Trapper Industries is a company that deals in Russian vehicles. To run their Gaz 67bs, they discard the original carburettor and use a Webber 34. They say that it is better for modern fuel. You can see a Gaz 67b here, running on "modern fuel" with the original carburettor - https://youtu.be/y3ygJmJf1qI
You can hear the backfire due to a weak mixture ( I think ). The main jet is adjustable and is fully open. So it appears the mixture is still too weak. I have checked for air leaks in intake/carburettor and believe there are none.
I would drill the jet larger, except I have no spares! So I am at the point of asking why the fuel is different.
GASOLINE DIFFERENCES FROM WARTIME
The Gaz 67b cylinder compression is 4.6:1, whereas the modern (petrol) engine is closer to 10:1. The reason for a low compression is to prevent pinking (ignition due to compression). Modern petrol is designed to stop pinking and has a high octane rating (85/87 - the higher it is the higher the compression possible). So the earlier gasoline would have a low octane rating.
For interest, I have a wartime Fordson tractor that runs on kerosene (octane 15-20) and has a compression ration of about 4.1). From that information it would appear that the Gaz 67b would easily run on the lowest octane.
GETTING MORE FUEL INTO THE GAZ 67b
Moving onto the jetting problem - the issue is getting more fuel through the same jet size. Doing a bit of research I see that kerosene is 11% more dense than gasoline, with 4% less energy. I also see that by adding up to 20% kerosene to a petrol engine with carburettor, that the power increases by 20%. The theory there is that the same jet would allow more fuel in, because the kerosene is more dense.
So is the answer to the Gaz 67b problem this? that the wartime fuel was fractionally distilled to a lesser degree and contained a quantity of other hydrocarbons? So to counter-balance the modern fuel, one needs to add kerosene? ANY THOUGHTS?
