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1Back to top Go down   Erratic idling 1987 K75C Empty Erratic idling 1987 K75C Sun Sep 04, 2022 12:18 am

Rockman

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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.

    

2Back to top Go down   Erratic idling 1987 K75C Empty Re: Erratic idling 1987 K75C Sun Sep 04, 2022 7:17 pm

moriarti

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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 Very Happy


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1984 k100 rs red/black VIN  0004449
    

3Back to top Go down   Erratic idling 1987 K75C Empty Re: Erratic idling 1987 K75C Mon Sep 05, 2022 1:46 am

Rockman

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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 Very Happy
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.

    

4Back to top Go down   Erratic idling 1987 K75C Empty Re: Erratic idling 1987 K75C Mon Sep 05, 2022 4:20 am

moriarti

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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. Smile


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1984 k100 rs red/black VIN  0004449
    

5Back to top Go down   Erratic idling 1987 K75C Empty Re: Erratic idling 1987 K75C Mon Sep 05, 2022 4:37 am

Laitch

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Your Brick has a recent history of trouble starting. That condition seemed to have resolved itself but its cause might be presenting itself differently now. Before embarking on a long distance trip like the one you’ve lined up, I'd verify the valve clearances, the air balance of throttle bodies, the condition of the spark plugs, the condition of the air filter, the condition and tightness of battery connections and ground connections, the condition of the fuel filter, and the condition of the fuel tank hoses and hose clamps.


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1995 K75 81,000 miles
Erratic idling 1987 K75C Usa-lo10
    

6Back to top Go down   Erratic idling 1987 K75C Empty Re: Erratic idling 1987 K75C Mon Sep 05, 2022 10:47 am

92KK 84WW Olaf

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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.


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1992 K100LT 0193214 Bertha Blue 101,000 miles
1984 K100RT 0022575 Brutus Baja Red 578 bought 36,000 now 89,150 miles
1997 K1100LT 0188024 Wotan Mystic Red 689 58,645 now 106,950 miles Deceased.
1983 K100RS 0011171 Fricka 606 Alaska Blue 29,495 miles Damn K Pox Its a Bat outta Hell Now 49,840 miles. 
1996 K1100LT 0233004 Lohengrin Mystic Red 38,000 miles currently 42,640 miles.

Past:
1968 Yamaha 80 YG1
1971 Yamaha 125 YAS-1
1968 Honda 125 SS
1970 Honda CD 175
1973 Honda CB500-4
Honda CX 500
    

7Back to top Go down   Erratic idling 1987 K75C Empty Re: Erratic idling 1987 K75C Mon Sep 05, 2022 6:59 pm

Rick G

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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 affraid


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"Man sacrifices his health in order to make money.
Then he sacrifices money to recuperate his health.
And then he is so anxious about the future that he does not enjoy the present; the result being that he does not live in the present or the future; he lives as if he is never going to die, and then dies having never really lived."   Dalai Lama


Bikes 2004 K1200GT 1998 K1100 LT, 1986 K75 GS.
    

8Back to top Go down   Erratic idling 1987 K75C Empty Re: Erratic idling 1987 K75C Mon Sep 05, 2022 9:03 pm

Rockman

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Erratic idling 1987 K75C 20220910
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.

    

9Back to top Go down   Erratic idling 1987 K75C Empty Re: Erratic idling 1987 K75C Tue Sep 06, 2022 2:24 am

Rockman

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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 affraid
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.

    

10Back to top Go down   Erratic idling 1987 K75C Empty Re: Erratic idling 1987 K75C Tue Sep 06, 2022 10:54 am

Point-Seven-five

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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.


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Present:
1994 K75RT
1991 K100RS


Past:
1988 K100RS SE
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

11Back to top Go down   Erratic idling 1987 K75C Empty Re: Erratic idling 1987 K75C Tue Sep 06, 2022 7:30 pm

Rockman

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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.

    

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