how bright does a spark need to be? - kilotank
I have posted before on this forum regarding a pierburg starting problem. I have been thru the ringer with this but are there any gurus that can confirm the following hypothesis.

I have noted that my sparkplugs are black as the ace of spades, especially the ceramic bit, although they spark fine out of the engine. Odly, if I am lucky enough to catch the engine it fires and then revs lovely, but it wont start at all on auto choke(apart from the once in blue moon I happen to have pumped the mixture to a state it can catch)

Is it possible that a totally sooted up set of plugs wont fire a cold engine with an over rich mixture, yet still be able to run smoothly once caught.

I have tried 2 carbs now but not yet put in a new set of plugs. I am hoping that a second hand carb (set up right) with a set of healthy plugs could be the answer I need.

I have tested all electrics (OK) and swapped 2 pierburb 1b carbs (i may have stuffed the settings on both) so I am thinking of getting yet another second hand carb and buy new plugs.

I stripped the 2 previous carbs but cannot find a test to check that mixture is getting thru to the chambers. The plugs always look dry no matter hwo long I turn the engine over.

this has become a bit of a mission so any further help much appreciated - BTW to all previous help given many thanks, and I did finally get the Haynes manual for carb...learnt loads, but still no solution.
how bright does a spark need to be? - bell boy
plugs can and do die try a new set ......
how bright does a spark need to be? - glowplug
Tricky one that. I had a strange problem with a Honda motorcycle, after a service it was terrible, poor idling, bad pickup etc. Tried all sorts of things, stripping and cleaning carb, removing inlet manifold and using blue hylomar on it for a better seal, checking and cleaning LT/HT system but nothing worked. The one day I was in a local dealers buying something else and I bought a set of plugs on impulse, fitted them and the bike was cured. The bad plugs had 1 extra suffix compared to the proper plugs. Seems stupid looking back but I trusted that the extra suffix was important and the dealer knew his stuff...

I'm not sure this is what you mean but a Gunsons Colourtune had a glass spark plug that allowed you to see the colour of combustion. Is that what you're after?

Xantia HDi.

Buy a Citroen and get to know the local GSF staff better...
how bright does a spark need to be? - kilotank
thanks steve, Colour tune may be the answer...I even have an old one at the bottom of my toolbox...never even used it.

This is where the forum is great

Thanks again
how bright does a spark need to be? - bell boy
a bright spark would still change the plugs
how bright does a spark need to be? - jc2
If you use the old"colourtune" instructions it will probably fail the MOT emission test.You will need to set it weaker than the old recommendations.
how bright does a spark need to be? - Cliff Pope
I think you might be trying to vary too many things at the same time. Unless you are starting with known new plugs, it's worth putting in a new set. Then you know whatever is wrong, it's not the plugs.
Then check the basics on the carburettor - ie check that the float valve doesn't stick or leak, the jets, float chamber and passageways are clear of dirt, and that the float isn't punctured. If there are linkages, basic butterfly setting etc to be set up, get them right. Then try basic tuning. The extremes of mixture adjustment are too rich = black smoke, "hunting", ie speed going up and down, coughing and spluttering and sending out smoke when you rev it. Too weak is usually described as "splashy". Weak, thin exhaust note, engine tends to slow down and cut out, reluctance to rev up.
Set it somewhere in between, at the point where it seems to run best, at the fastest idle. Then adjust the idle speed with the stop screw.
Take a look at the plugs. Clean them if sooty, and take the car for a run until hot. Look at the plugs again. If they are getting sooty then the mixture is definitely too rich, or the carb is faulty.
Tuning by ear/eye may well be good enough. I've never had a problem with a carbed car at MOT. But you can do more sophisticated checks like getting a CO meter. Personally I've always found long-term plug condition a good guide. I've set the two carbs on my Triumph by ear, balanced them, then looked at the plugs after a few hundred miles and tweaked whichever block of cylinders seemed to be a bit too sooty. If you go too weak you notice the performance going a bit off, so you can turn back a fraction.
how bright does a spark need to be? - Number_Cruncher
One further thing to note is that unlike the carbs on Cliff's Triumph (and possibly also his Volvo?) the mixture adjustment on a Pierburg only affects the idle - once the throttle is more than a little way open, fuelling is controlled entirely by the jets, which aren't adjustabe - as long as they are clean and clear, they are OK.

how bright does a spark need to be? - Cliff Pope
You're partly right NC. My second Volvo (other is injection) has a Cisak carburettor with fixed jets like the Pierburg, and a tamperproof mixture screw. When I got the car it was running badly. After cleaning out the carburettor and freeing the float I had a go at twiddling the screw, as I described. I was amazed when it sailed through the MOT with emission levels that were borderline good enough for a catalysed car.
But I think cleaning everything up without making any adjustment is always the best starting point.
how bright does a spark need to be? - sierraman
I once bought a car off a friend,as I had serviced it for him I knew the plugs were about six months old.It developed an intermittent misfire which I could not cure.I was discounting the plugs as 'known to be good',but in desperation I replaced them anyway and-problem solved.
Simplest way to set up a carb without equipment is turn the revs down as low as poss then adjust mixture screw until revs pick up,if they go too high use the throttle stop to reduce.Only make small adjustments and wait a few seconds for the engine to stablise.If hunting occurs slightly increase mixture(screw out).
how bright does a spark need to be? - Cliff Pope
Also plugs are easily damaged by shock. If you drop the box on the floor as you scrabble to get the celophane wrapping off, it's possible to cause a new plug to misfire even though apparently undamaged. Never pick new plugs from a rack where customers are free to rummage around.
how bright does a spark need to be? - jc2
If you set the the mixture to optimum then the CO would be too high to pass MOT;the trick is to set the mixture to optimum and then weaken the mixture till the revs come down by about 100 rpm.;this xhould give about 1.0/1.5% CO.
how bright does a spark need to be? - Micky
I can recall many years ago reading the results of attempts to improve the starting of an XT500 trail 'bike. Much fiddling with the carburettor, timing and valve lifter. The best results were obtained with a 24v battery, the variable settings became irrelevant. Must have been a bit of a shock (ha!) to the rest of the electrics though.


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