Misfire

[Waggles]

My bike developed a misfire on the way to work  thought it was going onto reserve but no, that made no difference. All exhausts were hot. tried to 'clear it' by holding higher revs, that didn't work either. I have no doubt I can trace the problem and fix it but my question is, as I have a lunch hour to try and sort it, what are the most likely causes?

I am thinking possibly plugs as she has just been commuting with no open road running for a while, if not, probably fuel contamination / blockage. Any more suggestions before lunch?

[SteveD CB500K0]

Only the usual...

Misfire on one cylinder - probably plug or HT lead
Misfire on two (1/2 or 3/4) - fuel flow to carb feeds
Misfire on two (1/3 or 2/4) - ignition LT or coils

Let us know how you get on.

[Waggles]

Thanks Steve,

Difficult to know at this stage which cylinder(s) were mis firing as all exhausts were hot ( mind you she was running OK for half the journey ) so will pull the plugs first to hopefully get a clue. It felt more like one cyclinder out as power was seriously down. Will check the points haven't come loose too.

Plugs are only a couple of hundred miles old, coils leads and plug caps all new too ( mind you the coils are pattern parts and I am getting a bit cranky with bad aftermarket stuff .... )

Will let you know.

[mick]

Hi it could also be a condenser going faulty i've had that before causing  a misfire  , cheers Mick.

[Waggles]

Cheers Mick,

Condensors and points also new, but will check connections

[Waggles]

OK well I think it was points closing up, not sure why. will investigate fully when ( if ) I get home!

Checked plugs, all looked fine, cylinders 2-3 not firing tho. Cleaned plugs and swapped 1-4 for 2-3, no difference. Looked at points and 2-3 set seemed to have a smaller gap. Cleaned then and gapped by eye, no difference. check wiring to coils, all seemed OK, drained float bowls, all carbs have fuel. Have no multi meter with me so just went back to the points, fiddled again and turned motor over to see if both sets were sparking, and whaddya know, it started and ran fine!

Perhaps its like quantum physics and the very act of observing the points changed things!

[Bryanj]

Bad connection in the rubber boot where the rear brake light and points wires join to the loom----regular fault after rebuid for some reason!

[Waggles]

Do you know what, you may just have hit it! I did chase the points wire(s) up to that point just before she started OK so I probably jiggled that connection! I will have it apart and clean up at the weekend. She got me to work OK again today so it seems OK but better safe than sorry. I didn't really find anything wrong that TOTALLY convinced me so I think you are probably correct, I award you one housepoint old boy and see matron for an extra rum ration 

[Bryanj]

I really dont know why but it's happened to me 3 times on different bikes, put engine, in connect every thing up and when I went to start it was a bugger and only ran on 2, eventually found blue wire(all 3 times) pulled out inside boot and 'cos it's hidden inside there you don't see it.

[Waggles]

Over the weekend I checked this connection. Bear in mind my motor has not been out of the frame, the points wire connections were together but rather on the loose side, so I cleaned and tightened them, reset points and timing and she is back to her old self again.

Can see why these wires can cause problems as they dont have any spare length at all so the slightest strain would pull them apart.

Will post this fact under 'hints and tips' as it seems likely to cause others problems, some more than others Bryan!