Ignition issues

[nzmike]

Hi all, since getting my S1 4.2 back on the road post engine rebuild I have been suffering from a misfire. It runs fine for around 50 miles then a misfire develops, and the only way of sorting it is a new set of spark plugs.

I have a 123 ignition and am not sure if I have the correct plug/lead/coil combo. I am using BPR5ES plugs, a Pertonix 3.0ohm coil, and standard spark plug leads. Is there anything in this plug/coil/lead combo that anyone could see causing me an issue?

[johnetype]

Yes, the standard plug leads are now having to cope with the higher voltage and faster rise time the 123 and new coil develop making them more prone to breaking down.

Replace the plug leads with 8mm silicone ones and a new distributor cap and rotor arm and see if the misfire goes away.

If you can't live with the aesthetics of the more modern leads, try them to isolate the problem and then fit new copper leads, quality butterfly caps and space the leads out.

[politeperson]

9/10 times it is an electrical issue.
However I have this happen to me once and it was a fueling issue.

The main fuel line was corroded, restricting the supply after about 40 minutes of running. This caused the engine to run lean, backfire and then stop.

Leaving the for half an hour car reset the obstruction.

The only way I could diagnose it was to pull off the fuel supply to the carbs when it misbehaved. I could then see a pathetic dribble of fuel as the fuel pump struggled to overcome the sludge.
A new fuel line fixed it. The tank was new.

I always fit an extra a large aluminium fuel filter (Bosch) (in line) before the fuel pump, hidden in the back and zip tied in place under the boot floor.
The one I have found that works best is for an old W107 Mercedes 500SL. If its a large one, you can forget about it once fitted.

[mtnjag]

If you are using Copper ignition wires they are not compatible with electronic ignition.
Switch to modern 7mm or 8mm Suppression wires. Same conductor, just more insulation in the 8mm.

Also, if you are still using old Champion spark plug boots check the resistance and the body for cracking.
Resistance can rise dramatically in the old caps and they can crack leading to poor running. May be good cold and expand a crack when hot with a possible short.

[malcolm]

I had these issues with a Hall affect system (replaced contact breakers with electronic device) and a switch to silicone leads solved it.