Misfire when cold

[Roadrocket]

Hello all

I have a misfire from cold (1986 R80 Bitsa with electronic ignition). Ignition timing is checked, valve clearance OK, fuel lines OK. What is the likely cause? Checked one carb, the needle had worn off the yellowish surface, and some wear could be felt. When warm, everything is OK.

Cheers, Otto in Denmark

[ME 109]

Hello Otto, welcome!

Can you explain the misfire in greater detail?
One cylinder? both?

Short, sharp breakdown or prolonged breakdown?

If we don't know the answer, we'll make one up.

[Major Softie]

If it's one cylinder, you're in luck: you just trade plugs, plug wires, carbs, and you can quickly eliminate systems from suspicion.

If it's on both cylinders, you're in luck: few problems effect both cylinders equally, so you can quickly concentrate on those.

So, you're in luck!

[Airbear]

So, you're in luck!

Ah, a new meaning for the work 'luck' that I hadn't encountered before. Thanks for that, Major.

[Roadrocket]

Thanks chaps, the misfire is not constant, it starts fine on the button, but when I take off, it misfires sometimes, making a haff-haff sort of sound. The engine of course stops pulling, and the bike judders as it goes. After a while things clear off, and it runs as it should. I installed the electronic ignition only last year, bought from Motorworks. I overhauled the carbs a couple of years ago with new gaskets and diaphragms, but not needles and jets. Air filter is admittedly a couple of years old, but does not look dirty.

[tagordon]

When did this issue start?
No backfire?
From the sound you described (a haff-haff sort of sound) I think is the engine gasping for fuel or air.
Are you using the choke when starting the engine?
If you are using the choke to start, when you move the choke lever to off make sure the carb's lever is returning all the way to the stop/off position on the carb. I had a chronic prob w this on one particulat carb. I had to make certain the area was free of grime,goop, mud, etc, or the prob would recur.
Then check the floats for proper height. To high, maybe?
Clean the vent on the engine side of the carb with a small wire.
It sometimes gets blocked. I didn't even know it was there untill I had a prob w it.
I had a difficult time finding the vent, gotta look close to see it.
Here's an image of the vent I speak of:

This will get us started with narrowing down the cause.

[Roadrocket]

Thanks tagordon, will check that vent!

[Duane Ausherman]

Unless the fault was obvious, our first procedure was to try a good set of plugs. Keep them in until you have it sorted out and then replace with the original ones.

Let us know what you find.

[Roadrocket]

Hmm, the vent was not clogged, will try a new set of plugs. Maybe the coil/coils?

[Major Softie]

Hmm, the vent was not clogged, will try a new set of plugs. Maybe the coil/coils?

Not impossible, but quite unlikely. The normal fail mode for coils is to get worse when hot, but I have heard that they can occasionally act the opposite way. I've never seen it though.