SOHC.co.uk Forums > CB500/550

Cold misfire

<< < (2/3) > >>

K2-K6:
Have you started, run short (while misfiring) then pull the plugs out to see what that tells you ?

Put up pictures of the four plugs if you can.

Lobo:
Air leak in the exhaust manifold area? (which closes up upon heating)

peterengland:
plug caps it was, renewed them and it's fine now. Just struggling to understand how they failed when cold but fine when hot? me no understand

taysidedragon:
One of the mysteries of life.😁
Temperature change can expand or contract materials so can make or break electrical contact
It also changes electrical resistance which could make the difference.

K2-K6:

--- Quote from: peterengland on October 22, 2022, 11:26:05 AM ---plug caps it was, renewed them and it's fine now. Just struggling to understand how they failed when cold but fine when hot? me no understand

--- End quote ---

I take things apart to understand failure as it helps with diagnosing/isolation of future problems encountered.

With a failure of circuit inside the cap it will still transmit the HT feed under certain conditions, the current just jumps across the gap that's there.

Not my photo, but shows internals nevertheless;-

[ Guests cannot view attachments ]

When it starts with a cold cylinder and rich mixture the plug gap effectively shorts to earth and the spark develops inside the cap at failure point (you can see a carbon track developed at site that shows where the spark is jumping across) with the system unable to then provide enough energy to spark at the plug electrode and essentially making a wet plug situation. The change that occurs to get it going is those cylinder/mixture conditions, as the engine runs on the other cylinders and back the choke off the conditions improve in the cold cylinder as it vents itself to exhaust without firing. Eventually, and usually on a overrun closed throttle (the smallest fuel amount added) the plug will start to spark again with that cylinder then picking up more continuous combustion as it warms.

The specific variable is the mixture/stratified charge has lower resistance (when at max fuelling and cold cylinder) than the cap failure point, such that the spark will shift location as these two sites go to parity and then bias to favour one or the other as the mixture shifts.

Doesn't seem to be particular to these bike twin coils as the same happens on single line distributor system too.

The first indication of cap failure may be that under highest throttle demand you get a slight but noticeable missing beat occasionally, then slightly more laboured starting. At this point the cap is starting to spark internally under maximum load and mixture conditions, but likely quite subtle and missed perhaps in sporadic mileage running.

Navigation

[0] Message Index

[#] Next page

[*] Previous page