My K100 is idling well and is fine under load but running quietly in town at 2000rpm the engine misfires irregularly. A slight opening of the throttle and it's fine, and an increase in revs to 2500 also seems to fix it. Any suggestions anyone ?

likely totally unrelated but i feel like 'expressing myself' as it were hahaha


The last few days ive had this weird kinda 'missing' or 'surging' - noticeable in all gears mostly around 3k - 4k revs. 

Very minor, the tacho remains constant etc. Its so subtle I actually question if its really there - but I know it is.

I think i discovered the problem today....

The under seat EFI computer's plug wasnt seated home correctly - i.e. the metal clip that holds it in place wasnt doing its job. I only discovered this this arvo while poking around in the area.

Anyways...point is ive had a few niggling issues (such as the bike just stopping in the middle of no where) caused by this connector coming loose.

good luck mate

That connector under the seat has been the source of quite a few no go bikes like Floyds post so I reckon before going at anything else suss that one out and make sure its al good. Loose connection here can mean no signal from Hall Sensors, Temp sensor and more.

Another potential issue is the plug front left of the bolted down gas tank [early tank, not the clipped one] that has the power supply for the fuel pump. This also can go loose creating occasional cut outs.l

92KK 84WW Olaf wrote:Another potential issue is the plug front left of the bolted down gas tank [early tank, not the clipped one] that has the power supply for the fuel pump. This also can go loose creating occasional cut outs.l

Yep, Olaf hit that one on the head too. I had issues with that fuel tank plug (the small round 4 pin jobby on the early bikes - pre 85)

Replaced it with the 'spare' from the relay box and also tightened the pins up, and its been fine since.

But the under seat EFI computer plug really needs an eye kept on it....even more so if the rubber shock mounts have degraded and it bounces around a bit...

Could be a number of things but it sounds like it could be electrical to me. Is the misfire getting more frequent?

I'd start with the spark plugs. 
You can test them with a dmm. Your plugs should be non-resistor type (NKG D7EA) and so should measure very low resistance... around half an ohm. If resistance is higher they're on the way out. If resistance is infinite, the plug has gone 'open' and is dud. They can still randomly fire in that state.

You can test the insulation by measuring from either end of the plug to the threaded body. If you get any reading the plug is dud.

Make sure you have the terminal nut screwed firmly on the end. The lead plugs don't like it when they're missing.

You can test the leads as well. They should measure around 5kilohm

Also check that all the throttle body clamps are tight and that the manifold rubbers aren't split just under the clamps.
Wornout spark plugs are also a common cause of eratic missfire.  Check leads and coil connections, check the inside of the plug caps for white powdery stuff and make sure the little bobbins are on the top of the plugs. Corrosion in the plug lead connectors on the coils is also a common problem.

Could also have a very small vacuum leak.

Only affects off throttle because so small, but lean mixtures are harder to ignite.

Could also have a very small vacuum leak.

Only affects off throttle because so small, but lean mixtures are harder to ignite.

Not saying this is likely, but entered for completeness.