Fuel consumption

[Hugh Barnes]

Everyone here is getting really good MPG figures. I kept an eye on things during the Eurotour. I did almpst exactly 1500 miles in the cabriolet, 2 up fairly well loaded. I reckoned I returned some 38mpg which I was fairly happy with. I run the car with a 1" OM SU which I've always though runs a tad rich, but I am happy to leave it like that...

[Ivor Hawkins]

Since I overhauled the Zenith 26va on the box Fuel consumption rarely goes below 40 mpg.

[John H]

Not everyone gets really good mpg!! Since the lock-down period, our '29 chummy has been using more fuel than (I think) it should. On the 120 mile run to Moreton in Marsh in July, we appear to have averaged 32mpg, cruising at 40-ish on roads that are not all that hilly either. The brakes are not binding, and the car's performance is fine, but fuel consumption is heavy and the spark plugs always look very black and sooty. Also, it hesitates when pulling away from junctions, something which it never used to do. The carburettor is a bronze FZ updraught. The float level looks good and all the jets are clear and standard size (measured with proper jet drills). The needle valve doesn't appear to leak. The engine has good compressions, the valve clearances are right, the ignition system is in good order. It starts and idles well, responding correctly to the slow running adjustment. My thought is whether there is an air hole or passageway in the carburettor that has become blocked, making the mixture go rich? Dunno? It always used to do 40+ mpg on long runs, so what has changed?

Any thoughts?

[Vancevr]

I live in a semi small town. (south lake tahoe CA.) And there are traffic lights every 1/2 mile or so. In that amount of time it takes me a calender week to accelerate to 30 mph. On a steep down hill ive never exceeded 45 mph. I live at 6200 ft so obviously the elavation plays a big part. I try my best to always take the fuel measurement on flat ground. As another person said that my fuel needle my be not working. I thought about it and even after i turn the gas off i still get gas leaking out of the air inlet of the carb. So it may be flooding the engine even while im driving. Any thoughts?

[David Stepney]

One of the things that will have a bearing on fuel consumption is the weight of the car. By and large (and I am not forgetting Dave Mann's figures for his RN) it seems that the lighter the car, the better the fuel consumption reported.

For instance, a 1931 tourer weighs is at 'approx. 8 1/2 cwt' according to the handbook, whereas the RP saloon at 10 1/2 cwt

The other factor is that the LWB cars had a lower back axle ratio.

Yet another factor will be the state of tune of the engine. A well tuned engine in good condition will be more economical than one where this is not the case.

[Vancevr]

How do you tune a seven engine?

[David Stepney]

How do you tune a seven engine?

Tuning an engine in standard trim: Make sure that the points gap is properly set to 12 thou. Set timing. If you have a manual advance, the timing should be 20 degrees before TDC fully advanced. Automatic advance engines should (if memory serves me aright) be set to TDC stationary.

Check that compressions are even.

Start engine and let it warm up. Set the mixture so that the engine runs evenly. I use a Colortune (which is a transparent spark plug) to get the mixture right. The combustion should be a nice electric blue colour. Set the tickover to about 500 rpm.

My experience is with the side draft Zenith. Updraft carburetors may need more attention, but I am sure there are those with the required skills to tell you how it is done.

Hope this helps.