Introduction #
The Weber carburettors we sell are built to 1960s and 1970s specifications. This can cause problems, for example, when installing a new fuel pump.
Pressure #
We received some Weber 28IMB back from some customers. The problem description was “adjusting the idle mixture screw made no difference”. We sent the carburetors to the distributor and they came up with the follwing problem:
The fault that they were returned with, was that adjusting the idle mixture screw made no difference. And when this happens, it means the carb is flooding. If the needle valve and float are OK and the carbs pass the bench test, then it means the fuel pressure is too high.
This really happens quite often, not only in the Fiat market, but also at Ford, Volvo, Landrover, etc. What we conclude is that modern, mechanical fuel pumps made in India, China, etc, exert much more pressure than the original pumps. This is the cause of flooding. For example, we know that modern Volvo mechanical fuel pumps can put out up to 7 psi (0.48 bar) and for a Weber that needs about 2.5 psi (0,17 bar), that is too much. I am beginning to think the same thing is happening in the Fiat market.
Being honest, I have had a couple of other Fiat specialists say the same thing, and what I have done is given them a test kit so that they can lend it to customers to check the fuel pressure.
Adjusting the pressure #
You can adjust the pressure by changing the gaskets around the isolator. Most fuel pumps come with 0.3 mm gaskets. You can replace those with 1.2 mm gaskets, which will lower the pressure slightly. Another solution is to shorten the fuel pump rod, but our advice is to start with gaskets first. The most expensive solution is placing a pressure regulator.
