Join Date: Dec 2012
Location: York, PA, United States
Quick and dirty test for air leaks: Start the mc and remove the oil filler cap. You now have a self-created vacuum leak. The mc should get lumpier/rougher. If not, you probably already had one. You didnt mention how the mc is idling. If the problem is only while running: put the mc on the centerstand. I usually put a board under the centerstand to get a little more distance between the ground and the wheel. Put the mc in gear and run until the problem reappears. Then remove the cap and see if there is a difference. Please do not run the mc into your garage wall.
My K75 had similar issues in freezing temps and high altitudes toward the end of its life. Fine in warm weather. Mildly irritating in cold temps at lower elevations. Never figured it out. Best guess was vacuum leak due to worn rubber which only manifes:-)ted under extreme conditions. This was before I knew about the oil cap test and the crank case breather hose.
Re: the TPS. Bosch manual for LE-Jetronic is available for DLing as a pdf at many sites. The TPS has two contacts inside. One indicates throttle off and the other WFO. That is it. Minimum gas and maximum gas. Two switches. The controller gets no info from the TPS when the throttle grip is in intermediate positions. The air flow sensor handles that job. At WFO, fuel economy and emission control are not the controlling factors. The control unit is designed to deliver a fuel mixture for maximum power. One reason for the throttle closed switch is so the control unit can shut down the injectors at the proper time when you go from throttle on to throttle off. Without this, in situations such as from WFO to closed, the fuel injectors would not decrease the amount of fuel in sync with the decreased amount of air. You would get a rich mixture of some sort. This results in the popping exhaust sound. You also get this if you fail to completely close the throttle when rolling off the gas. I did this regularly when I first got my K75. It was duck and cover eveywhere I went.
The point of this is the manual says failure to properly set the TPS can result in the popping noise even if you close the throttle completely when rolling off. In this case, you have set the TPS so the switch which indicates throttle off is not activating. So, the control is not shutting down the injectors in the way designed to prevent overrun. You do get decreased fuel as the air flow sensor sends its signal to the controller. But the specially designed shut off at throttle off does not occur. BUT THIS IS AT ROLL OFF, NOT ROLL 0N. As far as I can tell, TPS does nothing on roll on until you hit WFO. Unless you have misset the TPS so badly the WFO switch activates very early, missetting the TPS should have no effect on moderate roll on. I do not know what the effect of early activation of the WFO switch would be.
The click you are hearing with the throttle grip forward must be the contact closing. That means the controller should be shutting down the injectors to prevent overrun. Presumably there is a click at WFO to indicate that switch is working, but I havent tried that. If I remember I will try that tomorrow to see if I can figure out when the WFO switch closes.
I am not an expert in LE-Jetronic. I have studied the manual to get a basic understanding of the system. A guru may correct me here.
Is the cap rubber\vinyl? I havent run a K75 for 5 years, but I have a vague recollection of two small black rubber\vinyl caps that cover open-ended tube spars that came out of the main frame under the tank somewhere. One on each side? Mostly decorative I thought. Maybe kept water and dirt out of there.
Good luck. I would triple check for a vacuum leak. That is an easier fix.
Last edited by 97tiger885; Mar 31st, 2013 at 12:56 am.