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Bike Profile - Posted 19th November 2010

BMW K75RT Part 2
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Adjusting fuel injectors need not be a traumatic task. Well, not if you read the manual first. Steve MacGregor tried to cure his BMW's rough running...

My background is in engineering, and a favourite saying in that industry is; RTFM (the polite version is Read The Flippin' Manual). Often this is quoted just after someone has broken some delicate and expensive item because they haven't researched how to use it first.

And of course it's good advice when working on bikes too. Bikes are complex things and it would be stupid to work on them without knowing what you're doing. And only a complete numpty would try something like adjusting a fuel injection system without first reading the manual. Can you see where this is inexorably heading yet, dear reader?

Before fiddling. Notice mostly smoothish running. BMW K75RT

Having cured the wayward handling of my BMW K75RT, the next step was to synchronise the fuel injectors. The bike was down on power, wouldn't run much over 80mph and at high revs made the characteristic droning associated with unsynchronised carburettors or fuel injectors. Fortunately access to the fuel injection system is good on the K bikes, as the engine is laid on its side. Removing the left lower fairing panel gives unrestricted access to the injector bodies and linkages.

On the K75 the system consists of three separate throttle bodies linked by a simple, adjustable mechanical linkage. Each throttle body also has what looks like a mixture adjustment screw and a connection point for a vacuum gauge. I mean, how difficult could it be? Evidently all you have to do is connect the vacuum gauges and then adjust the linkages until the airflow is even. Simple, eh? Or not.

It was the work of a moment to connect up the vacuum gauges (ever noticed how smoothly and quickly things go when you're making a terrible mistake?). It was also very simple to adjust the throttle body linkages to get an even air flow registering on the gauges. I revved the engine. Oh dear. It sounded like a violently shaken a sack of spanners. It was also reluctant to rev and spat and popped on the over-run. I checked again - the gauges showed that the flow across the three bodies was even, but the bike was running much worse than before. Time to RTFM then.

A Really Bad Idea... BMW K75RT

I had downloaded a .pdf version of the official BMW manual soon after I bought the bike. I fired up the computer and opened the file, going quickly to the fuel injection section. There, right at the start was a diagram with large arrows pointing to the linkage adjustment screws (the ones I had just been fiddling with). In big, scary block capitals it said;


Hmm. This didn't sound good. I did some further research on the Internet and discovered that the linkages control the position of the internal throttle body butterflys and are set at the factory when the bike is assembled. If the adjustment is disturbed, there is no way of getting back to factory settings. One forum poster said ominously 'Do not on any account adjust the linkage screws. If you do, the only way to get back to smooth running is to replace the complete throttle body assembly.'

Stopping only to reassure my wife, who had become alarmed by the sound of gnashing teeth and rending garments coming from the vicinity of the computer, I read on with a sinking heart. What I should have done was to set the balance using the air bypass screws (the things I had mistaken for mixture screws) whilst leaving the mechanical linkages well alone. But now that I had disturbed the linkages there was no way of getting back to factory settings. The manual didn't have any suggestions as to what to do if you had inadvertently adjusted the wrong thing.

After rejecting the obvious solutions (sell it immediately on eBay, a mysterious shed fire, etc.) the next step was to turn to the Internet for assistance. I don't know about you, but one of the things I find most comforting about the Net is that I can usually find someone else who is as stupid as me. Sure enough, after some trawling on forums I came across the website for the BMW Owners Club of America. A K75 owner in California had done exactly the same thing that I had, and had posted a message asking for help. Some of the replies were frankly unhelpful ('You shouldn't have adjusted the linkages. Buy a new set of throttle bodies'), but I finally came across one message which seemed sensible.

By closing all the air bypass screws completely and making some other adjustments to make the bike idle, I could ensure that any differences in flow were caused solely by misalignment of the throttle body butterflys. I could then balance the flow using the vacuum gauges, which should roughly align the butterflys. This wouldn't be as precise as the settings applied at the factory, but should get close enough to allow fine tuning using the air bypass screws.

After fiddling. Can you hear the difference?... BMW K75RT
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To my relief, this worked. It took a lot of fiddling but I finally got the bike to run smoothly. Acceleration is now much crisper, power is noticeably up and the droning noise at high revs has disappeared. I don't know if it's as good as it would have been if the system still used factory settings, but it feels pretty sharp. I also suspect that I wasn't the first person to try adjusting the throttle linkages on this particular bike. However I could have saved a great deal of heartache and faffing about if I had just read the manual first. From now on I'll RTFM before starting any new job. Almost certainly. Probably.

Next, time to get rid of that topbox and those panniers…


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