced points will go 3000 miles no problem. Electronic needs no service. You can start & ride bike with dead battery
with points, not with EI. So it's a personal choice.
Regarding 1 or 2 carbs. I've serviced many Bonnies & Tigers. When I needed a commuter bike I bought a Tiger due to servicing was a little easier. A little easier to access intake adjusters, no carb syncing. I don't think 2 carbs actually make the bike less reliable. Just a little more to work on.
Rear chain wear can be an issue. You need to find a chain lube that actually reduces wear. I can't figure that one myself. This chain I've been using 85w GL5 differential fluid. A big mess, but seems to wear well. I'd like to do better though with less mess.
For sure I'd ride a vintage Triumph across USA. On a well prepared bike I'd expect to do the trip without a single break down. 60-65 mph is no problem on these bikes. I like 20t front sprocket. When temps get near 100f ping can be a problem. Never lug the motor, keep it spinning freely, even if you have to ride at 55mph all day in 3rd. West bound highway 10 in California with a 35 mph head wind comes to mind. Vicious!
My personal choice would be the 1970 Tiger, especially an early production with the plunger for shift cam, for the reasons I stated. But again all these bikes are very good well prepared.