K Bike Performance Improvements
By Rob Lentini
Here are the major things I’ve done to my K75S over the years…
Engine-
- Ignition timing has been advanced, resulting in better throttle response and more low/midrange power.
- Fuel mixture has been very accurately set using an emissions analyzer. In addition to setting the air flow meter idle mixture adjustment to 2% CO, I have also recalibrated the air flap potentiometer to ensure the mixture is about 1% CO at cruise RPMs. Yours may be fine already. Assuming you don’t have an analyzer, bring it to you BMW dealer and hook it up to theirs. Idle should be 2%, and full fast idle (start lever all the way CCW, about 4-5 K RPM) should drop to about 1%. Strangely, some Ks seem to be VERY lean which results in higher operating temperatures and less power. By the way, do the timing advance adjustment prior to setting fuel mixture. Note: adjusting the potentiometer is precision work requiring patience. If your bike is under warranty, you may void it. If still interested in “how to do”, let me know.
- Valve adjustment is not critical, but can affect overall performance. You should shoot for intake clearances that are on the tight side of the spec, and exhaust on the loose side. Any time you adjust valves you will need to resynch the throttle butterfly bypass screws. I use a carb stix, an inexpensive mercury manometer.
- Use your favorite brand of oil at the proper viscosity and change it and the filter every 2500-3000 miles. DO NOT use ANY oil additives! I know, BMW filters are expensive, and the dealers will advise otherwise, but I use a Fram PH3614 filter available for $3 at auto discount stores. You choice. Keep the oil level between the sight window center dot and the upper mark.
- Install a K&N cotton gauze air filter to achieve more air flow and a noticeable power improvement.
- Replace the stock muffler with a Staintune slip-on muffler. It will improve your throttle response, looks great, and is lighter than stock. Quality is first-rate, and the sound has a slight attitude, though barely louder than stock. Staintune is available from California BMW-Triumph. I just saw a used one advertised on the net today.
- That about it for the engine–nothing radical at all, just basic “getting things optimum”. By the way, don’t expect any K to put out its full power potential until after 20,000 miles. They really are tightly clearanced, and the Nikasil cylinders won’t smooth out until this much running.
Suspension and brakes:
- The K75S has a nice front end, but it can be improved. I think the stock fork springs are too stiff. I’ve replaced them with Progressive Suspension #1126 springs which are much more compliant to bumps. I use 5 wt fork oil, 280 ccs per leg, with 3-5% Dow Corning Gear Guard M added to the oil. This REALLY improves compliance and response to small bumps. Gear Guard M is a concentrated molybdenum additive, available from bearing supply stores in one quart bottles for about $20. Sounds expensive, but one quart will last you a LONG time. This stuff is highly recommended by Orlando “Oak” Okleshen, a leading BMW tech guru, for use in transmissions and final drives. I used to use Kal Gard Smooth Stroke fork oil with moly. Now I use Bel-Ray and mix in the additive myself, saving money.
- Replace the grabby front brake pads with SBS. They do less damage to the rotors, are very controllable, have good power, stop well in the wet, and the matching rear pads have lower friction for better controllability. If you have ABS the rear pad friction compound matters less, if at all.
- Rear suspension can be improved, likewise. The K75S shock works OK, but is nothing to write home about. There are two good options here. If you ride fast, and weigh 180 or more, Ohlins, Fox, White Power, and Progressive all make good aftermarket shocks. I currently have a Progressive “Adaptive” shock on my bike. It’s a little stiffer than I’d like, but is smoothing out with the miles. Prior to this, I ran a BMW Nivomat shock for a long time, and it worked GREAT. I only changed over because I needed to be able to adjust ride height for two different wheel sets I use. The Nivomat is self adjusting for ride height and damping. I think you really might like one. They may be found used in BMWON flea market. Even new, they are cheaper than many of the premium aftermarket units.
Tires-
- Certainly a matter of preference, but I want high mileage AND handling (who doesn’t?). I recommend Dunlop D401s, Elite IIs, or Metzeler ME88s front and rear. Dunlops are a better value, and even work well at the track. I ran a set of D401s (they’re getting hard to get, Harley sizes only now being produced) at CLASS at Willow Springs. They never lost traction and performed GREAT! I’ve run many different tire combinations and will mail you my analysis with the ignition timing procedure, if you want.
Transmission and Final Drive
- This is very important!!! Routinely clean and lubricate clutch and driveshaft splines, at least every year or 20K miles. I’ve had two spline failures at the final drive end. A good rule of thumb is to grease these EVERY tire change. I know, it’s a pain, but it’s not hard to remove four bolts, pull the drive back, and do your thing. It’s SOOOO expensive to face the alternative!
- Use 75-90 gear oil, again laced with 3-5% Gear Guard moly. You will notice measurable shifting improvement.
Summary:
Louis, you can see I’ve done nothing radical to this machine other than relentless preventive maintenance, peaking of specs, and, I might add, keeping the motorcycle spotless. Routine cleaning uncovers problems as they develop, keeping you ahead of the game. OK, I am experimenting with a wide KRS 18″ rear wheel with radials. Requires machining and some workarounds though, but the bike looks neat! So far, so good.
Good luck with your bike, and give me your address if you want the mailings. Happy wrenching! Work carefully and methodically and QC your work!
Later!!!
Rob Lentini
’87 K75S
Tucson, AZ