The basic way to find out how your carburetion is doing is to read spark plugs. This is covered elsewhere in the file, jets.htm. But I ran into a good little rundown while playing with an OCR programme the other day.

Let me quote from the Meriden Triumph shop manual:

"Spark Plugs

It is recommended that the sparking plugs be inspected. cleaned and tested every 3,000 mikes (4,800 k.m.) and new ones fitted every 12,000 mikes (20,000 k.m.).

To remove the sparking plugs a box spanner (-1.1- in. (19.5 m.m.) across flats) should be used and if any difficulty is encountered, a small amount of penetrating oil (set lubrication chart Section A2) should be placed at the base of the sparking plug and time allowed for penetration. When removing the sparking plugs, identify each plug with the cylinder from which it was removed so that any faults revealed on examination can be traced back to the cylinder concerned.

Due to certain features of engine design, the sparking plugs will probably show slightly differing deposits and colouring character- istics. For this purpose, it is recommended that any adjustments to carburetion etc. - which may be carried out to gain the required colour characteristics should always be referred to the left cylinder. The right plug will almost always have a darker characteristic.

Examine both plugs for signs of oil fouling. This will be indicated by a wet, shiny, black deposit on (the central insulator. This is caused by excessive oil in the combustion chamber during combustion, and indicates that the piston rings or cylinder bores are worn.

Next, examine the plugs for signs of petrol (gasoIine) fouling. This is indicated by a dry, sooty, black deposit which is usually caused by over-rich carburetion, although ignition system defects, such as a discharged battery, faulty contact breaker. coil or condenser defects, or a broken or worn out cable may be additional causes. To rectify this type of fault, the above mentioned items should be checked, with special attention given to carburetion system. Again, the left plug should be used as the indicator.

Over-heating of the sparking plug electrodes is indicated by severely eroded electrodes and a white, burned, or blistered insulator. This type of fault is usually caused by lean carburetion, although plugs which have been operating whilst not being screwed down sufficiently can easily become over- heated due to heat that is normally dissipated through to the cylinder head not having an adequate conducting path. Over-heating is normally symptomised by pre-ignition, short plug life, and "pinking," which can ultimately result in piston crown failure.

Unecessary damage can result from over-tightening the plugs, and to achieve a good seal between the plug and cylinder head, a torque wrench should be used to tighten the plugs to the figure quoted in "General Data."

A plug of the correct grade will bear a light flaky deposit on the outer rim and earth electrode, and these and the base of the insulator will be light chocolate brown in colour. A plug which appears bleached, with a deposit like cigarette ash is too 'hot-running' for the performance of the engine and a cooler-running type should be substituted. A plug which has been running too 'cold,' and has not reached the self-cleaning temperature will have oil on the base of the insulator and electrodes, and should be replaced by a plug that will burn off deposits and remove the possibility of a short-circuit. A plug that is heavily sooted, indicates that the mixture has been too rich, and a further carburetion check should be made.

( Ed. note:- Check also for spark erosion - the constant sparking will actually remove metal from one or the other electrode - this can be corrected to some degree by careful filing with a small file such as a nail file, but when this becomes necessary replacement should be considered the best option. Slight rounding of the center electrode tip can be improved by filing this aspect flat, however.)

To clean the plugs, it is preferable to make use of a properly designed proprietary plug cleaner. The maker's instructions for using the cleaner should be followed carefully."

Thanks to the Meriden Triumph Shop Manual!