Hi everyone.
I have an ignition issue with my 87 K100. Does anyone have a simple method of finding out if I have a faulty spark plug lead, or a faulty HT coil? The bike on at least one cylinder starts to miss slightly at low revs once the bike is hot (e.g. slowing down around a tight corner, then accelerating). Once the revs climb above 2500 revs, the problem goes away. Also, if the revs are kept high, the problem doesn't really occur.
Within the last year I have replaced all 4 spark plug caps (5k ohm as required) and the leads, as corrosion was starting to eat away one of the original leads/caps. There was some corrosion in one of the coil sockets as well, which I cleaned away. Plugs are less than a year old, gap set correctly. I am thinking that maybe one of the coils is starting to break down when they get warmed up. The problem doesn't seem to occur when I ride the bike to work in the mornings (temp approx 10 to 15 deg C, distance 24km), but usually in the afternoon on the way home (temp 20 to 25 deg C).
I did use standard HT cable from an automotive outlet, and wondered if the varying resistance of these would cause a problem like this? (I put them together myself, as genuine leads are very expensive!)

Any ideas greatly appreciated.

Thanks,
CV

Hi CV,
I was in Nelson for a flying visit on Thursday last week, lovely day, drove to Picton did the survey and drove back to Nelson really enjoyable would only have been better on the K.
Now to your problem you probably know this but just in case here goes, our K100's have two double ended coils and use what is called a wasted spark system in that each coil has two HT outputs so two cylinders spark every time, one on compression which lights the mixture in that cylinder and the other which is not on compression and hence does nothing (so it is said to be wasted).

That means if the coil is a problem then it usually affects two cylinders since when on compression each of the pair are sparked by the same coil. The K coils are prone to cracks in the insulation and that allows damp in and they can misfire. The leads are individual to each cylinder and a faulty lead can affect only one cylinder.

You say you used standard HT cable from an automotive outlet was that by any chance resistor cable with a black carbon core? If so you have double resistance when you add your plug caps to the mix and if by chance you use the plugs intended for the 16 valve K then they have the resistor built in so it is possible to have triple resistance. I mention that only in passing for background so to speak.

With the plug caps are they the type intended to clip onto the threaded spark plug top or are they intended to clip onto the little barrel screw on plug tops which used to come with every plug but now they don't and we have to recycle them. I ask because it is surprisingly common to find people using the plug caps where the little barrel screw on device is required but they have just pushed the cap on and now there is an extra gap for the spark to jump and to burn away the inside of the cap as well.

One of the common problems that affects K's is the Hall Effect sensors which have been known to fail and the commonest symptom is problems when hot which go away when cooled down.

First off take your high quality multimeter (an essential fault finding tool) and measure the resistances of both coils with everything unplugged and tell us if they are approximately the same. Do that cold and then pop them in the oven whilst SWMBO is out and heat them up to ~50 C and measure them both again. Have any of the readings changed significantly? If they have and the two coils are significantly different you have isolated the coil as a problem.

One way you can watch the coils sparking is with a neon with a wire wound round the spark plug lead and the other grounded the neon will flicker every time it sparks. An alternative is to use a clip on inductive unit for an automotive meter, if it is misfiring it will show lower revs on that coil lead than on the others.
Let us know how you get on and browse the download section for ignition related topics there is a wealth of information there.
Last but by no means least go to the new members page and introduce yourself to the group.
Regards,
K-BIKE

Thanks for your response.
Have measured the HT leads, with caps attached,and found quite a variation. 5.03k, 6.5k, 8.2k and 13k! Leads are 'Bosch Super Sports Inductive Core' which I guess is what you were referring to, though the variation in resistance is very odd - from 300 ohm to 8k. Obviously not a good choice on my part! I note my manual says leads should be between 0 and 1k. Replacing the leads looks like the best place to start!
Coils both seem ok at this stage (primary approx 2 ohms, secondary 10.5k on both units).
Can you suggest where to look for good leads? Auto-electricians and places like Supercheap Auto don't seem to have anything of any quality.

Regards,
CV

Check out www.realmengineering.com.
They have developed, with the help of beachcomber, replacement HT leads and rear shocks.
Note that the prices are for the set, not per lead and they post worldwide.

Hi there.
I experienced some ignition problems in december, checked HT leads resistance and found hig values, a good cleaning (sand paper included) to the cap contacts and all the values came to specification (5kohm as per Haynes), afther that, failure continues so HT coils were checked, fault was found: bad contact at the coil primary connection.
The first law for K´s electrical trouble rules: clean all connections before you start cleaning your wallet.
Best regards
JAP

Amen, JAP.

I think JAP has coined the mantra that defines all classic vehicles as well as K's pure succinct brilliance.

This deserves to be called the First Law of K Dynamics or JAP's Law
1 - Clean all connections before you start cleaning your wallet.

Regards,
K-BIKE

cv wrote:Thanks for your response.
Have measured the HT leads, with caps attached,and found quite a variation. 5.03k, 6.5k, 8.2k and 13k! Leads are 'Bosch Super Sports Inductive Core' which I guess is what you were referring to, though the variation in resistance is very odd - from 300 ohm to 8k. Obviously not a good choice on my part! I note my manual says leads should be between 0 and 1k. Replacing the leads looks like the best place to start!
Coils both seem ok at this stage (primary approx 2 ohms, secondary 10.5k on both units).
Can you suggest where to look for good leads? Auto-electricians and places like Supercheap Auto don't seem to have anything of any quality.

Regards,
CV

IMHO the leads are not really a problem. I run standard, cheap Bosch leads ($6 each) on my bike and have done so for a few years. No problems at all, I get about 250km from full-to-red light out of each tank of fuel. The resistance is approx 3k-5k ohms. What I did was to get all the leads to be the same length, so the resistance is the same. Also, you can add 5k ohm plugs if need be.

The standard resistance from memory is 5k ohms per lead, plug included.

Your resistance variation is huge. I would replace the 13k first with something close to 5K-8k to start with and see if that makes a difference.