misfiring Atlas

Those pistons are more than 8 to 1 , probably nearer 10 to 1, ignition should be 28 or a degree or 2 less .. Try a grade hotter plug . But not without backing off the timing first.

The ignoition timing is NOT the issue here. It could be the grade of plugs. You would need ones that are more for akin for town running for a missfire within a few miles from cold. The sooty plugs would be richness which would help cold starting. And rich running could easily be down to worn needle jets as the needles are already as low as poss.

Commando pistons are the usual replacements in the Atlas ,these are flat top. You have domed pistons ,so the ratio is going to be well over 9 to 1 ,  Check what is written about ignition timing with Commando pistons in an Atlas. And think on.

When the Atlas was first produced it was sold with two sets of plugs. One set for running-in and town work and another set for open road riding.

One set were KLG FE 75 or Champion N5 or equivalent.

The second set being KLG 80 or 100 or Champion N4 or equivalent.

I never said worn needles....The NEEDLE JET wears far qicker than the needles. The Concentric wears 3 jets before the needle needs changing!

Higher than std compression pistons would normally need a colder plug. But i dont think we have the full picture on your bike . If the plugs are fouling then its probably oil or fuel and a hotter plug may make starting and warm up easier but risk burning your pistons once running hard. 

This has always been an area of confusion. If the plug runs HOT it is usualy for town usage where the heat of the plug causes the engine to run cool by taking the heat away.

Go faster-racing plugs- are for track usage and run cooler as they cool the hot engine.

So hot running plugs for towns and cooler running.

Cold running plugs for 'hotter' engines used on the track to keep them cool. But are they hot plugs?

Champion, NGK and KLG (and more) have ranges of plugs, but the heat numbers often go in the reverse direction than you think and to the other make of plug. Guaranteed to confuse.

It might be stating the obvious, but the coloured pages in our old manuals showing the different plug colours only applied to leaded petrol.  They have been irrelevant (or incorrect) for a very long time now.  Modern plugs have a wider heat range than in the old days.  How many people nowadays start a cold engine on hot ('soft') plugs and change to cooler running ('hard') plugs once it's properly warmed up and ready for a hard run?

There is a very interesting article here:-

http://mgaguru.com/mgtech/ignition/ig101.htm

That I found when doing some research on plugs a few years ago.

If you don't want to read the complete article, please read this extract.

The most important feature of a spark plug (and maybe the only important feature) is the heat range, and that may not be the same optimum for every engine, maybe even different for engines of the same spec and build parameters. In reality you may have to experiment a little to figure out what works best for you. If the plug runs cold it can misfire or collect oil and foul out. If the plug runs hot it can cause pre-ignition or detonation which can damage internal engine parts or even physical failure of the plug itself. When in doubt you should start with a cooler plug, and if it works well stop worrying. If it fouls out try a hotter plug. Black deposits on the plug usually mean it's too cool. Stark white color or a cracked insulator around the tip would mean it's too hot. Good operation makes the ceramic tip insulator an even light tan color, possibly just a little darker on one side because of variations in temperature in different parts of the combustion chamber. A badly worn engine might leave black deposits on the plugs regardless.

I recently did a plug chop on my 99.  Troubled by signs of weak mixture  just off tickover but still suffering sooty plugs , I took my bike to a long steep  and twisty hill in kent .A run down and up to warm up and a set of clean but used plugs fitted .The throttle was taped and marked up to indicate the fueling phases .  The very  fast run up was a bit too exciting ,the 99 was not fazed by the hill at all and using both sides of the road on the footrests  I frightened myself at 80 mph still accelerating in third and had to run in top to avoid going any faster . The ignition was cut at speed and the plugs checked. .The plugs showed good colour at all positions . After much playing with carb settings I concluded that carb body wear was causing the odd POP and found  that  an ancient  Amal needle and a too weak pilot setting would give reasonably clean plugs. If i switched from Synergy Plus non eth to lower grade  E5 plugs ran cleaner , The additives in premium fuels  could be making sooty running  or perhaps the weaker eth fuel runs cleaner . I am still fidling with it and no , I dont like eth in my tank !.

Needle jets wear out faster than the needles.