[Roger]

If I take a new RE 120 miles over a mountain range 4 times a week how long could I expect it to last before major engine work would be required?   There's about 2500 feet of elevation change over this path.

[Arizoni]

The answer depends on a lot of things.
The first thing is to make sure the engine/transmission has been broken in right.

After that, except for regular maintenance it should not have a bigger problem than most other motorcycles of a similar size would have.

The altitude change won't be a factor as far as the air/fuel mixture is concerned.  The fuel injector body has a sensor that tells the ECU about changes in altitude and the ECU changes the mixture to accommodate the change.

Hopefully you realize the Royal Enfield UCE has only 28 horsepower so you will need to plan on riding at slower speeds on those mountain grades.
If the engine sounds like it is really working hard it is a good idea to shift down and ride a little slower.

[Ice]

 The UCE has a full roller bottom end with a forged con rod and a high volume oil pump. The EFI is made by Keihin in Japan. It's mapped from 350 ft. below sea level to 20,000 above and tested to 18,500. 

 You'll be fine just keep up on the oil changes and look after the chain.

[gizzo]

it's a royal Enfield, so who the hell knows? could be 10 years, could be a week. at least you might have fun finding out. all the best.

[Ice]

Roger, how many miles are on your Bullet ?

[mattsz]

it's a royal Enfield, so who the hell knows? could be 10 years, could be a week. at least you might have fun finding out. all the best.

The correct answer...

[Narada]

You tube is overflowing with videos of people riding Royal Enfields all over the Himalayan mountains. Many of these roads are the highest and dangerous on earth.

They have been riding those mountains since 1955 when the Indian army purchased 800 of them (due to thier excellent reputation as trials bikes) to fend off Pakistani Jihad in the mountains of the Punjab.

It seems to work for them. 

[wildbill]

i'd say about 3 days.. it's an enfield so it must be ridden carefully or a week on the back of a trailer

jokes aside I think it would be worth your trouble documenting the experience for other forumites to view. if I was in your position I think i'd put it down.
plus it could be a good challenge for the bike and I see no reason why it couldn't do it if ridden sensibly and not have the guts flogged out of it.
no doubt it would be working a bit harder motor wise than the normal bike so i'd schedule regular oil and filter changes.
some bikes have record 50,000 klm or 30,000 plus miles and still running.

[Ice]

 ScooterBob tested a UCE by running wide open continuously for 60 miles out and wide open for 60 miles back.

[Roger]

Roger, how many miles are on your Bullet ?

I don't have one. I own a Ural so I don't need any more trouble.   

[Magoo]

2012 C5 just shy of 78,000km here. Daily commuter rain or shine.
Mostly trouble free...mostly....I've never been left on the side of the road anyway
Keep up the regular maintenance and she'll be fine.

[wildbill]

magoo
that's a great run you've had on that C5 of yours -will be good to see you crack the 100k mark

[gashousegorilla]

    Excellent motors.  I would say at least 50k miles if taken care of.... And probably MUCH more.   Chassis/frame and electrics getting better every year .

[REpozer]

I don't have one. I own a Ural so I don't need any more trouble.   

Thats too bad Roger, but glad you own a Ural. If you can wrench on a Ural or even a lawn mower, you should be able to affordably get many fun years from a RE. Stop messing around and find a good used one.

[Ice]

 Roger, I'll risk bring repetitious by mentioning that the UCE lump is Superior to the older engines. It is a quench chamber design with stainless valves and unleaded seats. The hydraulic tappets and gaskets are USA made. The oil filter is made by Purolator.  Pressure die castings are used instead of gravity sand casings of the earlier engines. The transmission is a slightly modified form of the previous 5Speed with the shafts stacked instead of laid out. The disc brake is made under license form Brembo.