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Bike Profile - Posted 22nd December 2010

BMW K75RT Part 4
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Steve MacGregor suffers one of those mysterious misfires which means that he ends up fixing absolutely everything on his three-cylinder BMW 750...

With the fuel injection sorted on my BMW K75 there only remained two minor sources of irritation. One was very loud backfiring when slowing with a closed throttle. This was easily cured by leaning out the mixture. The only difficulty was finding the mixture adjustment screw - it's buried inside the airbox, accessible only once a rubber bung is removed.

The other issue was an intermittent low rev misfire. This took the form of one cylinder cutting out at low revs. At rest, this meant the idle speed see-sawing up and down, and the need for a flurry of revs to pull away smoothly. Not very dignified. When rolling, it meant that the bike would cut onto two cylinders until the revs climbed to almost precisely 2500, then the third cylinder would chime in.

Not the end of the world, but irritating. Niggling. It turned my normally stately progress into a series of jerky stumbles. Most of my twenty-five mile daily commute to work is on open, fast A- and B-roads, but the last couple of miles are through city traffic. It was during this last part of the journey that the misfire was most apparent.

Monarch of the Glen. Is this the city traffic part of the journey?

It didn't require the services of Hercule Poirot to deduce that the problem was probably ignition related. I narrowed it down to the rear cylinder with the spit test (spit on all three exhaust header pipes. If the spit on one pipe bubbles less vigorously than the rest, that pipe is cooler than the others, so that's the misfiring cylinder).

I had replaced the spark plugs when I serviced the bike so I assumed that these were OK. A quick check revealed that the rear cylinder had a slightly dodgy-looking replacement HT lead and plug cap while the others still had their original BMW parts. Oh ho! A quick phone call to Motorworks and a genuine BMW plug lead and cap were winging their way to me. It might have been prudent to order a full set, but three plug leads and caps would have cost more than one third of what I paid for the bike. Not cheap these BMW bits! But never mind, I fitted the new lead and fired it up.

Perfect. Nice steady idle, no misfiring. I took the bike out next day and by the time I arrived at work, the misfire was back, just as before. Bum!

That evening I pulled all the connections on the HT and LT side of the ignition circuits and cleaned them thoroughly. I put everything back together and fired up the bike. It idled smoothly. I left it idling for some time just to be sure, but there was no sign of the misfire. I took it to work next day. By the time I arrived, the misfire was back. Ho hum.

When I got home, I checked everything again. It all looked good. I started the bike. The idle speed see-sawed up and down. I turned it off. Could I have fitted a faulty spark plug when I serviced the bike? I had a new Bosch plug of the right size and grade in the shed, so I fitted it in the rear cylinder. I started the bike. It idled smoothly. No longer willing to be so easily fooled, I took it out for a run. It ran without misfiring and idled perfectly. Hurrah.

K75 bits on Right Now......

I took it to work next day. By the time I arrived, (did you guess?) the misfire was back. I was starting to get seriously disgruntled.

As I rode home with the bike coughing and misfiring every time the revs dropped, I began to wonder if my initial diagnosis of an ignition fault was correct. That evening I re-checked the fuel injection synchronisation. I set it up very carefully so that there wasn't a whisker of difference between the three cylinders.

I also checked the throttle position sensor (TPS). The TPS cuts off the flow of fuel to the engine when the throttle is closed. It has to be adjusted to ensure that it's operating correctly. I checked it, and made a slight adjustment. I started the bike. It idled smoothly. I took it out for a long run. No misfire.

I took it to work next day. By the time I arrived, (this is getting predictable, isn't it?) the misfire was back. My state of mind was now heading for something approximating homicidal rage. The misfire (or rather, my inability to fix it) was ruining my enjoyment of riding the bike.

Bisch, Basch, Bosch BMW K75 Coils

I got home and checked everything again. The only thing I could see was that one of the connections to the coil for the rear cylinder was slightly corroded. I couldn't think of anything else to do, so I went online and had a look at the price of a new ignition coil from Motorworks. Then I had a bit of a lie down. When I felt stronger, I checked out eBay for a secondhand coil. I found one that came from a "guaranteed running bike". I bought it for not very much money at all, and two days later it arrived. I opened the package to find that the ceramic section where the HT lead plugs into the coil had broken off. The coil was useless. With gritted teeth I contacted the seller, prepared for an acrimonious exchange. To my delight he apologised profusely and immediately posted another coil out to me at no additional cost. What a nice man.

The new coil arrived and looked fine. I fitted it and started the bike. It idled perfectly. I took it for a very long run. No misfire. I took it to work next day. By the time I arrived, it was still idling perfectly and running without the misfire (hah, didn't expect that, did you!).

I have now done over 5000 miles since fitting the replacement coil, and the bike is still running perfectly. So, the issue was obviously the coil. But I can't understand why everything else I tried seemed initially to cure the problem, only for it to recur within fifty miles or so. I also can't understand why a faulty coil should cause such an intermittent problem. In my experience, coils either work or they don't with nothing in between. I suppose I'll just have to add it to my list of reasons to loathe and distrust all things electrical…


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