It happened to the owner of a BMW R69S and the youthful driver of a Honda CBF 125 from 2012: Both engines were hit by the heat. Not by a lock or a clamp or something scary, but by vapor lock. Or in good Dutch: 'a vapor lock'. But who still speaks Dutch if he can finish it with the English term?
When it is nice and warm outside
The outside temperatures and driving conditions caused the gasoline to boil in the BMW float tanks and in the Hondaatje injection system. And a block does not really want to run smoothly on gasoline vapor. A float does not float on gasoline vapor either.
Gasoline is hard stuff
Gasoline is the fuel for ignition or Otto engines. It is essentially a mixture of hydrocarbons whose boiling range is between the 25 and the 220 ° C. It is a complicated mixture of more than 100 components. All hydrocarbons in gasoline have their own mutually different boiling point. High volatility means that a relatively large part of the gasoline is already boiling in the low temperature range.
It also happens with cars
In cars, this cooking often happens on hot days after the engine has stopped after a long drive or during prolonged slow driving. The heat under the hood then rises. The same applies to motorcycles, but 'without the hood'. The heat radiated by cylinder heads can sweat the carburettors usually mounted nearby. Where the problem with cars is often that they do not want to start after a long drive, the mixture provision on motorcycles can also get confused during (slow) driving so that the engine starts running very poorly or stops. Motorcycles with a lot of plating or fairing (Goldwing) can be more affected than 'naked bikes'. We know a Goldwing owner who solved the problem during his Spanish vacation by purchasing and installing a vacuum cleaner hose. But it is also bizarrely hot on the Spanish plateau.
Under what circumstances can vapor lock occur?
- At high outside temperatures.
- By heating, for example, the exhaust, cylinder heads or other hot engine parts.
- By using metal pipes instead of rubber hoses due to heat conduction.
- When driving in the mountains which lowers the boiling point.
- More volatile winter gasoline if it is still used in the summer.
- By the heat released after a long drive with a parked motorcycle / car.
The solution
Vapor lock will dissolve by itself if you take the time. By cooling, the vapor form automatically turns back into liquid. You can speed it up by parking in the shade or working with wet cloths. If the problem occurs near an ice cream truck or a terrace, you can enjoy explaining that you are in the process of fixing a fault in the fuel system. Find a place in the shade on the terrace. Because a sweaty forehead can be laughably dismissed with “I have a leaky head gasket”. But you really have to watch out for sunburn and heat stroke.
What can definitely help is shifting the fuel line. This can even be done with a return pipe, possibly into the fuel tank (which is usually mounted in a cool place), but if not, the normal fuel pipe can also be placed along some cooler sheet metal.
I occasionally suffered from a vapor lock with my engine until I realized that a small piece of fuel hose was on the block (not even two centimeters). First I drove a piece of foam between block and fuel hose for a while until I was sure that this had solved the problem and then shifted the fuel line.