TPS and vacuum switch question

[Puccini]

I have a mid year 1985 K100RT.
I read that late in 1985 BMW removed the Vacuum switch and the TPS.

Can I do the same to mine?

Anyone know why BMW removed them?

[Laitch]

I read that late in 1985 BMW removed the Vacuum switch and the TPS.

Can you post a link to that text, Puccini?

[Puccini]

I read it on line at another forum but I understand that it is factual.
I don't have the actual link.

I just want to see if anyone else knows of the issue and resolution.

[Laitch]

I read it on line at another forum but I understand that it is factual.
I don't have the actual link.

Here are two images concerning the topic you've brought to discuss, Puccini. They're from a widely published source. Maybe it was these you have read. Please note that neither of them states that BMW removed the throttle position switch. Also, neither of them has information about what—if anything—was needed to compensate for the vacuum switch removal. Memory isn't always reliable, that's why I asked you for the source.

You can always remove that vacuum switch yourself, plug the lines and tell us how it goes, but maybe someone will come along who has done the switchectomy and will let you know.

[Puccini]

Yes, on reading again it does not say that the Idle switch was removed just the vacuum switch.

I guess the easiest thing to do is to just remove the hose that goes to the vacuum switch and plug up the ends of the tube and the port. Then essentially the switch is out of the circuit.

I'll give that a go.

[K1300S]

Why would you remove it?

[Laitch]

Why would you remove it?

For adventure, Marshall. I'm now inspired to start removing engine components from my bike to alleviate ennui.

[Puccini]

I just have a feeling that BMW wouldn't remove the vacuum switch unless there was some advantage to doing so.

THE EFI and fuel injection system is already complicated and I wondered if they thought this was a way to simplify it.

[Laitch]

I just have a feeling that BMW wouldn't remove the vacuum switch unless there was some advantage to doing so.
THE EFI and fuel injection system is already complicated and I wondered if they thought this was a way to simplify it.

That could be.

I read your past posts about your engine stumbling at take-off, especially after long cruising stretches. Did you ever follow the advice about looking for air leaks, checking valve clearances and performing a throttle body balance, Gerry? If you haven't yet, now may be the time. It would be helpful if you put the outcome in that other thread for owners who may have a similar condition with their bikes.

It's also helpful to readers if you put the year and model of your bike in your profile so it will show up in each of your posts. Adding your general location is useful because willing helpers may live nearby, as far-fetched as that may seem.

[Puccini]

I read your past posts about your engine stumbling at take-off, especially after long cruising stretches. Did you ever follow the advice about looking for air leaks, checking valve clearances and performing a throttle body balance, Gerry? If you haven't yet, now may be the time. It would be helpful if you put the outcome in that other thread for owners who may have a similar condition with their bikes.

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Well, I've had mixed results. I did find a cracked and dried vacuum line from the manifold to the Vacuum switch and that the idle was too low.
I understand that low idle is a problem and that the K100 should be at 1200 rpm. I think I'm at 1000 now.

All the other visible lines and ducts are in good shape.

The bike ran very well for about a week and then , at night, coming home, it faltered once for about 3 seconds and then recovered.
I am feeling confident that it is the fuel system as there was no indication of lights dimming or any dash lights coming on.

The idle , after warm up, is very smooth and stable but I still want to check TB balance.

I'll report what I can find.

[Laitch]

You haven't checked valve clearances then balanced the throttle bodies?

[Puccini]

my understanding from many others who have K100's is that valve clearances are not an issue till about 50K miles.

I have at lease two others that have checked their valves at 35K and found no variations.

Mt 85 K100 idles fine and has lots of smooth running power. I find it hard to believe that's an occational  3 second loss of power is valve
adjustment related. If it was it would show up in many other ways.

[kennybobby]

According to notes on a schematic from Bert in 2009, the vacuum switch was used to adjust ignition timing at mid-throttle, but was removed by late 85.  It connects to pin 15 of the ignition control unit and opens/closes the circuit to ground. 

i can't imagine that signal would matter much in normal riding, except a cracked hose would be an unmeasured air leak and it would cause the idle to be low due to a leaner mixture.  The same would be true for the vacuum line to the fuel pressure regulator that is not visible without removing the air filter housing.

[Puccini]

I've been considering the fuel pressure regulator also.
I haven't looked at it yet. This bike has had a few vacuum lines replaced due to age and I wonder if the vacuum line to the fuel pressure regulator may also
be old and cracked.
Is it a rubber hose to the fuel pressure regulator?

I wonder if fuel pressure isn't sufficient enough to allow the bike to accelerate off of idle when starting to move and then after the hesitation, the pressure is high enough to feed the injectors properly.

I don't know what would happen if the vacuum to the regulator drops because of a leak.

[kennybobby]

You said you changed all the visible vacuum lines--so i figured you didn't change the line to the fuel pressure regulator (FPR).  i'd bet that your line is cracked in the sharp bend where it exits under the regulator.  The line originates at the rearmost throttle body port and is covered with a coil spring wire, all the other throttles should have a rubber plug covering the port.

FPR operation:
The FPR maintains a constant difference between the fuel rail pressure and the intake manifold pressure just below the throttle butterfly valves.  This constant is the regulator set point which is 2.5 bar or 36 psig.  Internally the FPR has a spring-loaded safety valve to provide fail-safe operation in the event that the feed line is cracked and leaking, and the safety release pressure is 4.7 bar or 68 psig.  In addition the fuel pump has an internal relief valve at around 70 psig. 

This design ensures that fuel is always flowing in the rail at a pressure about 36 psi greater than the pressure in the vacuum tube line whether it is intact or leaking.

[Puccini]

OH, I didn't know that was the line to the fuel pressure regulator.

Yes, I did replace that line with new hose. The coil spring around that hose was missing when I got the bike.
I'll have to go back in and check it again.

One note, I've never had the bike backfire on deceleration so I am reticent to think the initial acceleration problem is timing related.
As I said before , it starts, idles smoothly, runs with strong power but just occationally, on initial start from a stop acts like it shut down for a few seconds. I did have a low speed episode once when it felt like it missed one or two cylinders for aecond.

Puzzling.

[kennybobby]

Another possibility is water or sludgy ethanol in the bottom of your tank.  Catch a slug of that and the bike will stumble for a few seconds.

It's fairly easy to drain the tank if you lift it up and back slightly to expose the front left fitting (return line), loosen the clamp and pry the hose off the tube and push it down and out the side into a catch jug of sufficient size.  Then remove fuse 4 and 6, and use a wire jumper from the right side of fuse 4 to the left side of fuse 6 to run the fuel pump and empty the tank.

Examine the contents at the bottom of the catch jug.  If it contains water with dirt, black floaters and jelly then that may be your culprit.  Remove the gas gap cover to examine inside the tank.  Clean, rinse and repeat as necessary.  Don't re-use all the old fuel--you don't want the junk at the bottom to get back in there.

[woodgeek]

I have an 85 with the vacuum switch. If I remove the switch, do I want to close the connections or leave it an open circuit? Of course I'll cap the vacuum connection. This is what Haynes has to say. Just wondering what others have done.