Bonneville to Scrambler in 145 Easy Steps: Part I

Almost a year ago, I found myself the proud owner of a 2008 Triumph Bonneville Black that’s seen about 34,000 miles pass under its wheels. Bonnie Black 3 Andy ParkerAs a 10-year-old, the bike was in excellent shape, and still in near-stock condition. Almost the same as when it rolled off the showroom floor, but with a few minor modifications. The two mods I was told about were a new speedometer at 7,800 miles, (the current one showed 26, xxx), and the conversion to LED “idiot” lights. Another change which was easy to spot was a pair of heavy bar-end weights, and the final visual change was the comfort seat. A change that was only detected after a ride was the 19-tooth counter sprocket at the front of the chain drive. Other than that, the Bonnie looked like a new bike.

Bonnie Black 7 Andy ParkerThe moment I rode it I knew it was going to have to up its game a bit. Like all Bonnevilles, the handlebars are weird; they cant your wrists at an odd angle and leave you with a feeling of not being completely in control of where the bike is going. The throttle is very forgiving and a bit ‘fluffy’ in terms of engine response, and there’s a black hole in the rev range just about where you’d expect things to get a bit more exciting (well as exciting as they are likely to get anyway, this is a Bonneville we’re talking about after all), and the suspension is strictly a cost saving exercise so the handing suffers as a result.

Thinking back to conversations in pubs thirty years ago about British motorcycles (which were long out of production) there was much talk about how the T120 Bonneville was a speed machine, a bike that was light and handled pretty well, it could propel you up to 100mph with very little in the way of modifications needed to make it happen. It was the preferred bike for the working-class Rocker back in the sixties and seventies and earned its brass on numerous rides from the Ace Café around the North and South Circular which skirts Central London; a fast rider could do the trip and hit the ton while doing so. It was the tool to have wherever the lads gathered to talk bikes and race on the streets.

The modern equivalent still retains the same engine configuration and even attempts to copy the look of the old unit twin that shares its name; Bonnie Black 1 Andy Parkerit does however, have the benefit of modern engineering and components. It still resembles the old bike, it shares some of the beautiful lines of the original tank shape and side panels, it has period foot pegs, levers, turn signals, fenders, chrome headlight and spoked wheels. For those who are not the wiser, it looks like a late sixties to mid-seventies motorcycle. Any Bonnie owner will tell you of the conversations at fuel stops and parking lots somewhere, that a guy will come up and ask “What year is that?” and go on about how they used to have one. To all intents and purposes the modern Bonneville is the bike that every rider wanted fifty years ago. Hell, if the old ones were anything as good as the new one we’d all be riding British steel today – The big four would never have got a foot hold and the UJM would never have been developed.

The question I had to answer was “What am I going to do this bike to sharpen it up… a lot?” First of all, do I want a Café Racer, a Tracker, a Scrambler, a Brat, or a Motard look? A few days and nights perusing the interweb and the answer was pretty clear in my mind. Although I already had two bikes, a new Monster 1200S, and a newish liquid cooled boxer, the R1200GS, a third motorcycle wouldn’t be too many and could easily find a home between the other two. Bonnie Black 2 Andy ParkerThe monster was used for ripping up and down the Natchez Trace and riding too fast on any twisty road I could find around Middle Tennessee. The GS was my long haul mule that also let me get away with mild road off-road diversions on forest roads and nature trails. However, the GS was too heavy for serious mud, deep water or rock-strewn trails, so the question really was about “What’s the real use this new bike is going to see?” It was never going to be able to out-perform the Monster in any of its strong areas like power, handling, braking, ride quality, or even the aggressive good looks of the Italian superbike, so there was no point trying to go Café Racer when the 1200 was at home. I thought about just keeping the Bonnie as-is but ditching the Secondary Air Injection and investing in better pipes, a jet kit, free flowing filter, better handlebars, and some proper suspenders to make it a bit sportier, but if I was going to do all work it might as well have a theme.

