Running problem - help please

[quee2646]

Hello,

My MM was running generally fine, but I noticed that the idle wasn't exactly even - it fluctuated slightly. I thought it might be a mixture problem (?), so I turned the mixture nut (the bottom one?) fully anti-clockwise until sounding bad, likewise clockwise, and then back half the number of flats. Seems to have improved slightly (I think it might have been slightly too lean).

However, on switching off the ignition, it kept turning for a good 2 seconds afterwards!

Can somebody please help me out?

[palacebear]

Assuming ignition timing is correct, plugs clean, plugs and points gapped correctly and no other ignition component defects. Running-on usually caused by over-weak mixture or very high idle speed. Rule of thumb for adusting SU carb mixture is to turn the adjuster nut fully clockwise then turn it back 12 flats of the nut (3 full turns). Should get you in the right ball-park. Fine adjustment will probably be needed to get it spot-on

[quee2646]

Thanks. How do I know how high to set the idle? I see figures of around 500rpm or similar, but how do you do it without a tachometer?

If I set it really low, it can sound like it's about to die!

[firedrake1942]

I was always taught to set the idle to make the ignition light just glow. The dynamo cuts in at about 1000 rpm as I recall.

[palacebear]

If you're planning on doing routine maintenance yourself, you could invest in a dwell meter, which will help you set up idle speed, ignition timing, dwell angle etc. There are some on a popular auction website from around £20.
As firedrake has posted above, you could set the idle speed the following rule of thumb. If your car has a DYNAMO use the ignition light as a guide. Get the engine warm and make sure the choke is off before adjusting idle. Ignition light should be dim/flickering. Bright ignition light means idle probably too slow. Igntion light off means idle probably too fast.

[oliver90owner]

Dwell angle is generally not a problem with 4 cylinder engines, there being adequate time for the current rise, in the coil, to provide sufficient spark energy at the plugs.

If you have fitted a V8 into you car, it may be another matter. That twenty quid would go a fair way towards an electronic ignition module instead!

So, even if the points are a little to widely spaced, it does not matter too much. An excessive amount of contact gap or a worn distributor cam could cause a dwell problem but, apart from that, 4 cylinder engines revving to the usual Minor levels is not important enough for buying a dwell meter, IMO.

RAB

[quee2646]

Thanks. Engine no longer running on, and I think I've got the right mixture - by trial and error.

The idle is pretty low. Ignition light not on, but it usually only lights first thing in the day (I have an alternator, if that affects things). How low is too low? It's ticking over - very quietly - but will it take the engine if it's too low?

[Trickydicky]

My advice re the idle is be careful setting it too low, when you press the clutch this creates drag through the engine so you could end up stalling the engine.
Set it so that with the clutch pressed it just keeps running and then with the clutch up that's your best idle speed.

[oliver90owner]

The alternator lamp should be on whenever the engine is not running and the ignition is switched on. The current provides excitation for the rotor if there is insufficient residual magnetism.

Regarding idling speed, pick it where the engine runs smoothly (but without the oil pressure warning light illuminated!). I have a tractor which will run so slowly that the inertia trip on the magneto catches but still runs smoothly, apart from that. Needless to say I do not leave idling that low. Try flooring the throttle, momentarily, from idle. If the engine picks up cleanly it is OK; if it stalls either increase the idle slightly or alter the carburettor mixture if the idle sounds too high.

As long as the engine has sufficient oil circulation and the timing is not too far advanced you will not damage the engine.

[palacebear]

The idle is pretty low. Ignition light not on, but it usually only lights first thing in the day (I have an alternator, if that affects things). How low is too low? It's ticking over - very quietly - but will it take the engine if it's too low?

As you have an alternator, the ignition light should illuminate every time you switch on the ignition, but should always be extinguushed whilst the engine is running, even at idle speed.

[quee2646]

Yes, my bad. Light is on when the engine isn't actually running.

Engine seems to be pootling along fine now. Thanks for your help!

[quee2646]

Oh, the only other query is that, at idle, bits under the bonnet can vibrate quite a bit. Should I be doing something about this? Wedged a bit of rubber between the horn and servo!

[firedrake1942]

Increase the idle very slightly to the point that the vibration abates acceptably