[tsetse1986]

Originally Posted by asneddon

I did a little experiment in the weekend.

First, background on why Im not confident in my timing torch. Its a cheap one from Helfords (cant remember the price, but under 100), and it has a dial on the back so you can adjust the strobe. In theory this makes life easier as I just put a dab of whiteout on the TDC mark on the front pulley, then I adjust the dial on the torch so that the timing mark on the cam cover is pointing at this TDC mark. I can then read off the dial what the timing is. Well, the feel of this dial just doesnt fill me with confidence. You can wind it way past zero backwards, and there are no notches or anything to give me the feeling its a digital dial rather than an analoge one.

So this weekend I did two things.
Built a 'step' curve (ie, right angle jumps, not curved) for the 123. so 0d at 0 to 1000rpm, then 10 to 1500, then 20 to 2000, 25 to 2500rpm. No vacuume for anything

I then used my torch and reved the car and adjusted the dial to make sure it jumped in the steps I expected. This actually worked really well, and I got the 10, 10, 5 degree jumps I expected.

Next was to make sure the zero was correct. This is set by having the engine at TDC on number 1 and disconnecting the neg from the coil. You just turn the 123 clockwise until a little LED just comes on. Pretty easy.

Since my normal curve has 10degrees at idle, that means the total idle advance should be 10 degrees. Both my torch and a borrowed one (VERY old and dodgy looking) agreed within 2 degrees. I then loaded a curve with 5degees at idle, and tested again. Same thing.

so, timing is where I expect it to be, and I can probably trust my torch now.

I have put new plugs in, and replaced all the HT leads (with fancy spiral core 8mm ones). I'll check them again next time I get a chance and see if its fixed itself.

What does the colouration of my spark plugs mean though? Its like its running a little rich at the base of the plug, and a bit lean at the top. Is this to do with the heat range of the plugs? I'm using NGK BP6HS plugs with gap at .028

I've been reading this:
http://www.4secondsflat.com/Spark_plug_reading.html
and it seems to suggest I'm not far off, I've jsut never seen the distinct colour change up the earth leg before.

This thread is getting quite old, so you probably have things sorted by now. If not, hope this helps...

Your plug looks on the rich side of OK, but don't forget it's the position of the SU needle in its jet at the RPM / throttle position you were using for the couple of minutes before the plug was 'read' that you are looking at. If the engine was idling for any length of time before it was turned off, then you are looking at the idle mixture. If you cut the engine immediately after a foot to the floor run up a long hill at 5000 RPM the plug colour will reflect mixture strength produced by the narrow part of the SU needle. This is the basis of the classic 'road test' method of calibrating SU needles.

Fluctuating idle can be due to wandering ignition timing - hopefully not in this case - and air leaks around carb flanges / spindles and inlet manifold.

Poor MPG means either the mixture is set too rich or the engine is set up so the throttle has to be opened more to get the power needed. The latter can be caused by ignition advance being too retarded. The general rule seems to be use the most advance you can at all times to get best power / economy but not so much that you get preignition / detonation. That is why vacuum advance is important for maximising fuel economy under part-throttle (cruising) conditions where cylinders are only part filled and the rate at which the mixture burns is slower - it needs firing up sooner.

Des Hammill's book on tuning dizzy based ignition systems gives loads of good info on how advance curves work and gives rules of thumb to help 'curve' your dizzy.

I have just bought myself a 123Tune for my Amazon rebuild. I hope to get it on the road by Christmas, but there is lots of welding & rebuilding to be done first! Can anyone help by providing a set of advance curves to get me going? My engine specs are: B20 2l bored +30 thou, K cam, twin 1 3/4 SUs, cast 4 into 2 ex manifold with Simons sport exhaust. Any help would be appreciated.

All the Best,
Tim