Having ridden the GS along the Tennessee Dirt Devil and around Land-Of-The-Lakes a few times where the surfaces vary from hard packed dirt to gravel to muddy patches, puddles, dirt strewn with pea gravel and even some shallow river crossings, I learned the bike was very capable but I would need more dirt oriented tires to really get up some speed and the confidence to slide the back around and hop the front over bigger rocks. Ultimately I was not prepared to do that given the BMW’s primary job of hauling me out west or as the last trip, up to Maine.

I needed an old-style trail bike; so I started off with the intent to create just that, a hybrid Scrambler/Commuter that had a responsive throttle, enough power and gearing to rip away from lights or spin up the back wheel on dirt, and still be authentic to the styling cues of the modern Bonnie. I sat down and drew up a list of things I’d like to upgrade, remove, replace, or modify. It turned into about three strings of lists that were pretty much all dependent on a previous step to get the max return on the investment. Take the Air and Fuel system as an example; if I wanted a more responsive throttle I needed to change the jetting, which meant I probably needed a new air filter, and a new snorkel, and that meant if I was going to do that I might as well do a new pipe at the same time. Then there was the Tail Tidy; that meant new turn signals, and a new rear light, which led to a front turn signal relocation bracket, an R&R and Horn relocation bracket, and an Ignition Switch relocation bracket which in turn led to a new gauge bracket with integrated Ignition Switch, and new headlamp brackets. Bonnie Black MotoADVRThe shortest list was new tires, new bobbed front fender, sump guard, and fork gaiters. At the end of the list compilation with the cost of the suspension upgrade at the rear, and the progressive springs in the forks, the list for the build was over the actual value of the bike, that being close to around $4,000. Luckily the bike had only cost me $100 in raffle tickets, $1.13 in TN taxes and $29 in title fees. I started searching for parts and looking at websites regarding jetting on the 865 motors. The build list would have been significantly more if I had bought all the parts needed but I had a good stash of parts from a previous Bonnie I’d briefly owned, and a whole host of parts I’d bought for other projects and not used.

Within a month I’d decided things weren’t getting done whilst internet browsing, so I dug in and started with some basics. I got the bike up on the stand and took off the front and rear wheels to have Shinko 705’s fitted. I pulled off the seat, the tank, the side panels, fenders, and finally the carbs. They were nice and clean. I’d ordered a Dynojet kit, a Sprint air filter, a pair of carb boots, a TTS air funnel, and I’d decided to go with the TEC 2-1 Desert High Pipe, which also meant new side panels were needed. The rectifier and horn were relocated, and the new turn signals fitted with the Tail Tidy.

After three attempts at pilot and main jet combinations and DynoJet springs I couldn’t get the bike to perform well. I went back to Square one. Bonnie Black 4 Andy ParkerSome more fiddling and reading of tried and tested combinations and I arrived at 2.75 turns out for the air screw, OE pilot jets, OE springs and needles, with a DynoJet 112 main jet (Keihlin equiv 122.5). The bike idled nicely on and off choke, pulled hard up to redline with no flat spot on either partial or full open throttle, and the engine braking improved, it backfired occasionally, and ripped forward again as soon as you got on it. The plugs are a nice color and it sounds awesome with baffles in (although they are drilled a bit…), but without the baffles you’re saying, ‘What did you say?” a lot.

To Be Continued…

About desmoderek

A Civil Engineer
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4 Responses to Bonneville to Scrambler in 145 Easy Steps: Part I

  1. Went through similar with my 08 Black; wound up becoming a 10,000 mile/year Tigered-Bonnie so sold @ 24k miles & went full Tiger. Yours is looking good.


    • MotoADVR says:

      Andy has really put a lot of work into it. I helped him do a few things I expect to find in the next episode. I’m curious where he’ll call it done or if it will evolve like Rosie the Scrambler.


  2. Pingback: Bonneville to Scrambler in 145 Easy Steps: Part 2 (of 3). · Moto AdventurerMoto Adventurer

  3. Pingback: Bonneville to Scrambler in 145 Easy Steps: Part 3 (of 3). | Moto Adventurer

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