Royal Enfield Bullet 350 Classic

Royal Enfield Bullet Classic Diesel Review

From golf course green to motorcycle

Do you fancy a motorcycle that can achieve in excess of 150 MPG? Then you need to speak to Trevor Williams.


For many years this remained a dream until in the late 1980’s, the Robin diesel engine found it’s way into a Royal Enfield motorbike and Trevor started to see the way forward for his dream. He saw how a diesel power plant could be successfully mated to the flat backed four speed gear box of the iron-head Royal Enfield engine.

Then, at Christmas 2006, Trevor saw on a website that a German enthusiast had in fact successfully put a Yanmar small diesel engine in a Royal Enfield Bullet.

This was the catalyst Trevor needed; he immediately set about buying a Royal Enfield project bike. Almost instantly he found a mint condition, two year old 500 Bullet Classic for sale, that had only done 45 miles! This the original standard Royal Enfield Bullet with the iron head, four-speed gearbox and kick start.

The exact donor bike Trevor needed!

Trevor wisely did not tell the seller what he intended for the bike. In fact prior to the conversion to diesel power, Trevor admits to never even starting the bike’s existing petrol engine. Royal Enfield motorcycles in themselves are no strangers to green motoring with MPG figures of 90 MPG plus being achieved from their existing petrol engines.

As soon as Trevor got the bike back to his garage, he immediately ripped the petrol engine out and thereafter started much head scratching! It took several months and a few sets of trial brackets before Trevor was able to design the perfect brackets to mate the new donor diesel engine to the Royal Enfield’s frame and gear box.

The engine brackets

Beautiful hand crafted stainless steel exhaust system

Initially Trevor hand cut, filed and drilled the early design brackets, but the final versions were laser cut by an engineering company in Worcester, based on templates provided by Trevor. Trevor had five sets of brackets made as this was more cost effective. So tight were the tolerances when fitting the small diesel engine into the Royal Enfield frame that there was only a 1mm gap between the back of the engine and gear box. This gap was taken up with a gasket.

However the inner primary drive casing had to be changed for a Taurus diesel unit from India, to achieve a perfect mating of engine to gear box. In addition the main shaft of the gear box has to be changed to one of German origin of the required length.

On cost grounds, Trevor has in fact used a Yanmar clone engine from China, which Trevor maintains is near identical to a Yanmar diesel engine, but approximately Ј1,000 cheaper!

The small diesel motor used, is a 406cc engine which produces approximately 10 BHP, revving to a heady maximum of 3,600 RPM. It uses a direct injection system, with no heater plug. Trevor says despite the absence of a heater plug, there are never any problems getting the engine to start. From hot the engine always starts on the first kick and from cold never more than 3 kicks on the Royal Enfield’s kick start are required.

Unfortunately space did not allow Trevor to retain the modern electric start; but this is a green motorcycle after all, so a kick start seems a fitting way of starting the bike.

Trevor was advised that beefing up the clutch would be a good idea, due to the enhanced torque of the diesel engine. So he has installed a new five plate clutch, the same as fitted when pulling a side car.

The engine casing sprayed matching gloss black

The fuel filter, stop lever and throttle rack arrangement

Trevor did not want to alter the frame of the bike, which accommodating the electric start would have necessitated; so that at any time the original Royal Enfield petrol engine could be reinstalled.

Trevor has yet to fully prove that using the Chinese clone engine will be a reliable choice, but 700 miles of incident free riding seem to say it will be.

All the electrics are as on the new diesel engine, but Trevor found it straight forward to connect these into the Enfield’s wiring loom.

No significant changes where necessary for the fuelling, the Royal Enfield petrol tank being cleaned out and filled with 100% bio-diesel which is fed through a Kubota filter.

It has taken Trevor an amazingly short period of time to achieve this green miracle. He stated the conversion in January 2007 and finished the project in August 2007.

Trevor had the the beautiful hand made stainless steel exhaust system, crafted by Dave Leonard of Bristol. This had to be specifically designed to accommodate the exhaust outlet position of the diesel engine.

The hardest technical challenge was modifying the engine to accept a motorcycle throttle as the means of increasing the fuel into the engine and acting as an accelerator. Although the diesel engine chosen does not have a governor its power is controlled by small balance fly weights and a combination of springs which pull the rack up and down.

