WTK: Pre-ignition

[Rory Phoulorie]

Make, Model and Year: Mitsubishi Lancer GL (Local CK1) / 1997
Engine: 4G13 (Carburetor)
Kilometreage: 178,000km
Symptoms: Lots of black smoke upon starting when engine is cold; pinging under heavy acceleration; and pre-ignition occurs occasionally when shutting engine off.

The car began to idle rough a couple days now, shutting off in traffic today. I just pulled in by a mechanic (not my normal mechanic) to see what could be the problem. Three out of four spark plugs were black at the tip. The mechanic put in four new spark plugs and "balanced" the carburetor (his words not mine) by adjusting two screws towards the back of the carburetor. The mechanic also checked the engine timing and said that it was spot on.

The above solved the rough idle, but when coming home I still heard the engine pinging under heavy acceleration and the engine pre-ignited when I shut it off at home.

Could anyone shed some light on what else could be the problem?

Your assistance with this issue will be sincerely appreciated.

Thank you.

Kindest Regards,

RP

[Red Dawg]

The black spark plugs would indicate an over rich condition (too much fuel).
Normally engines pre ignite and knock when they running lean, not when they are running rich.

There is no simple answer, especially since I am trying to diagnose without actually seeing the car. I can think of a number of possible causes. I listed them in the same order that I would do the diagnostic.

The easiest diagnostic is to inspect the spark plugs closely. The spark plugs can tell you exactly what is happening inside the engine.
Spark plug diagnostics here -
http://www.spark-plugs.co.uk/pages/technical/diagnosis.htm.

1. Carburettor
I cant diagnose without actually seeing it but basically the carburetor may not progressing properly from idle to acceleration, basically the mixture is tuned well at idle but it is going lean upon acceleration. Black smoke is also indicative of carburettor problems. Only a proper disassembly, clean and rebuild will solve this but there is no guarantee. I cannot tell you how many carbs I have rebuilt only to end up tossing them and replacing.

2. Ignition
Specifically the distributor is not advancing correctly or smothly. Have the mechanic check the internals of the distributor, make sure that all the internals of the distributor are free and not stuck. also check the springs on the advance weights. If one of these springs is broken the weights will advance early causing pre ignition but will read correct at idle.

3. Heat
Is the engine running hot, heat will cause the engine to ping and run on when switched off. Check the Thermostat, Temperature Gauge and Fans. Also check the heat range of your spark plugs. Spark plugs that are not dissipating the heat fast enough will create hot spots and cause pre ignition and run on. A close inspection of the plugs will determine if they are overheating.

4. Oil fouling
If the engine is worn, engine oil can pass the piston rings and cause smoke and fouling of the plugs. Engine oil also reduces the effective octane of gasoline. This will cause engines to knock and run on.

5. Carbon build up.
A carbon build up in the combustion chamber can create hot spots that can cause fuel to pre ignite. These hot spots can cause engine to run on after it is switched off.
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[Sanctifier]

Great write up dawg. Thanks.
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[Rory Phoulorie]

[Rory Phoulorie]

Can anyone recommend a good mechanic who can do all of the above? Location is not an issue when it comes to taking care of my car.

Thanks.

RP

[Red Dawg]

Check Raj K Singh auto supplies in Arima for an Aisin Carburettor. I cant guarantee that he has one, but he usually has some of those around.
Somebody on tuner had a new Weber for sale recently but it is not a bolt on and go item, it might need to be jetted for the application. Sherrifs auto supplies has new new Weber carbs and all the Weber parts kits and adapter plates, but they can get a bit expensive.
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