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Still rocking and singing
by Anders Dinsen ... January 11, 2009, 05:22:10 pm
The subject title of this blog entry refers to the wonderful sound from 3 to 5000 rpm, where the four inlet barrels are singing in full concert with the exhaust! Smiley

I have experienced a bad smell of petrol when the car was cold, and I suspected a leak somewhere, but now beleive that it comes from the tank vent, which has to let out petrol fumes when the engine heats up and the air in the fuel tank (and the fuel itself to a small extent) expands with the increasing temperature. The vent exits below the car, and fumes are drawn in through the cabin ventilator, which I usually have running full speed these days.

The tank sits on the cooler side of the engine, so in normal warm weather, it's probably not heated too much by the 90 degree warm engine, but these days the temperatures are around 0 degrees here,  the air inside it is heated a good deal more. The fuel tank vent is a bi-directional pressure valve, which opens at a certain pressure only (either way).

I used to have a more or less permanent smell of fuel from the car, but this was from the Solex carb which used to leak out fuel from the acceleration pump channel, where the o-ring seal has a tendency to break. The webers have a nice carburettor-typical smell of petrol, but nothing bad.

The idle speed has been increasing somewhat from the 1000 rpm to about 1200-1500 and even upto 2000. The increase has happened particularly in cold weather and after straights on the motorway. I think moisture in the air forms ice in the carburettor venturis preventing them from closing completely. With the single Solex, I had the same problem, but fixed it by connecting the warm-air device. I can modify the airbox to take the thermostatic vacuum valve and connect it with these carbs too.

This photo shows the warm air device connected to the single carb. It sits under the air filter housing.

But I was also suspecting that the carbs have gone a bit out of syncronisation: The right carb is the one that has the throttle stop, and if the left one goes out of sync, the result is vibrations and either a lower or higher idle speed.

I check the syncronisation visually by unscrewing one progression hole inspection cover on either carb and carefully adjusting the syncronisation screw so that the throttle plates are in the same position on both carbs. This can be done quite precisely.

And my suspicion was right: The left carb was indeed a little more open that the right one. Hence the higher idle speed.

It's now fully adjusted, so I'm ready for a week's more "road testing".

My future plans still include going to the rolling road, trying out F16 emulsion tubes, and probably also trying out 34 mm venturis, but I haven't ordered anything or scheduled a session on the rollers yet.
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