However with the initial addition of two springs Trevor found this overrode the fly weights and caused the bike to smoke heavily as to much fuel was being introduced. For those motorcyclists not familiar with diesel engines (many of us I’m sure) the rack opens or closes a slide which regulates the fuel coming from the fuel pump.

One needs to appreciate that these small diesel engines are generally designed to run at a fixed revs of between 3,400 to 3,600 RPM all day long, rather than the variable rev requirements of motorcycle riding; so it’s very easy to over fuel the engine for little increase in power output. Hence it was critical that Trevor got the action of the rack correct, so that more fuel can be fed into the engine in the right amounts to achieve a smooth increase in power with most, if not all of the bio-diesel fuel being burnt.

Trevor has finally settled on a combination of three springs of the right tension, there is a tick-over spring that initially holds the rack in tick-over position so the bike can be started. The further two springs being utilised to make the engine accelerate. However Trevor has had to add a new cable and lever to the engine which in fact acts as a shut off cable to kill the engine when it’s needed to stop the engine. This shuts the rack off, cutting the fuel supply.

However the former petrol choke lever and cable has been removed as this is no longer needed.

The addition of the stop lever is the only discernable addition to the normal motorcycle controls.

Royal Enfield Bullet 350 Classic

I’m sure many will want to know how the performance differs from the petrol engine. The standard 500cc iron-head petrol engine, depending on the gearing used, should top out around 75-80 MPH.

Trevor claims that bike will happily cruise along all day in fourth gear at 50-55 MPH and will easily top 60 MPH; however Trevor has had to increase the size of the gear box sprocket from a 17 to 18 tooth gear to achieve the 60MPH plus top speed.

The bike pulls 45 to 50 MPH in third gear, before changing up into top is required. Trevor says that the single cylinder engine pulls the bike away from rest cleanly. But what Trevor was at pains to express is the sheer grin factor and pleasure that comes from riding his unique green creation.

Plus the enhanced attention the bike gets where ever it is parked or ridden is a bonus.

Trevor had set himself a performance bench mark of the BSA M20, but Trevor is happy that the performance offered by his diesel powered Royal Enfield exceeds this. The BSA M20 was a 500 cc side-valve engine-ed BSA. Production started just before the Second World War and continued throughout the war years.

126,000 M20’s were produced for the military alone.

The engine runs on 100% bio diesel supplied by Brandsford Bio Fuels Ltd. When Trevor was running in and testing he has been achieving an amazing 135 to 140 MPG. However he expects normal usage MPG to be between 160 to 170 MPG.

Careful use of the throttle could nudge this figure towards 200 MPG, giving the bike the phenomenal potential range of approximately of nearly 700 miles before the tank needs refilling.

Surely has to be one of the greenest motorcycles in the country, particularly as this amazing holy grail of MPG is also being achieved using 100% bio-diesel?

One minor downside though is the increased engine noise. Trevor claims it sounds like a cement mixer coming down the road. Trevor kindly let me briefly ride his green miracle at the conclusion of the interview for this article and yes it sounds like a cement mixer!

Additionally the smell from the exhaust is not your normal diesel fumes, but more akin to the smell from your local fish and chip shop, thanks to the bio-diesel. Though many may prefer this aroma!

The finished project

I can state that the finished project has been done to a very high standard and does not in anyway look cobbled together, in-fact so slick is the end result you would think the bike rolled off the production line in India.

However Trevor is not content with producing one green miracle he now intends to convert both a twin and a three cylinder petrol motorcycle to run on corresponding diesel engines. He is thinking of converting a BMW R75 or R80 to run on a three cylinder diesel engine from either Kubota, Yanmar or Daihatsu as his next project.

Additionally Trevor is happy to assist other budding diesel converters with help and advice and possibly even a kit to convert their Royal Enfield. Trevor estimates the cost of conversion is about Ј1,200 excluding the cost of the donor Royal Enfield machine. He would also be interested if any far sighted entrepreneur wanted to take the idea forward to commercial development. So if you’re interested in making your own green miracle or helping Trevor create more, please forward an email with full detail via or by email .

Article and Photos by Jon Booth – – The UK Biker Site. Note all performance figures, weights and technical specifications are as claimed by the respective manufacturers

Thanks to Trevor Williams – for his time and co-operation in the writing of this article

Royal Enfield Bullet 350 Classic
Royal Enfield Bullet 350 Classic
Royal Enfield Bullet 350 Classic

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