Greetings helpers. A misfire diagnosis has led to an ignition coil (aftermarket) that displays only a yellow spark at the right plug. Plugs and leads have been swapped and replaced along with the NGK caps. Left side displays bright blue spark. Is it possible that only one side of the coil weakens or fails? The coil is a single unit with dual outlets. Disconnecting the left lead has the engine stop when relying on the right side and continue running when relying only on the left. Bike idles sweetly but at 3000rpm under load all hell breaks loose. Carbies have been cleaned with new needles jets and diaphrams and vacuum balanced. We are in Cajamarca Peru and have begun organizing a New coil. Hope this is the cure.

Any ideas?

Ken and Carol Duval

Yes.

At higher RPM and with load the compression increases. The mixture is harder to ignite the higher the compression gets.

Many thanks for your reply. We have a couple of options and the ball is rolling.

Ken & Carol

Yup.. sounds like dodgy coil..

Could try swapping them over and see if it happens on both sides just to be sure !

Or do BMW's only have 1 coil ????????

__________________
Did some trips.
Rode some bikes.
Fix them for a living.
Can't say anymore.

Yes...just one coil..

However it might be worth looking for some alternative locally, a popular option is to use a 20$ Harley coil, which is said to work just as well as the OEM if not better. Might be worth posting on the old's cool forum over on ADVRIDER, they have a section just for cheap alternative parts, it may be possible to find something in bolivia which will work. Good luck and safe travels. A.

Yes - that might happen.
One of my dual Kawasaki coils died on me in rush-hour traffic...

However, what is your real question?
You know the coil is bust - but what is your question?
Are you looking for new coils, 2nd hand coils, building a replacement kit to get you back on the road, or what?

__________________
Drive Safely,
Albert

Our problem still exists. Testing so far.

Replacement of leads, spark plugs and caps.
Testing of leads and caps with multimeter.
Swapping of leads at coils. Problem still existed only on right side. Surely if the coil was failing on the right side this would have been transferred to the left when the leads were swapped!!

Started engine and removed cap on right side and engine ran perfectly of left cyclinder. The reverse had the right cylinder splutter and stall.

I am not convinced the coil is dead although we will replace this item when it arrives.

Yesterday removed the right barrel and checked piston, rings, head and valves. Despite a heavy carbon build up due to the misfire all checked OK. Also replaced leads with new silicone ones. The misfire continues.

Today we will recheck the carburetor and a few other things. The carbs already have new diaphrams and o rings.

We have nothing else to do while we wait for the coil.

My "question" is after all this testing..." What is the problem?"

Obviously I have missed something and will persevere in the search.

K & C

Seems like the coil is okay!

Just a guess, the main-jet starts to get in operation around 3000 rpm, remove it (including the atomizer and all the other brass parts) and clean it. You should also check that the opening below the main air-intake to the atomizer is open.
What about float-height?

On the other hand:
Is it still different sparks on left and right side? Do you test both plugs at the same time?
If so it’s still probably an electrical matter.

Ok.

Now do this.
Pull the HT-lead cap off the plugs.
Used your multi-meter and measure the 2nd resistance across the coils by putting the test pins(?) in each cap.
The reading you should get is about 25kOhm (+/-5kOhm).

What you measure is cap resistance 5kOhm each = 10kOhm. Secondary resistance should be 7.5-9.15kOhm according to Clymer manual p.438, which is alsow stated by BMW Workshopmanual.
You will need to allow for added resistance in HT-leads and conncetions - hence about 25kOhm reading.

Since you have a dual-coil it is not as interesting to measure the primary coil as one of the two secondary coils are operating ok.

However, do not continue running your engine with a faulty coil! Get a new dual coil or two 6V coils:
Singe coil (6V):
Primary reistance: 0.67-0.77 ohm
Secondary reistance: 3.7-5.4 kOhm
Dual coils (12V):
Primary resistance: 1.15-1.32 ohm
Secondary resistance: 7.5-9.15 ohm
ANY coils that meat these specifics will do -
Kawasaki coils are great, neat, small, easy fit and usually very cheap 2nd hand, though extremly expensive if bought new. Their resistance does not match the BMW specifics, but works very well in spite of this.
Car-coils are bulky and hard to sqeeze in / onto the bike, otherwise they are excellent.

The caps should have 5kOhm built in resistance - the ICU (electronic ignition control unit) sort of demands it... I've used plugs with no resistance and all negative I had with it was being scoleded by my mechanic when I arrived - his radio did warn him that I was arriving - -
There are stories, non-verified, of ICUs breaking due to non-resisted caps (faulty resistance in HT-coil-plug circuit).

As for plugs:
Svenska BMW MC Klubben • Visa trÃ¥d - Super 4 vs BUR ET

__________________
Drive Safely,
Albert

Problem Solved.

After checking the coils and replacing electrical components several times and removing the right cylinder and head for a full inspection we switched back to the carburetors.

Today we found the right carburetor's needle was 2-3 mm shorter that the left. The clip system on this bike is less that perfect, inserting the needle into the clip and twisting into the selected notch with a small amount of pressure. It appears the needle was on the correct notch but the clip had pushed up into the piston. This needle /jet replacement was done at least 16,000 kms ago and no serious running problems were detected until we got into altitude. I note that later models have a circlip and all is held into position by a screw. A much better system. Carb overhauls I usually do one side at a time and in twenty one years have never had this problem.

Lesson learnt. Will do both carbs together and compare needle lengths before reassembling.

Many thanks to everyone for their help in this matter. The colour of the spark still has me baffled and we will be carrying a coil expecting the worst. We generally carry very few spares... two up on one bike we have no space!!

The journey continues...

Ken and Carol Duval

Soooo glad it's sorted guys !!

It could of been sooo many things but the important thing is , its fixed and now you are even wiser to your machine..

The different colour spark is telling you that you will have a problem in the future......

Your coil deffo needs an eye on it...

But for now... rock on ! your trip is AWESOME !!!!!!!!!!!!!!!!!!!!!!!

__________________
Did some trips.
Rode some bikes.
Fix them for a living.
Can't say anymore.