|The MGA With An Attitude
Setting IGNITION TIMING (practical application) - IG-116
(The Short Version)
At 11:26 PM 2/7/2007 -0700, Francis Camilleri in Malta wrote:
"What is the dynamic timing on the MGA 1500 engine? Is it the same as static, i.e. 7 deg BTDC? "
No. When static timing is set to 7d BTDC you should expect a dynamic timing reading to be a little earlier, like maybe 12d BTDC. This is because mechanical advance starts at very low engine speed with the stock distributor, so here may be some mechanical advance already happening at normal idle speed. Mechanical advance may be more at 1000 rpm than at 700, so where you set idle timing may depend on condition of the distributor. This is a moot point anyway, as the factory spec for timing was very conservative to allow for some of the worst grades of fuel anywhere in the world in the 1950's. Modern motor fuel is considerably different, and you can use considerably more spark advance to good advantage.
With engine at slow idle, well under 1000 rpm (maybe 500 rpm) there will be no mechanical advance and no vacuum advance (vacuum ported on the carburetor), so you don't need to disconnect the vacuum line. At 1000 rpm there is likely to be some mechanical advance, but no vacuum advance (maybe, depending on other tuning factors). Set dynamic timing at 20d BTDC at 1000 rpm using the strobe light. This may seem like a lot, but your MG will like it. You should notice a nice improvement in low speed throttle response and perhaps a small increase of power as well. Once timing is set you might also set or reset idle speed to about 1000 rpm to have quicker throttle response from dead idle. Only worry about setting a slower idle speed if you spend a lot of time with engine idling and may be concerned for fuel economy at idle speed (which is a very small difference).
When you drive the car try a short acceleration run at full throttle in 3rd gear from 2000 to 3000 rpm. This is the condition giving maximum dynamic pressure in the cylinders. If it pings audibly with full throttle around 2500 rpm you might try using premium fuel. If it still pings you might retard timing a few degrees. If it still pings there would be something amiss with your engine, like abnormally high compression ratio or hot spots in the combustion chamber. Some engines cannot be made to ping under any circumstances, no matter how much you advance timing, so using ping as a reference to set timing may be a useless endeavor.
As long as it doesn't ping you can use the cheapest and lowest octane grade of pump fuel available. Low octane fuel may cause run-on after ignition shut off. This is aggravated by high running temperature and by fuel containing alcohol which vaporizes easily (the same things that cause vapor lock). Setting idle speed slower can reduce a tendency to run on. Otherwise just stick it in 3rd gear and let the clutch up to kill the engine after shut down, and don't worry about it.