Okay:
RePost:
“I didn’t want to be the first to say that but can you explain what plug cuts you are refering to”
"As I seem to remember:
Find a suitable stretch of road (quiet and straight) on a dry day. With the engine at normal temperature, take it up to say 6000 rpm in third and hold it there for 10 seconds. Then in one continuous operation (and very carefully) depress the clutch and turn off ignition (mind any steering lock!), engage neutral and coast to a stop.
The plug can now be removed for analysis."
The object of the exercise is to best establish what is happening in the combustion chamber at that precise point in time. Ten seconds or so at those revs, should give a reasonably clean/new plug a chance to ‘take on’ a true colour… obviously such things as points gap, timing etc must be spot-on (and don’t forget to check that the advance curve is correct, and that no air is leaking into the inlet manifold, … etc, etc).
Effectively, you are at the end of the timing/carb balancing/mixture cycle, and are using the “plug cut” to analyse what is happening at these high revs… it gives you a check on how good your setting-up has been.
For analysis try:
ngksparkplugs.com/techinfo/spark_plugs
or:
dansmc.com … click on Index and choose spark plugs.
(hope the links post okay).
In very simple terms, & assuming there are no problems with the carb(s)
themselves, you should then be able to “tweak” individual mixture screws to achieve a good plug colour. BTW, I do mean “tweak”. If you have to make serious adjustments, then either your initial setup was way-out, or something else is afoot.
As an example, this idiot spent half a day setting up his new Elan, fitted a set of NGK BP7es and went off for a test … result: all black … Yipes !!
Whatever I did, I could barely improve the colour.
I used to use these plugs on my Lotus Cortinas, but they were Stage 1/2 road-tuned engines requiring different plug heat settings.
Substituted them for an ‘old’ set of Champion N7YC’s and the colour is MUCH better … without making any other changes. So the correct grade of plug is critical to the engine’s performance. These may well prove not to be the final plugs I use … we’ll see.
As I recall, the higher the plug number, the hotter the plug (Champion), but the reverse is true for NGK.
FWIW (and setting aside the petrol additive question) I intend to use the same type of petrol (Shell Optimax) whenever possible … just to try and eliminate another variable.
Speaking of variables… the Manual states 25 thou as the plug gap.
However, extensive dyno-testing shows that 20 thou gives the most power for a twink.
On the other hand, plug manufacturers say not to bend the earth electrode away from its’ preset gap ('cause electrodes would be out of parallel).
Personally, I intend to stick with my 20 thou, and change plugs more often if needs be.
HTH.
Regards.