Any suggestions on this:
My 1987 K75C runs very well, but the idle is erratic. It will idle well at about 1100rpm for a while, then it'll suddenly drop to about 500 rpm or less. Mostly I can catch it and give it a few revs. Sometimes then it will return to 1100 rpm, sometimes not.

Occasionally it goes the other way and will idle at about 1500 rpm for a while then drop back to 1100. All of this mostly happens when sitting waiting for traffic to move.

I have a 4000km trip planned starting the end of September so I'd like to sort this out before I go just in case its something that might go awry while I'm away.

Hi ,First of all is the plastic oil filler   cap seated correctly  .second inspect the Z pipe crankcase to airbox.Im thinking air/vacuum, problem

moriarti wrote:Hi ,First of all is the plastic oil filler   cap seated correctly  .second inspect the Z pipe crankcase to airbox.Im thinking air/vacuum, problem

Checked both. Filler cap is good. Z pipe is slightly perished at the outer end. I rubbed a little silastic into the crack but I dont think it was the problem. I'll take it for a run tomorrow to find out - the problem seems to occur after a bit of a run.

Just to be clear, when engine is cold it runs ok, problem occurs when at running temp, if this is  correct then the ICU is SUSSPECT.

Check that the throttle mechanism is totally unobstructed and moving freely and dropping to idle freely.

Also check the throttle position sensor is correctly set and functioning.

Balance throttle bodies.

I had that type of issue on my RT, the above three items resolved it.

Phil the engine seems to exhibit characteristics of running a touch lean try turning the air idle screws in a 1/4 turn see if that improves its manners. If not go out to the wood heap and show it the axe

I noticed that the No.1 and No.3 rubber connectors from the air box to the throttle bodies are a bit oily, No.2 is clean. I'm thinking I have a small air leak on these which gets worse when the motor is warm. I'll try to tighten these up and see what happens.

50/50 on whether I take the K75 on a long run or not at the moment. We'll see.

Rick G wrote:Phil the engine seems to exhibit characteristics of running a touch lean try turning the air idle screws in a 1/4 turn see if that improves its manners. If not go out to the wood heap and show it the axe

No need for the axe just yet.

I tightened up the clamps on the throttle body rubbers a bit, then took it for a run. No difference. When I got back I turned the throttle screws in ~ a 1/4 turn as Rick suggested and it idled perfectly at 1000 rpm for a good 5 minutes before I turned it off. The next thing is to balance the throttle bodies, check the air cleaner, valve clearances (not in that order), and see if it is still ok. One thing I should have mentioned is that it has the 4 hole injectors from Tills. This might have affected the system - dunno.

Edit: I might have this whole thing sussed and the starting problem and the idling problem are related .. When I got the bike it had a modified muffler - someone had cut the end out of a standard muffler and neatly welded in a bigger outlet pipe. The bike ran well and idled properly, but tended to run rich at idle and flood and sometimes not start. For reasons I cant remember I changed back to a standard muffler and immediately noticed that the idling went up to 1500rpm from the previous 1000rpm, so I adjusted it back. Then the idling problems started. Richening up the fuel mixture seems to have fixed that.

So, what I think is that the modified muffler was in fact more restrictive than the standard one and this caused the starting problems. When I changed to the standard muffler it was now running a little lean, causing the idling problems. What I need to do now is balance the throttle bodies and adjust the mixture. The balancing is no worries, but I'm not entirely sure how to properly adjust the mixture.

There is no starting point setting for the air mixture balance screws on the throttle bodies that I know of, so I set mine all to about 1 1/4 or 1 1/2 turns out from lightly bottomed before I connect my balancer. 

I then balance the throttle bodies by matching them to #3 or #4(the last throttle body).  I do this because my balancer matches two throttle bodies to each other, so I use the throttle body connected to the pressure regulator as my standard.  I adjust the other bodies to match it.

Overall idle mixture is controlled by a screw that controls the bypass in the Jetronic airflow meter.  This is an adjustment I avoid messing with because it is such a PIA to get to, and there is little reason to fool with the factory setting.

About exhaust systems.  The OEM has a large engineering department with a lot of expensive equipment and experienced exhaust engineering specialists to design exhaust systems that not only meet noise regulations, but do it while maximizing torque and power output.  To think that some guy out in the driveway with a hacksaw can improve exhaust performance is ridiculous. 

The stock exhaust on your bike has a perforated baffle tube that gases flow through.  Shortening it will increase back pressure and change the mixture and intake flow characteristics.  If you consider that the OEM exhaust is matched to ignition timing, fuel mapping, cam timing, compression, etc. it easy to see that if you change it, there are a number of other variables that also need to be adjusted. 

The only improvement you will get from an exhaust change is cosmetic and/or acoustic.  These are entirely subjective, while the performance which is a measurable objective result is almost guaranteed to suffer.

Thanks, I'll try 1 1/2 turns out as the whole system is now back to stock.

As for your comments on the modified exhaust, I agree and that is probably the reason I changed back to a stock muffler. The modified on did sound quite good though, even if it was rubbish.

UPDATE: I started at 1 1/4 turns out and then balanced the throttle bodies. A little further out on cylinders 1 and 2 (<1/8 turn) to match 3. All seems good now.