A few weeks ago Japie started to carry with him a motorcycle fuel tank with some small stones in it, and he would shake it up and down until the complaints or the noise got too much to bear. Older readers will know why – he was cleaning a rusted fuel tank and could not bribe or cajole anybody else into doing the shaking.
The story really started when he bought an old Honda XL125. It did not take him long to discover that it habitually spluttered to a stop. We left him alone in the interest of good teaching practice and were rewarded by a very quick and correct diagnosis; there was dirt in the fuel. He stripped and cleaned the fuel tap with its little filter, but then faced the task of cleaning the insides of the tank. This appeared a bit rusty and Hennie told him to put some stones in it and shake it about for a few days.
When the tank was clean Japie fitted it and started to experience the joys of riding a motorcycle that’s small enough to take commands from the rider, instead of the other way around, as is the case when riding a bigger ’bike.
His Honda has a singlecylinder overhead-cam engine that thrives on revs and is very robust, but still falls victim to the laws of nature, as Japie soon discovered. When he left to go home one afternoon the ’bike refused to start and, because he’d been working on a lawnmower that afternoon, he did what lawnmower mechanics do by instinct – he checked the spark plug.
The plug was wet with petrol, so he assumed there was no spark. He then persuaded a pump attendant to hold the HT lead close to the engine while he operated the kick-starter. He was rewarded with a fat spark, but also noted something very peculiar; every time the piston got near top dead centre there was a spark. He expected this to happen only every second time, because the engine is a four-stroke.
I was still in my office so he tackled me and I put his mind at rest by telling him that many Hondas used the so-called “dead spark” principle. It was designed to supply a spark near every top dead centre to keep the electrics simple. The wasted spark on the exhaust stroke did not matter.
Back at the ’bike, Japie remembered that he had not opened the fuel tap before trying to start it, because it had already been open. He must have left it open all day. He then checked the oil level. The sump was overfull, which meant there was petrol in the oil, most likely because the worn-out needle and seat in the float chamber could not resist gravity’s pull on the fuel. He drained the oil, filled the sump with fresh oil, dried the spark plug and set off home.
He did not get very far – and experienced readers will know what he had neglected to do. He didn’t squirt some oil down the bores, with the result that the engine seized because the excess petrol had run down the bores into the sump and washed the oil away.
Japie took the setback surprisingly well. He most likely could have polished the bore and cleaned up the piston, but by this time I had taken an interest in the ’bike. I had one just like it in my younger days, so I bought him a new piston and barrel and justified it by praising him for being such a quick learner.