Loud banging (misfire)

Solved this problem but thought post anyway case others wanted to know or have the issue.

Today was on the bike and was doing a casual speed of 40... when disaster struck!!! There was:...

• Loud banging noises (misfires)
  
• Revs dropping dramatically
• Enging cutting out (still power i.e headlights etc)
• Speed not exceeding 20mph or 3500rpm

So after some kicking hong kong fuie style we discovered that the spark plug was buggered! So a quick swap to a CH8E from halfords if i remember rightly, and problem solved!


Side note:
Does anyone find their front spark plug behind the radiator not seat properly? Compared to the rear one under the fuel tank. Got it to slot in but really barely takes any effort to pop the cap back off. Doesnt really feel right.

[Spikenipple]

Take a look inside the plug cap and check that the gold element-amajigger (technical term) is the right way round. They have a tendency to spin around if you move the cap about a lot.
Unless you mean the actual spark plug? It should tighten up thumb-tight then a quarter turn (provided it has a spring washer on it, which it probably does) and no further. It should be getting pretty difficult to turn it by the end of the quarter turn.

[yorkie_chris]

It's not a spring washer. And it should NOT be difficult to turn it that quarter turn. If you are having to use lots of force near the spark plugs then you've got problems.

What you are doing there is crushing the washer slightly to form a seal, next time that plug is removed, don't use the 1/4 turn as it's already been crushed slightly, just give it a bit of a nip.


I like to use a little smear of copperslip on the threads, to prevent siezure and galling of the threads if the old plug was all rusted up and nasty.

Noticed when took the old spark plug out, it there was no gap at all, was flat against the flint(i think its named) No damage to it visually, but what would cause the spark to be flat against the plug?

[yorkie_chris]

That's very odd. If it's running fine now I think you can rule out mechanical damage. Was it covered in $hit? As in carbon fouled across the gap?

Thought that to, I've been told to check if the piston may be hitting it? But dont think enough damage caused for that really.
The spark plug was all black

[punyXpress]

? anything that could have fallen into that cylinder & been knocked against the plug ??
Or perhaps the centre electrode dropped down ?
Neither very likely, but SOMETHING caused it

[yorkie_chris]

Hmmm indeed. I'd advise a compression test.

how do you do a compression test please?

[yorkie_chris]

Get a compression tester, screw it into spark plug hole, ensure HT leads are earthed (or disconnect coils), hold throttle wide open and crank engine.