Spark plug for a short-stroke model 18.

[swordfish]

My 1965 model 18 short-stroke goes through plugs at a rather inconvenient rate - they last about 1000 miles before starting becomes gradually more difficult and the engine occasionally peters out when pulling away - usually in heavy traffiic or some other time when it causes maximum embarrassment! I've tried a variety of plugs to little effect, NGK 8BES seem to last marginally longer, but are certainly not the answer.
The bike is dead standard (actually an ex-South African army bike, engine never opened up from new) 7.3 : 1 compression, runs excellently, carb settings right, etc. I also have a '64 350 G3 on 9 : 1 compression using the same Champion N5C it had when I bought the bike a few years ago and this goes on and on without any problems at all - I never even clean it! Incidentally, I tried a N5C then a N3C in the Ajay and they lasted a few hundred miles.

[paul knapp]

Running too hot or too cold!..You don't say if you have done any checks re the engines ignition timing or the carburettors jetting. Fitting a slightly colder NGK plug and noticing an improvement could indicate running too hot....(ignition retarded or wrong slide or jets fitted in carbuterror)

[Stuoyb]

In my days in the motor industry, vehicles sent to South Africa always had carburettors set to cope with the high altitudes encountered. They were jetted to cope with the rarified air conditions around Cape Town. Therein may lie the problem!
Suggest you check jet sizes, needle position and slide against standard UK spec.

[swordfish]

Thanks for the suggestions lads. Ignition timing is spot-on and carburettor settings are as per English bikes. The bike was fitted with a brand new Mk 1 Concentric when I got it, but I fitted a 389 monobloc - a second-hand one, admittedly - and jetted it accordingb to the manual. However, the slide is rather worn but I wouldn't have thought it would have made such a difference - to be truthful, most of my bikes have worn slides! Anyway, I'll try it with a less worn one and we'll see how we go.

[Rob Harknett]

It is always forgotten, you do not have 1960's fuel, so the standard plugs are not correct, you need one to suit 2013 fuel. also plugs cleaned are no longer any good they will quickly fail, as the centre electrodes have not been glazed for yeras, if cleaned they very quickly contaminate. At present I cannot find my book that gives old type plug plus the type now required to suit 2013 fuels.

[robcurrie]

In my days in the motor industry, vehicles sent to South Africa always had carburettors set to cope with the high altitudes encountered. They were jetted to cope with the rarified air conditions around Cape Town. Therein may lie the problem!
Suggest you check jet sizes, needle position and slide against standard UK spec.

Cape Town is at sea level, unless you mean to top of the mountain which is 3 600 ft. I stay in Pretoria which in on the Highveld (Highland) and is 4 500 ft.

I use B7EV plugs which have a gold palladium electrode and found that their gap stays consistent longer than the standard electrode plugs, starting is also easier even with slow kicking the mag ignition engine.

Rob

[en7jos]

Hi Vic,

Could well be worth trying one of the fancier NGK plugs as Stuoyb suggests. I'm running a B8EGV which has a fine palladium (from memory) electrode; tried it out initially as a possible solution when my mag was weak before being rebuilt, but have since found that the bike starts more easily and idles better using it rather than the standard B7ES. The other advantage is that they are much more hard-wearing than the standard copper electrode plugs which might be where you could find them handy. There isn't a number 7 plug (i.e. a B7EGV) available in this range, at least not here in Hong Kong, but my bike runs fine with the slightly cooler plug which probably suits my occasion pootling around town. The downside is that they're about 5x the price of standard plugs, but seem well worth it.

Avoid the iridium NGK plugs (B7EIX etc) as they all have inbuilt resistors now (even though there is no 'R' in the code) which aren't good for mags. You might find something useful in my witterings here, although this does admittedly need updating in some parts now:
http://matchlessclueless.com/mechanical ... ark-plugs/

Regards, James