85 K100 ...all of a sudden backfiring and stalling when HOT! Still not fixed!

[daveson]

Some thoughts about why the vacuum switch was deleted, just guessing.

If we think about the carby days, we had a distributor with mechanical advance, vacuum advance, and points. These parts would wear out and become increasingly inaccurate over time and use, and often fail. The ideal gradual increase in advance required for increased RPM is not identical to the gradual change with the springs in the mechanical advance, but it was considered good enough at the time. So they weren't even accurate when new. Computers can use sensors with fewer parts that wear out, and can be programmed to provide the ideal amount of advance. A vacuum sensor was used for vacuum advance. Then it was realised that the computer can be programmed to provide the correct amount of vacuum advance without a vacuum sensor, due to the inputs it gets from the hall sensors.

BMW often made parts backwards compatible, in this example there would be no need to change the part number, anyone wanting to buy an ignition control unit would be supplied with the new version.

In later models with a motronic computer, it was realised that the fuel pressure regulator doesn't need vacuum assistance to reduce fuel consumption in high vacuum situations, again the computer already has enough inputs to predict the ideal amount of fuel required, and provides it according to its program.

[Atabeach]

So my venture into used coils:
 the orange cap coils from flea bay;  one good, the other O/L ….all plugs still wet with gasoline.
Placing spark against engine block, swapped good coil with bad  , BOTH SHOWED GOOD SPARK, at all rpm’s.
New set is about $400 ( need an opinion here )
Another flea bay set bench tested O/L WTF? These came from running bikes?

All battery wires and grounds gone through ( AGAIN ) good 13.6 volts at 3000k, good 12.7 battery ( ignition off)

Lastly,
Air flow meter female pins…ohms tested well, door moves well. Changed with used one I had…same.

What am I missing? Bike starts and runs rich!

[Edward]

did you balance the tb and reset the tps?

[Atabeach]

So this happened suddenly, as if something went wrong. I have NOT balanced the throttle bodies since this event. The Throttle position sensor clicks. I also removed the clip and tested the contacts...seems fine but i will check again. Thanks for the input.

[Laitch]

So my venture into used coils:
 the orange cap coils from flea bay;  one good, the other O/L ….all plugs still wet with gasoline.
Placing spark against engine block, swapped good coil with bad  , BOTH SHOWED GOOD SPARK, at all rpm’s.

One possibility is that one coil was working well and the other coil was working, but not well. A crack could show an open circuit when tested with a multimeter, but the high current could jump the gap while running, which would be about zilch millimetres.

As demand and running time on a coil increases, resistance in its wires increases. If a wire or wires in the coil have a thin fractures the engine will run until the resistance in the defective wires increases causing an open circuit.

[Laitch]

When testing, is the coil with the O/L always used for the same cylinders, or is its position swapped?

My suggestion is to contact the seller, show the meter reading and ask for a partial refund. If the seller doesn't offer refunds, eBay has a refund policy.

[Atabeach]

So I have swapped the coils and wires several times.  Note: on the good coil … no matter the location , the spark plugs …. Are wet. I ran the bike till the fan clicked on. All 4 plugs wet. Bike starts right up and runs… constant smoke.
Let’s assume the coils are good. Air meter bench tested , door moves smoothly….changed!  fuel regulator changed. One thing I have not done was go through the fuel system.
The pump, filter, all lines and injectors were purchased and installed NEW 3k ago.
… open for suggestions. I’m missing something.

[Laitch]

Plugs can be wet when there is too much fuel, when the fuel is the right amount but there is not enough spark, when the fuel injectors seize open, when the fuel injection control unit is defective, when the ignition control unit is defective, when the hall sensors are defective, ignition timing is incorrectly set, when the airflow meter or its temperature sensor is defective, when the coils or their wiring  have defects, when the coolant temperature sensor is defective, when the spark plugs are the incorrect type, or when the spark plug wire connection is the wrong type for the coil wire.

The current assertion seems to be that almost every critical part in the ignition system has either been replaced or passed testing. The assertion includes the observation that the plugs are giving a "healthy" spark, every part in the fuel tank is only ≈ 3000 miles old and the fuel pressure regulator has been replaced.

What is the fuel pressure in the line from the tank directly to a fuel pressure gauge? What is the fuel pressure if a T is fitted to the line from the tank to fuel rail and the pressure gauge is attached to the center leg of the T? What is the model number printed on the coolant temperature sensor? Where was the coolant temperature sensor purchased. Has continuity of both coolant sensor wires been verified?

I couldn't work with a multimeter that repeatedly gives unexpected O/L readings—e.g., O/L on a coil connection that is firing a "healthy" spark—until I understood why it was registering that measurement.

The upside of this situation is that you apparently have a high functioning R1100 to ride.   It might be time to cruise down to Blowing Rocks and take a break. You've earned it.

[Atabeach]

Thanks Laitch, the only thing I haven’t done ( at least that I can think of ) is fuel pressure. I will test and update. The ignition module may be faulty but I want to be sure it is not an easy fix here. Thanks again.

[daveson]

So I have swapped the coils and wires several times.

Everytime you swap something, roughly, that will be another item of unknown quality.

Have you replaced spark plug leads with a K75?

With a K100, the high tension circuit jumps two spark plug gaps, one wasted. Original K75 leads have an air gap. The high tension circuit in a K75 only jumps one spark plug.

A K100 with K75 leads would mean that the high tension circuit would need to jump three air gaps.

I'm not home for a month so I can't post the part numbers for K75 spark plug leads. It might help if you post the numbers. Alternatively, spark plug leads with an air gap will measure O/L.

It's a bit of a long shot, but you have a K75, so I thought it might be worth mentioning.

[Atabeach]

I have not swapped the K 75 wires. For the record… when a “ swapped part” results in no change, I put the original part back. Unless it is new. I purchased used coil and wires from k 100 series bikes. Although the coils showed O/L ( 3 quality multi meters) repeatedly swapping proved fruitless. I’m goin to go through every wire and fuel line and electric clip. I’m missing something Thanks

[daveson]

Goodo, good procedure. Have you tested the resistance of the spark plug leads?

[Atabeach]

Yes 5 ohms on the shorter ones 8 on the longer. Thanks.

[daveson]

Ok, two coils are installed, if either of them test O/L, that's a test fail.

[Atabeach]

Understood. But reasoning would be those two coils would be dry. Not wet with fresh gasoline. Please confirm that reasoning

[daveson]

Your reasoning is good. I don't know your problem yet. I'm thinking a weak spark might not be good enough for combustion, that would lead to flooding. I should look back to see where you're at.

I'm thinking you have one good coil, and one test fail. Then only two cylinders should be wet. Then swap them to see if the problem follows the coil.

If the spark is erratic at all cylinders when one coil is good, to me that means the problem would be upstream of the coils.

Look at the videos I posted, they are very quick and easy tests to do, and will help to pinpoint the problem area.

I don't remember how to post the video, instead of the link, I might try again later.