MG Car Repair: Mg fuel, spark plug heat range, high octane fuel

Question
QUESTION: I have a  1970 Mk III midget which runs on after I switch off. Should I use a hugher octaine gas, and ahouls I use a lead additive?
alastair

ANSWER: Hi Alastair,
Several things need to be looked at before you can decide what to do about it. The following are things that can cause "Run On" after the ignition is switched off.

Idle too high (over 1000 RPM)
Engine over heating.
Spark plug heat range too hot.
Excess carbon in the combustion chamber. (water cleans it out if done correctly)
Ignition timing too far advanced.
Low octane fuel used.
Excess oil burning.
Wiring wrong and charging system back feeding into the ignition circuit.
Gasket protrusion into the combustion chamber.
Float level set too high.

Any one of more of these can cause "Run On", so ALL must be checked. If you suspect low octane fuel as a cause there are octane boosters available at most auto parts stores but be sure all of the other items are addressed too. Lead is a poor octane booster. Alcohol based additives are better if available if high octane fuel is not available.
Howard

---------- FOLLOW-UP ----------

QUESTION: Thank you very much.
The idle is OK and I have suspected that I should do a valve grind becasue of lack of power. Your suggestion of carbon deposits sounds right.
thank you again
alastair
New ealand

Answer
If you have a lot of carbon build up it can be removed with water but you must be careful. Remove the air filters and take a 8 oz small neck bottle and fill it with water and hold it up side down with you thumb over the opening. Set the RPM at about 4000 RPM and allow the water to dribble into the intake of each carburetor switching back and forth. You will find that you can control the RPM with the water. DON'T LET THE ENGINE STALL TO A STOP Very little water at a time will do the trick. It will make the engine miss and sputter as it runs. If it is hard to keep it running have someone open the throttle more as you do it. Keep moving back and forth between the two carbs with the water until the bottle is empty. It is easy on a single carb car because one person can do the job with one hand on the throttle and the other with the bottle of water. Some mechanics disconnect the muffler so as not to dump all that carbon into the muffler and if it is a later model car with a CAT then the CAT must be removed as it will for sure clog up a CAT.
As for the valve job, you should either do a compression test or a leak down test on the combustion chambers to see if it is needed. A dry and wet compression test will test rings too. If you are not familiar with the procedure let me know and I will detail it.
Howard