Cold misfire

[peterengland]

Start 550 up from cold and it misfires badly for exactly 1 mile then it's perfect after that and doesn't do it again until next cold start, pulling out choke has no effect on misfire and all headers seem to be the same temperature. New plugs, points and timing set. Does occasionally have a hanging idle, but not all the time.

[McCabe-Thiele (Ted)]

It's my experience that cold starts put pressure on the plug sparking process. When the engine warms up the plugs spark with less effort.
My suspicions would start with the cheaper easy bits first, check condensers, plugs, plug caps, leads & finally coil.

[K2-K6]

Try starting it and running for 30 sec, switch off and check pipe temperatures then to identify which one is cold.This should help narrow it down.

I had one with internal failure (on a car engine) of plug cap, it wouldn't fire cold but as it warmed the cylinder came to life and ok until next cold, rich mixture, starting then same again as here.

Problem was inside the cap which had HT separated in line but ran fine when cylinder was warm and mixture normal (off cold start system) took me a while to find it though!

[peterengland]

I have done this and headers are all the same temperature

[McCabe-Thiele (Ted)]

Cold engine compression check - are all the same?

[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]

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

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;-

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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.

[Johnny4428]

That’s interesting Nigel, and might go a little way towards the explaining zero resistance on old plug caps. I had two on the 750 but the bike definitely wasn’t running on two cylinders only. This was prior to replacing plug caps and refurbing old coils.

[K2-K6]

Yes, interesting that they'll "work" with significant problems in them.

Of course if you check resistance at low voltage through the cap it will often show toward open circuit if compromised inside. The HT will just jump that gap though when in use without real problem, as long as it can't escape to earth on the way to the plug then it may not be apparent in use under many circumstances.

Obvious here is the car system of distributor where the rotor arm to distributor cap transmission is made on the fly as it rotates, with a gap between the two as well. It's really not a significant problem in the grand scheme of things, although you'd not check a car system through that cap/rotor joint but just the descreet leads to assess resistance of each component. Given a little condensate inside the cap though, and it'll be tracking all over the place.

It's not commonly considered, but the spark duration shifts constantly based on the conductivity of the combustion atmosphere and compression effects etc. There's interaction with this aspect and HTsystem resistance that affects combustion, but not a topic for discussion on thus forum