cancel
Showing results for 
Search instead for 
Did you mean: 
GRP_Photo
Instructor

Bonneville Woes

I have a 1971 Triumph T-120 (AKA “Bonneville”). It has Amal Concentric Mark II carburetors. I’ve been working on it for a while and finally got to the carb rebuild process. When I was done, I hooked up the throttle cables and nothing worked. I determined that the cables were pretty crapped out (a Technical term). I measured the old cables and bought new ones.

 

Once I hooked them up, I discovered that the cable sheaths were about 10” too long for the bike. That was bad enough, but the inner wires were also several inches too long and would not activate the carbs. I discovered that the former owner had fabricated the old cables from swap-meet parts and shortened the inner cables to get them to work.

No problem, I thought. Triumph first used the Mark IIs in 1979; I’ll just get some 1979 cables.

 

Not so fast. In 1971, Triumph used individual cables to the carbs. In 1979, they ran a single cable from the throttle to a junction under the gas tank and then had separate cables from there. The 1971 throttle assembly may (or may not) work with just one cable, but it’s gonna look weird.

 

OK, just a little more money. I’ll get a 1979 throttle.

 

Not so fast. In 1971, the throttle assembly and the right-hand switch cluster were two separate items. In 1979, Triumph combined the two. Since I recently spent several hundred dollars getting accurate ’71 switch clusters, that idea was a non-starter.

OK, Triumph made lots of bikes with only one carburetor. I’ll just get a throttle from one of those.

 

Well, they did make a variety of bikes. They also made several varieties of handlebars. I measured mine at 15/16” diameter, but when I checked for parts, I discovered that the only bar that size was actually 1” in diameter with the right end swaged down to 15/16”. Since I had measured the left end, that didn’t parse out well.

 

I pulled the old throttle and checked the right end. Turned out I had made a mistake in using the calipers (maybe I’ll remember next time) and the bars are actually 7/8” diameter (whew!).

 

I also used a section of electrical wire to figure out how long my cables need to be. Now I need to talk with a vendor on eBay to make sure his stuff is what I want, but I think I’m in the home stretch. Looks like I need an after-market single-pull throttle coupled to a 1979 “high-bar” throttle cable assembly (yeah, the export bars sit higher than the ones made for the British market).

 

Now if I can just figure out where those extra brown & green wires under the seat belong.

1 REPLY 1
Kyle
Moderator

Wow, what a saga. 

 

Having built my own throttle cables by modifying what was commercially available, I now often find that easier than tracking down the right part number in applications like yours. I hope the eBay seller you found has just what you need. If not, a small drill bit and a dab of solder ought to get you back in business!