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MG MGA - unleaded fuel

Is there a rule of thumb for Hi octane unleaded fuel ? ie., does its use require richer or weaker needles ? My 1500 A with long branch exhaust manifold runs rich MG needles and the plugs are a nice grey. Where as my 2.5 '50s Riley runs Riley weak needles and the plugs are still shading towards blackish. Both engines run SUs, have good compression, run 180 deg. thermostats, and don't blow blue smoke. I'm afraid to go weaker in the Riley as they are prone to burning exhaust valves. It has twin cams and a cross flow head. Am I comparing apples with apples , I don't understand the difference in mixture requirements. Thanks Sean
S Sherry

i would have thought it more a question of timimg than needle settings. I tuned my engine on 98 octane. I sometimes refill with 95 and occasionally get pinking because the ignition is too advanced.

Steve
Steve Gyles

Sean As Steve said other things can change plug color as well as needles. A couple of things that spring to mind are spark plug heat range, compression ratio and also the Riley had a very old fashioned air filter setup compared to the MG that can cause a richer mixture.
Also the Riley needles, as well as the jet may well be worn a bit.
Denis
Denis4

Thanks for the comments gents. Carbs have been rebuilt, new jets and needles, bodies bushed with new shafts etc. The timing is 34 max on the mechanical in the MG. It is over 40 at 4000 rpm on the Riley. If I retard it to the mid thirties the engine is a bit flat in the driving range of 2 to 3000 RPM. It doesn't ping even with 95 octane. As it is essentualy a pre war engine with a crank shaft and flywheel that account for half the weight of the rest of the car, I can drive it all day in 4th gear Some people fit a Peugoet vacumn type distributer, said to have a more suitable advance curve. Probabaly why I need more advance with the origonal unit, set up for the low octane pool petrol in the fifties. I'll try hotter running plugs in the Riley, maybe that is the answer. Thanks again Sean
S Sherry

This thread was discussed between 25/12/2009 and 26/12/2009

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