Originally Posted by Work Horse
My mate has twin 600Vac's on a tunnel ram, on an angry alloy headed 302W in a street driven XM ute.
The set up is a bit old school but looks very tuff, and is quite streetable.
The problem is in very cold weather, the car runs like a dog and uses heaps of petrol. Last week he drove out to the country, used about 20 litres of fuel. Came back in the evening temp dropped to 2-3C and he used 35 litres for the same distance.
The carbs have no chokes and run fine the rest of the time.
Anyone had any experience with this?
Check the plugs and smell the oil. If it isn't fouling the plugs and oil, then it is burning it. A look into the tail pipes can also tell you if it is running too rich, look for soot that is usually more wet than dry around the exits.
What happens with a tunnel ram on a cold day is that fuel "de-atomizes" on its way down. Depending on how cold it is, you can also have icing happening. If the works sticks out of the hood or is receiving direct cold air from a hood scoop, then it is possible that not enough heat is staying in the manifold/plenum area to keep the fuel atomized unless operating at fairly high RPM like above 4000.
Usually this will not happen at any ambient air temperature above about -5C, but it somewhat dependent upon the relative humidity.
A good choice is to buy a single 4bbl top and switch for longer trips. A bit of a PITA, but will run much better all the way around.
Something else to keep an eye on would be the power valving in the carburetors. Use a vacuum gauge after inspecting them to see if they are staying open too much in the cold weather. It could be the best possible answer to why it is using too much fuel, though it is more likely to happen at lower RPM than highway speeds. What RPM did the engine average for the lengthy portions of the drive?
I'll also bet that the engine is very hard running/starting on a cold morning!