Perfect your ignition timing
QI recently fitted a rebuilt and correctly-calibrated distributor to my Land Rover Series 3. It’s a 2.25-litre petrol with a Weber 34ICH carburettor. I believe the static ignition timing should be 3-6° BTDC. I set it to 3° using a test lamp and started the engine. I warmed it up, then tried to finetune the timing at idle. I found there was a large range of adjustment between the start of misfiring due to too much advance and slowing down due to retardation. I ended up with a best idle setting of almost 18°. I put it back to 6°. It didn’t idle as smoothly, but at least it was on the scale! Can you explain this and give me any hints as to how to improve on my method?
Bill Blazeby, Dover
ASetting the timing by ear with the engine at idle is a very approximate approach. It may run satisfactorily over quite a large range of settings – as you’ve discovered.
The nub of the problem is that idling does not represent the normal operating range of the engine. It’s designed to run under load and much faster. When it’s in this ‘normal’ operating range, the mechanical advance mechanism will be giving maximum ignition advance. This is the crucial figure, rather than the partial advance at idle and zero advance at rest.
The best method is to set the static timing to the figure stated by the manufacturer, then fine-tune it on a road-test. Advance the timing a little at a time until one of the following happens: 1) The engine pinks while climbing a hill in the middle rev range, or pinks for longer than a second when accelerating. 2) The engine pinks or shudders unhappily under moderate load at low revs (eg, when you’re accelerating in top gear from 30-40mph). 3) The engine ‘shunts’ on and off the power when driving under a light load in top gear at 30-40mph.
All you then need to do is back off the
ignition until the unwanted effect just disappears.
This will give the best ignition timing for the engine in the state of adjustment that it’s in at the time. It will depend on carburettor float level (fixed jet) or mixture adjustment (variable jet), and the valve clearances being correct. Check these first.
If everything is set up correctly, the engine should feel willing under all conditions and rarely (if ever) pink. A ‘booming’ sound from the intake or exhaust when pressing the throttle accompanied by minimal acceleration suggests something’s still awry. If you can’t get rid of this ‘booming’, then suspect inadequate fuel supply due to an ailing fuel pump or a constriction in the fuel lines, fuel filter or tank vent.
We suspect that your 18° BTDC idling ‘sweet spot’ suggests that some other mischief is afoot. Look for small air leaks from vacuum hoses and around the carburettor’s mounting flange and spindles.