On my 85 K100RT, when at idle fully warmed up, on the center stand, and I blip the throttle, the engine rpm hesitates for a second then takes off.  It starts fairly well but takes two or three tries, with the choke on the number one position, to get it idling nice.  Once running and warm it idles very smooth, at about 950 rpm, and runs smooth at all rpms.  I checked for vacuum leaks at the rubber boots to the throttle body, all ok, and replaced all the rubber vacuum lines and caps on the throttle bodies.   The crank case breather Z tube looks new so I didn't replace it.  I removed, cleaned and checked the spray pattern on all the injectors.  I replaced the incoming and return fuel lines and the air cleaner.  Checked, re-gaped and cleaned the spark plugs.   I used a pressure gauge and checked the fuel pressure, and that is good.  I also checked the synchronization of the throttle bodies and all are matched very close.  Overall the engine runs great, just hesitates if I apply the throttle quickly.  I removed and cleaned the ground wires under the tank and at the rear of the bike.   It would be nice to get rid of the hesitation if possible.  I purchased this used last year so I have not ridden it except at the test ride.  It has about 30,000 miles.   Any ideas?   Thanks,  Jim

The MAF barn door might be sticking. Did you check that?

The inertia inherent in the mass of the MAF barn door in the Jetronic system naturally introduces a slight lag in the throttle response.  When the throttle is snapped open, it takes a small amount of time for the intake strokes to pull enough air to open the barn door which then provides the signal to add fuel.  The delay is something on the order of a couple milliseconds, short, but still noticeable.  In normal riding situations where the throttle is rolled on, the lag can't be noticed.

This lag isn't present in the Motronic systems where snapping the throttle open instantly activates the throttle position sensor that replaced the MAF.  Fuel is delivered instantly to the cylinders with the corresponding response with the Motronic.

Is this just pulling the throttle from idle or at all revs? Stumbling if the throttle is wanged open from idle seems to be normal.

To wang or not to wang—that is the question.

My stumble is just if you, wang as you say, the throttle off idle.  So, sounds normal.  The second part was the slight difficulty in starting.  With the choke to either position one or two, it takes two or three tries to start it.  Once started it runs nice when it is warm.   I thought I had read somewhere that there are two signals that tell the fuel injection control unit to richen the mixture, to help improve cold starting.  The first is the coolant temperature sensor and the second is the start button.  I don't see either of these situations discussed in my Haynes manual.   Is this correct?  How would I check these?  Thanks.

As I understand, the coolant temp sender will change resistance readings as it warms up.  Does anyone have the ohm readings of what it should be when cold?  Thanks

flyinglow wrote: Does anyone have the ohm readings of what it should be when cold?

You find a diagram in the troubleshooting guide at the portal.

flyinglow wrote: The second part was the slight difficulty in starting.  With the choke to either position one or two, it takes two or three tries to start it.    How would I check these?  Thanks.

From reading another of your threads, it seems like your bike has a discontinued fuel cap that can cause hard starting. Maybe this is part of the problem.
From the parts fiche at MAXBMW:

Thanks, I did find the coolant temperature sensor readings in the trouble shooting guide, when cold, 68 degrees F, ohm reading should be 2.5k.  When hot,175 degrees F, ohm reading should be 300 ohms.   Also, I had checked my fuel pressure reading earlier and they looked good, about 36 psi but I decided to replace the fuel filter and internal fuel lines just in case.  The lines were very checked and fuel filter mostly plugged.  So, this may be the real reason for poor idle throttle response.  Also, the fuel pump vibration gasket was turning to goo so I ordered one of these.  It's only money!

It was definitely needing replacement along with the cracked hoses.

flyinglow wrote:So, this may be the real reason for poor idle throttle response.

Let us know if your wanging improves!

Most forum readers are aware of the erratic manifold vacuum readings at idle.  Since this vacuum is connected to the Fuel Pressure Regulator to help control fuel pressure, the fuel pressure is also erratic at idle.  It does smooth out as rpm is increased.  Mine varies between 35 - 20  psi at idle.  When you disconnect the vacuum line going to the FPR and put a plug on the manifold port, the fuel pressure holds rock steady at 35 psi.  I've read on the forum that others have removed this vacuum line and left it off.  Most said they experienced better starting and smoother running, especially at idle.   Any thoughts on leaving this vacuum line off the FPR?  The attached pic shows the solid fuel pressure reading at idle .  

flyinglow wrote: When you disconnect the vacuum line going to the FPR and put a plug on the manifold port, the fuel pressure holds rock steady at 35 psi.  I've read on the forum that others have removed this vacuum line and left it off.  Most said they experienced better starting and smoother running, especially at idle.

Please provide links on the forum where you've read others have done this. I'm interested. Is it the disconnection of a vacuum switch that you've done?

There are many links that discuss the Fuel Pressure regulator.  This is the one that I thought was the most helpful and insightful, titled " things-that-bmw-put-there-to-annoy-me ".  See link  Fuel Pressure.   https://www.k100-forum.com/t934-things-that-bmw-put-there-to-annoy-me .