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Bike Profile - Posted 23rd August 2010

Harris Matchless G80 Carb Saga
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Derek Pickard wanted to upgrade the carb on his 1980s 500 single. He started with an Amal upgrade but swiftly moved on to Mikuni...

The 1980s Matchless G80 with the 500 Rotax engine is as weird as it is rare*. The Euro engine was originally designed to be an off-road enduro powerplant for KTM but for a while also found itself being fitted to various other makes like this Devon-built, Les Harris Matchless. For a few short years in the 1980s, this incredibly compact and strong 500 engine powered bikes as varied as MZ, CCM, Aprilia, Armstrong and Jawa. Then it became the basis for the new BMW mass-production single cylinder unit and the Austrian maker was forced to ignore their low volume customers.

Typically European, the 500 engine has a few infuriating problems like the belt drive to the single overhead cam, the left side kickstart and the big 36mm Dell'Orto.

Big 36mm Dell'Orto not shown... The rare 1980s Matchless G80 originally had a 500 Rotax engine fuelled by a 36mm pumper Dell Orto. It is too big, hard to start and a smaller replacement works better.

It wasn't until I happened to be taking the motor apart one day and had a dismantled Honda XL500 on the shelf that I realised how much of a copy it was of the three years earlier Honda motor. All the basics like bore/stroke, valve angles, sizes, ports, cam timing, etc, are from the big XL. And as the Honda was such a sweet road engine and used a 32mm Keihin carb, I began to think about converting the carburation on my Matchless.

The stock fueller on the G80 is a 36mm Dell'Orto pumper as fitted to many sport V-twin Guzzis. The problems on the Matchless were difficult starting caused by a non-progressive choke when cold, far too enthusiastic an accelerator pump leading to easy flooding while attempting hot starting, and the fundamental flaw of a 36mm carb fitted to a 35mm inlet mount. Understandably, it went like a rocket from medium to full revs, but that is not where I rode the bike. I wanted easy starting, good bottom end power and plentiful mid-range torque. How much BHP it peaked at on the red line was of no interest to me.

A Harris Matchless with a carb of some sort, yesterday...
Rotax BMWs on Right Now......

The obvious solution would have been to fit an XL500 32mm Keihin but Honda's use of a built-in tall bellcrank arrangement on the top of the carb made it impossible to fit under the Matchless frame. So the retro development began by Brit-ifying the rare Matchless, firstly with a 32mm Amal Concentric Mk1.

We all know this famous Mk1: it is cheap to buy, is incredibly compact and is very easy to tune because it is crude in design and manufacture. Predictably, I couldn't get it to idle reliably. I accept that is partially the fault of the minimum flywheel effect from the dirt bike type Rotax engine but I wanted a good tickover. Apart from that one failing, the engine went well: starting was okay, pick-up was good and although I may have lost some BHP at the top end, the all-important mid-range torque felt about the same.

The fitting of a chrome plated brass slide from UK's Amal specialist, Surrey Cycles (, gave a reliable tickover but with incurable increased popping in the exhaust on over-run. That was not acceptable so I went further down the path of 'smaller is sweeter' by fitting a 30mm Amal Concentric Mark 2 ( Interestingly, the CCM version of the Rotax had a 30mm Mk2.).

Although the Mk2 uses similar carburation technology to the Mk1, it has the advantage of a plastic coated well-fitting slide to give a reasonable tickover, but getting there demanded some very careful tuning as this later Amal is definitely a technology upgrade on the old Mk1. It too allowed some popping in the exhaust on over-run but it was the choke action which infuriated me. While all was well from cold and when hot, it was fussy during warm-up which gave the frustrations of always having to give it a touch of choke but not too much. As no tickler is fitter, the cure would have been to do the normal motorcycle trick of setting the carburation slightly rich so the engine is happy very soon after start when the choke is switched off. But it was time to try the next carb instead.

Some carbs, yesterday... A tale of four carbs: the stock 36mm Dell Orto only really works at medium to top revs, the 32mm Concentric Mk1 is crude but effective, the 30mm Concentric Mk2 a little bit sweeter but with a fussy choke, and the VM32 Mikuni delivers the lot albeit at a price.

The Mikuni VM series is famous for being the best. This is such a precision-made instrument that the superb fit of the slide in the body bore has to be felt to be appreciated. After mucking around with other carbs, it is a delight. Chatting to a Mikuni specialist in the UK revealed that many of the Rotax 500 bikes running around there were happiest on a 32mm and they had learned what jetting to supply. So the cash was splashed and a nice new VM32 was mailed in and nailed on. Needless to say, the Japanese thing went superb from the off and the Matchless is now a better bike. It not only starts easily and runs better sooner, but the reliability of the tickover makes me ask myself why did I put off lashing out the biggest money in the first place?

Too often, being happy with a classic bike is all about accepting compromise, and the early Amal is part of it.


On Tickling…

A true mis-match on the stock G80 is the use of a small diameter twistgrip pull operation for the 36mm carb which demands a wrist movement of 120 degrees. Ridiculous.

The old 'tickler' was a marvellous device. It allowed a rider to quickly raise the fuel level in the float chamber and so richen the mixture for easy cold starting where even over-richness was instantly cured by opening the throttle. And as the engine warmed, the float level progressively lowered to the correct setting. All achieved with only one more moving part. Simple, easily managed and very smart.

My old Velo Venom had about the same claimed power and performance as my Matchless Rotax. But the Velo used only a 30mm carb to do nearly 100mph in classic racing trim. I can still hear that gorgeous exhaust.


Technical Data

32mm Amal Mk1: 25 pilot 2 turns out, 3 slide, 2 ring needle centre notch, 106 needle jet, 220 main

30mm Amal Mk2: 25 pilot 1.5 turns out, 3 slide, 241 needle centre notch, 105 needle jet, 200 main, 50 choke jet, 3.2 air jet

VM32 Mikuni: 27.5 pilot 1.3 turns, 2.5 slide, 6DH3 needle centre notch, P-2 159 needle jet, 185 main


*although it is considerably less rare in the UK than in Derek's Australian outpost! Plenty of RC readers in Blighty own and run Harris Matchless 500s powered by Rotax motors, and yet more have MT500s and MT350s which use a very similar version of the engine


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