TOO MUCH CARBON
Twostroke Charlie Thompson is a motorcycle enthusiast, most likely because his father, Thumper Thompson, is a Harley-Davidson man. The old boy owns a number of these charismatic mechanical disasters and he has the appearance to go with it.
Twostroke doesn’t look like his dad. He’s small-bodied and loves two-stroke singles instead of big V-twins. His 125 cm3 off-road Yamaha is a regular sight in the forecourt and is well looked after. I spent a great deal of my younger days on motorcycles and always make a point of chatting to him. Our conversations bring back pleasant memories and remind me of the bond that sometimes develops between man and machine. It must be similar to the bond between humans and dogs or horses.
The other day, Twostroke appeared behind the wheel of a bakkie with the Yamaha on the back. The engine had seized-up. I felt his sadness because I have experienced it. In my youth, two-strokes were so temperamental that many riders, myself included, always carried a spare piston with them.
On a two-stroke, the cylinder head and barrel could be whipped off in a jiffy and the piston replaced in no time. I called Hennie and he proceeded to remove these parts to take a look at the piston. It had seized and the rings were stuck in their grooves.
Hennie was unfamiliar with the mixture and timing settings, so he contacted Twostroke and asked him whether he had recently changed the spark timing, the mixture, the spark-plug grade or anything else that could cause overheating. He received a negative answer to all these questions, so decided to tackle another task and let his subconscious take over.
That night, he was lying in the bath and wondered what Twostroke could have done without realising that it could be detrimental. Did he use the correct oil in the Yamaha’s oil tank? The next day, he phoned the lad and asked him if he changed to another type of oil recently. “Yes,” he said and gave a chuckle. “I started using some of my brother’s marine two-stroke oil because it’s more expensive, is supposed to be the best and I don’t think he’ll blame me if his can is empty.”
“Well, that was your undoing,” said Hennie and proceeded to explain: “Two-stroke outboard engines are usually cooled by circulating seawater around the cylinder barrel and head. This means they run a lot cooler than air-cooled motorcycles or even lawnmowers. Consequently, they need special marine two-stroke oil that is formulated for lower temperatures. This oil cannot cope with the higher temperatures that a motorcycle will experience. It will get fried, carbon-up the rings and the excess heat will cause piston seizure.”
BIODIESEL BLISS
Chippy September, who owns the local fish-and-chips shop, found out the hard way that you cannot believe everything you read in the newspapers. He had seen a number of reports about people running their diesel vehicles on waste oil, so he decided to run his Isuzu turbodiesel on the used oil that he would normally discard. He filtered it very carefully to get the bits of food out of it, filled the fuel tank with the stuff and started to drive the bakkie with the smug look on his face that somebody would have if he was getting free fuel.
Initially, the engine started quickly because there was still some diesel in the fuel lines but, as the weeks passed, the engine took longer to start. In autumn, when the ambient temperatures dropped, there came a day when the engine refused to start and we had to tow in the bakkie.
Chippy did not harbour any secrets. He told us what he had done and we teased him about his gullibility. We phoned one of the oil companies for advice and it enlightened us about the pros and cons of using vegetable oil in a diesel.
Some automotive diesels can run on pure plant oil, but the viscosity and surface tension must be reduced by preheating it. Failure to do so will result in poor atomisation, hard starting, incomplete combustion and carbon build-up. Overseas, there are kits that consist of a heat exchanger for the fuel, a separate tank for the plant oil and a three-way switch. The engine is started on diesel, switched over to vegetable oil as soon as it is warmed up and switched back to diesel shortly before being switched off to ensure that no vegetable oil remains in the engine or fuel lines when it is started from cold again. In colder climates, below-freezing temperatures causes the fuel to become very viscous and even solidify.
In general, this fuel is more suitable for indirect injection diesels, such as the W123-series Mercedes-Benz models. The Isuzu has a direct-injection engine and, for these, the fuel has to be heated to a higher degree than can be done with a heat exchanger.
These facts, as well as the extra complication of a turbo, makes Chippy’s fuel usage a very risky proposition. He was lucky he stopped using the stuff before the engine carboned-up completely. As it is, we had to decarbonise the engine and have the injectors cleaned.
