Pre-start checking the Yamaha LS3

Yamaha 100 LS3 1972

The first thing I did was looking for a new ignition switch. I found a nice NOS one on eBay. Luckily the wiring wasn't butchered so it was just a matter of rerouting and connecting the wires.

New NOS Yamaha LS3 ignition switch

Old and new ignition switch

Wiring inside headlight Yamaha LS3

Speedometer Yamaha LS3

The Yamaha LS3 has a 6V circuit, the stop light, flashers, "dashboard" lights and horn are supplied with electricity from a battery, the head- and taillight from the flywheel magneto. 6V globes are a bit harder to get, especially the odd shaped head light bulb.

6V bulbs Yamaha LS3

After reconnecting the clutch cable I found out the clutch wouldn't disengage. This normally happens when an engine hasn't been running for a long time and who knows how long the bike has been sitting in the shed.. So it wasn't a big surprise.

I also changed the transmission oil, better save than sorry. Yamaha advised Yamaha oil or 10W30 but that was 42 years ago and the additives in most oils changed. So I bought Castrol MTX 80W, it's designed for 2 stroke transmissions and also prevents the wet clutch from slipping.

After changing the trannie oil I also checked the compression. Normally a 2-stroke engine (50-500cc) should have a compression between 120-190 psi. So the 135 psi is OK for now.

Measuring the compression

Next thing to do was cleaning the exhaust (inside), fuel cock and tank. Luckily the tank was really clean, no rust at all.. But the fuel cock was filled and completely blocked with dirt.

The muffler (silencer) was also completely blocked with oil, dust, rust and whatever more..

Blocked fuel cock Yamaha LS3

Cleaning the Fuel cock

The fuel cock was also leaking a bit, right behind the lever. After polishing the inside edges and bending the round spring a bit extra the problem was solved.

Clogged silencer Yamaha LS3

Clogged silencer Yamaha LS3

Silencer after some cleaning

The Yamaha LS3 has a Autolube. This is a little pump that will deliver oil to the 2 stroke engine. The amount of oil delivered depends on the throttle opening and engine speed. The Autolube is driven by the engine and is also connected to the throttle. Unfortunately it was pretty common in those days to disconnect the Autolube by removing the throttle cable and to mix the petrol and oil in the fuel tank. The reason for this was that people often forgot to refill the separate oil tank and back in the days premix was available at most petrol stations,  So a new hard to get cable is on my wish list.

Autolube system Yamaha LS3 1972

I made my own premix: 40 parts of fuel and one part of 2 stroke racing oil (40:1) and put 0.5 L in the fuel tank. Time for the big moment, Starting the LS3.

It started almost immediately and produced lots of smoke :)

The clutch was still not working so while the bike was on the centre stand I revved the engine and started to put some pressure on the rear brake. After the third try it came loose.

Starting the Yamaha LS3 for the first time after many years