My 1957 Nash Metropolitan is new to me, just had some work done to make it run nicely, new carb and brakes and stuff. I put mid grade gas in and she diesels a bit when I shut engine off. should I put high grade or just stay with mid for the next tank?
It sounds like it needs the points and/or timing adjusted. It shouldn’t diesel. When’s the last time the engine was reworked? It could be carbon deposits in the head.
Dieseling was a problem on certain engines back in that time period. Certain Studebaker engines had that tendency. If the ignition point and spark plug gap are set correctly,the timing isn’t over advanced and the idle speed is correct, then it shouldn’t diesel. One thing we did back in those days that would help was to use Casite Motor Tuneup made by the Hastings company. We would add a can to a tank of gas and after driving around for a while,take the car out on the highway, accelerate it rapidly up to 70 mph or so, back off and then do it again in a couple of miles. For more severe cases, we poured the Casite Motor Tuneup slowly into the carburetor while the engine was running, and then take the car out for a fast run on the highway. I don’t know if Casite Motor Tuneup is available any more, but there is probably some other product that will do the same thing.
We did the same thing with a small amount of ATF, for carbon in the combustion chamber. After setting the points and timing, it might be worth just taking it out on the freeway for a short run, revving it up through the gears (an ‘Italian tuneup’).
An MG powertrain as I recall…Set the idle speed down to around 600 RPM and it should not diesel when you turn it off…Running a bottle of Techron or BG 44K through the fuel system might help…Fuel octane will not make much difference…