Sparkplugs

[ahebron]

Have been working on fitting Mk1 Amal Concentric carbs to a non Triumph, I think I mentioned this a while ago.
Interesting that the sparkplugs have not liked us fiddling with the carbs and started to make the car almost impossible to start and we kept working on the carbs thinking it was them causing the problem. We have been taking the plugs out and cleaning them but still not that great a difference even though they still had a healthy spark from the magneto.
I got a couple of sets of new plugs and put a set in and the car started instantly so we have to undo all the carb alterations and start afresh.
With new plugs it is now a tuneable engine.
I have been told by mechanics that they do not clean plugs anymore just replace them as they cause problems once fouled.
 

 

[Pete Lewis]

there some clues in this posting about insulator failure when sooty etc

Pete

 

[Casper]

My early instruction included:

"If it won't start until it starts - it's electrics.  If it starts but won't run, it's fuelling"

Not always true, but a decent start to diagnostics.

C.

[Unkel Kunkel]

I have not heard that about spark plugs.

I can see it might apply to modern cars where plugs seem to last huge mileages so if they are  thought troublesome, then just replace them.

In their day, ancient cars, needed their  “sparking  plugs” cleaned very frequently -  before every outing - and this is still the case now.
In which case, one would get through an awful lot of plugs!

Old cars with their  magneto ignition and carbs like Claudel-Hobson, early Zeniths and  Autovac things etc can be a bit pernickety -if ignition or mixture is a bit out, then the state of the plugs becomes the  critical factor.

In such a case, If they aren’t super clean -( or new ) then that small change can have have a big effect and you can crank away  until you’re blue in the face, they won’t start.

Don’t know much about Amals, but they are in effectively “ two carbs in one”  (or three for later ones)  and both bits have to be set.

There are guides on-line for setting these and there is an Amal  guide which is printable and makes into a little cut-out with a little wheel to turn  to aid settings.

Magnetos should give a good fat spark  at cranking speed.

Magnetos can gradually loose  power  over years as their  magnets weaken with time, but  they can still produce a visible spark  which gives some false reassurance- At least outside the engine, ie under atmos. pressure  but not under the compression of inside  a cylinder.

A cheap adjustable ignition gap  tester will show how long the spark is (based on  30,000 v per cm. at atmos pressure)  so say a spark is visible  - great, but only I mm  that spark is only 3,000v and would be very marginal to fire an engine.- really need a spark that will jump  a gap towards a cm.

( very old cars with induction , “trembler “ coils, rather than magnetos,  when properly set up can produce quite impressive sparks  of over 2 cm and then arcing  state of the cable and plug insulation can then be the issue)

The points gap in the mag. is critical.

The condenser in the mag can short internally and fail though older mica insulated ones seem to last.

Very old magneto armatures were insulated with shellac and set in petroleum wax or bitumen and this can degrade causing shorts between the wires.The insulation can sometimes be seen to have liquefied and even dripped out.

I think I would check the ignition.

Then get an an Amal guide and set the carb(s)

Good Luck!

[Pete Lewis]

and to   add   when it comes to plugs with an R in suffix ans our old HT systems  there is this

worth a browse

 

[ludwig113]

Many years ago when mucking about with motorbikes i realized that just because they spark doesn't mean the engine likes them.

I had a VFR400(NC30) which was very fickle about which plugs you put in, it really didn't like Nippon Denso(oem) but loved NGK's.

I've had similar experiances with 2 stroke engines.

[avivalasvegas]

If you can find them, W78 plugs are excellent. (No R or X). Long discontinued but old stock is available every now and again. 

[Pete Lewis]

the previously liked NGK are fine if you dont over choke or do repeated short runs so they never get cleaned up.

many plugs are now  not glazed  and do not like  overfuel/sooty conditions  

my 2000 happily starts and runs perfectly well on BP6ES  plugs   No 'R' and no faffing around trialing anything 

whilst the plugs are the life and soul  its more to how the car runs that contributes to probems with the plug spec. 

Pete