Restoring a 1971 Triumph TR6C Trophy - Part 4


| September/October 2007

Note: This is part 4 of a 4-part article covering the restoration of a 1971 Triumph TR6C Trophy. Start at the beginning with part 1 here, or catch up with part 2 here and part 3 here. 

Six months ago we turned the first wrench on our Triumph TR6C project, and it’s now finally drawing to a close. Sharp minds will note, however, that we didn’t say it’s done. Why? Mostly because we won’t call it finished until all the little details (missing side cover caps, front turn signals, etc.) are tended to. But it’s 99 percent of the way there, and better yet, it runs. And surprisingly, it wasn’t that hard getting it there.

A little air, please

In our last report, we noted some issues installing our Mikuni carb conversion. Namely, the Mikuni wouldn’t work with the stock air box – unacceptable to us, as we think the air box side covers are a key part of the bike’s look. We couldn’t believe it, but a few calls to some Triumph specialists confirmed what we’d discovered: It won’t work. Apparently, most folks doing this conversion either don’t like the stock air box or simply don’t care. We do, and decided to see what it would take to make the stock air box work.

Sizing up our options, we turned to eBay for a used air box we could hack on without feeling too bad if we ended up destroying it. There were a few sets available when we went trolling, and the first thing we discovered was that boxes for twin-carb 650s were perfect for our planned modification. Why? Both the single-carb and dual carb inner box are made up of two mirror-image pieces that bolt together around the main frame tube. The single-carb box, once installed, has an oblong hole at the front for a rubber sleeve to join the assembly to the stock Amal carb. But the twin-carb box has a blank front, the air tract running instead from the outer air box covers. With no pre-existing hole, we could start from scratch.

Fifteen dollars and three days later, we were ready to begin the modification. With all our pieces in hand, we discovered it was a pretty straight-forward process of mounting the twin-carb inner air box, the conversion manifold and rubber spigot, and then lining up the Mikuni carb to the spigot. Carefully eyeballing the Mikuni’s position (we couldn’t install it in the spigot because the Mikuni protrudes into the inner air box), we scribed lines on the air box marking the top and bottom of the Mikuni’s intake throat. Following that, we simply scribed the Mikuni’s opening on the air box, allowing an extra eighth-inch all around for a rubber seal, and cut away with a Dremel tool.

It took a bit more fiddling after making our first cut, but everything went together perfectly, and we ended up with exactly what we were looking for: A superior Mikuni carb without compromising the original look of our classic TR6C.

8/23/2014 11:21:13 AM

The Mikunis are nice once you get the jetting sorted out. Be wary of going too lean, these engines were designed to run a bit rich to aid cooling. I did the conversion on my '71 Lightning. Two carbs involves even more adaptation. In order to retain the original side covers I ended up making cut-outs in the front side panels and using Uni angled filters. Amal MkII carbs would be a better choice, a bit smaller and easier to tune.

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