ignition timing for Xflow

[Graham Sewell]

Having just received a timing lamp for Xmas, I checked the ignition timing at idle (about 800rpm semi-warm) and found it to be reading 12'BTDC. The build manual states 14' but does not mention at what engine speed.

 

So a double barrelled question that I should already know the answer to.

 

What should the engine speed be? I think it is anything below about 1200rpm (i.e. before the advance curve kicks in)

 

What difference does this 2' make in reality - it is an unleaded dizzy and therefore I am assuming that 14'BTDC is safe for any fuel if I make this adjustment. But bearing in mind that this represents a 1' tunr on the dizzy - is it worth trying to adjust?

 

Low tech luddite - xflow and proud!

[Graham Perry]

Have a look in the archive as there has been some excellent advice from Mr King on this in the past. Static timing depends upon the advance curve of the mechanical dizzy. When I had one I ignored this and just timed it in to get max advance at high revs which should be about 35 degrees. However. this is not always the safe thing to do (I have since found out) so this whole area can be a minor minefield. The advice in the archive covers most scenarios

[Roger King]

If it is one of Caterham's unleaded distributors the correct figure is 14 degrees BTDC at idle (around 900rpm). However, this setting will probably then only achieve around 26 degrees at full advance, which is way too little for a Crossflow. Around 35 degrees is typically what is needed at full advance (but engines vary according to spec so this is nothing more than a guide).

 

As you can see, the unleaded distributor has a rotten advance curve in it and if you set it to 35 degrees at full advance you are more than likely to cause detonation damage at lower revs.

 

We have long ago found out that most engines are quite happy to run with a 'leaded' type advance curve, even on 95 octane unleaded fuel, so ideally, fit a different distributor. THIS IS A GENERALISED STATEMENT ONLY AND CANNOT BE TAKEN AS CORRECT FOR ANY INDIVIDUAL ENGINE WITHOUT TESTING.