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MG MGB Technical - Timing help, engine preignition

I have recently had my MG engine rebuilt and all is going well but I seem to have a lot more overrun than I used to. The timing was setup with a timing light but there was too much pinging so there was some manual adjustment nessecary to find a happy medium with the timing. I have been told that overrun is not harmful, although it sure doesn't sound too good and the engine can cough and splutter for up to 30secs, before stopping with an awful puff of fumes out of the bonnet (unburnt fuel ?)

I have put up with it for a while but can't stand it any longer, trouble is I have forgotten which way to adjust the distributor (clock wise or anti clockwise)
to reduce overrun. From memeory you go one way to reduce pinging and the other way way to have more pinging and less overrun ?
Can someone refresh my memory. I am obviously not a mechanicical genius !

Craig
Craig

Hi..

I guess that the head was skimmed during the re-build, which has increased the compression ratio a little, which will increase the tendency to run on.

There are lots of threads on this board about the subject, but here is a summary of my own thoughts:

1 Fit colder spark plugs.

2 Make sure that the front number plate (if you have one)isn't blocking the airflow under the bumper.

3 Provide a supply of cold air for the air filters.

4 Use the best petrol you can get.

Don
Don

Craig,

The distributor shaft rotates anti-clockwise, so to delay the spark (retard the ignition) you will have to rotate the body anti-clockwise too - I hope that's right. Yep, I'm sure that's right, the rotating components are going to reach the contact heel a fraction later. The amount you rotate is very small - don't go turning it through 20 degrees, you probably won't be able to start it. A good tip would be to score a mark on the dizzy and the baseplate with a sharp tool before you start so you will know how much you've turned it, and can put it back if it all goes wrong.

Ideally find a timing gun you can borrow (or even buy one). I reckon you will want to set it at at least 13 degrees BTDC at 900 rpm idle. before you do that, check the points gap is correct.

I wouldn't be happy about prolonged running-on as it stresses the engine mountings terribly and causes hot spots in the combustion chambers. Like many others, I have never eliminated it completely, but habitually put the car into 4th gear before switching off, and lift the clutch as I switch off. Much kinder to everything.

Mike
Mike Howlett

Run-on is mostly caused by bits of glowing hot carbon deposits on the piston tops, valves, and
combustion chamber walls. Much like the glow-plugs used on diesel engines, they ignite the raw
fuel-air that is still being drawn into the engine while it is slowly rotating down during shut-off.
Re-setting the ignition timing, either way, will have little effect on this (pure technically speaking).

Advancing the timing (turning the distributor clockwise) beyond original specs will have a tendency
to make pinging, and run-on, more likely - especially when using today's lower-grade fuels. With a
slightly more advanced time, the engine will tend to accelerate, and idle, more freely and happily.
Extra energy that is stored in the flywheel will help the engine spin a few extra times at shut-down.
This will also draw in a bit more raw air-fuel. This combination will make run-on more likely.

Retarding the timing (turning the distributor counter-clockwise) from original timing specs will lessen
pinging. It may even lessen "over-run" or "run-on" - by a small bit - but this is only because it is
generally more difficult to maintain a smooth idle speed on an engine with retarded timing and there
is less energy stored in the flywheel to spin the engine after the ignition has been switched off. This
is not a fix and is only a "fake" cure and at best it is unpredictable. The other catches here is that it will
also dull your acceleration, lower your fuel mileage, increase engine temperatures, and increase
pollutants.

Both "pinging" (plinking) and "run-on" are definitely not good for the engine.

Pinging can burn holes through the tops of pistons, and can crack pistons and rings, and burn valves
and combustion chambers.

Run-on subjects the main and rod bearings, rods, pistons, crankshaft and timing chain to all sorts of
sudden loads and jolts of uncontrolled acceleration for which they were not designed to handle with
any regularity. It also regurgitates junk through the carbs and aircleaners, and fouls the sparkplugs.

Somebody once suggested that I rig a cable-operated valve onto the intake manifold (connected
to an aircleaner) so that I can manually lean the air-fuel mixture just prior to shut-down. Sounds
plausible, but I've never gotten around to figuiring out all the details and parts list for this.

Presently, at shut-down, I engage 3rd or 4th gear, and slowly release the clutch while simultaneously
switching off the ignition, much like switching gears during running. Sometimes, merely depressing
the clutch imparts just enough load on the engine at shut-down to prevent run-on.
Daniel Wong

This thread was discussed on 20/04/2003

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This thread is from the archive. The Live MG MGB Technical BBS is active now.