Royal Enfield diesel conversion with Yanmar L100

I have been considering building a diesel bike for a long time after reading many articles on the topic. I decided to go for an Enfield frame with a Yanmar L100 or a clone of this.From I bought the donor bike, a Bullet 350 with first EU registration in 1980. The first registration date also need to be before 1987 to get it approved later on due to restrictions on diesel smoke and exhaust noise.

The Yanmar L100 diesel engine has a quite heavy fly wheel on the right side and are wider than the Enfield petrol engine.

A few builds on the internet uses the original gearbox mainshaft but in that case, the diesel engine is placed far to the right side.

I prefer to have the diesel engine in balance ( “engine gravity vertical center line to align the bikes frame” ) and placed the engine center line approx. 12 mm left to the bikes frame. Thus the gearbox mainshaft must be swapped to a longer one. The clutch push rod inside the mainshaft needs an extension according to the longer mainshaft chosen.

The original mainshaft is 11 inch long, and diesel mainshaft are available in 2 lengths, 12 3/8 & 13 1/8 inch.

Original and diesel gearbox mainshafts

I got the 13 1/8 inch version but had to cut down to 12 3/4 inch. An Enfield Taurus inner primary chain case is needed as well, because it includes a bearing to support the longer mainshaft. I bought the mainshaft, primary case and bearing from :

Original and modified inner primary chain case

A spacer between the engine and the Taurus chain case is needed, approx. 8 mm thick with 12 1/2 inch mainshaft and approx. 24 mm with the 13 1/4 inch mainshaft.:

To have an adequate clearence between the secondary sprocket and the Yanmar throttle cabling, I kept the original 16 teeth 350 Bullet sprocket.

The Yanmar engine tops out at 3600 rpm, and with a primary engine sprocket of 30 teeth the bike should top at 97 kmh. The distance between the engine output shaft and the gearbox mainshaft adds 5 mm, compared to the original distance, in total a 94 pitch primary chain is required.

The process of making the engine support frame is a kind of stepwise designing and welding from the gearplate to the front. The gearplate is made of 6 mm steel plate, the bottom under engine frame are steel tubes of 30×15 mm with 2 mm walls. The gearbox plate includes a fuel stop arm arrangement. The frame front part is made of 30×8 mm steel.

Engine frame and fuel stop parts. The fuel stop lever pulls back the fuel control lever.

The original Yanmar throttle controlplate was changed:

The engine speed ( fuel amount ) is controlled by an internal centrifugal fuel regulator. The centrifugal fuel regulator uses an external counterforce in terms of an applied spring force. The larger, yellow spring sets the applied force on the fuel regulator, more spring force is higher rev. at max. load. From the engine manual, I used the 3-B setting, inner hole on the top lever ( picture shows outer hole setting ) and second hole on the bottom lever.
Yanmar fuel control plate mod.png (216.25 KiB) Viewed 14071 times

And a throttle cable and adjust plate was made:

Throttle cable , max. rev. screw ( at no load ) and idle speed screw
Engine in frame, ready for riding. Kick starting the bike:
The battery was moved to the empty space behind the gearbox, the two side boxes are removed, and the brake light switch is now a Dnepr-Ural type :):
Battery braces to hold the battery behind the gearbox.
Top and bottom view of the battery in its new place
Ural brake light switch using M8x1.0 thread

Finally got rid of the missing battery charging. I found an original NOS 1992 “Dynamo assy” 12V / 15A Yanmar sparepart no. 114351-78290.

With the original Yanmar “Dynamo assy” and the Yamaha XTZ 600, 34L regulator I am using, it starts charging the battery at 1500 rpm with all lights on. :)