1988 K75 - Tech Help Requested

Hi All,

Apologies if this is somewhere in the myriad of posts, but here goes...

Having some issues with the 1988 K75, ~53k miles...

Initially the bike would run pretty good for the short rides I would do (less than an hour). Then went out on a longer ride and after a couple hours it started to sputter/hesitate/miss low rpm up to about 3500 rpm then it would run ok above 3500. Thought maybe bad gas, but...
This weekend I went on a ride to Oshkosh and same thing, after about an hour of riding. Except this time it got worse. Sputter/hesitate/miss through all RPM. It very gradually would increase speed and once up to speed it would hold it, but still sputter/hesitate/miss, no good acceleration, occasional bursts of power for a second where it seems like it wants to go, then more sputter...
Today it started the same thing after about only 5 mins of riding.

Any ideas or questions to help troubleshoot?

Thanks!

K75 Newbie

Welcome to the forum!
I’m far from an expert on this bike. The next time it does this, somewhere quite, open the gas cap and see if you can hear any type of vacuum “whoosh”. Your description sounds like it’s starving for fuel.
Good luck.
Gary

Have you ever replaced the fuel filter? Sounds like fuel starvation.

I had a genuine BMW filter plug solid with varnish. Not a spec of dirt in it, but you could not even blow air through it.

BTW, Autozone has a filter that is the exact same size and shape (all metal) for a fraction the cost of the BMW part. Sorry no PN, but take the stock filter with you, it is obvious which one it is.

Scott

The previously mentioned options were my first thoughts also. (fuel starvation)

As Gary suggested, if the tank vent is clogged there will be a vacuum in the tank that reduces the amount of fuel that is delivered. Presuming the tank on a K75 is similar to a K100, there should be two tubes that exit the bottom of the tank. One is a cap drain to redirect water from the cap. The other is the tank vent. You can attach a short hose to these to test airflow. The cap drain should pass air freely. The vent should allow some airflow until the tank is pressurized, then allow the air to exit once the air source is removed.

The fuel filter, as Scott suggested is also a prime candidate for restricted fuel flow. It can be removed from the tank and tested after it has had time to drain and dry out inside. It should pass airflow with almost no resistance. Any resistance would suggest at least a partially clogged filter. If it hasn't been replaced in a while, it's probably easier to install a replacement while you're in the tank rather than wait to test and consider the need to replace it.

Disclaimer: There are lots of more qualified experts here - I'm only relaying information that I've found and used in the past.

Hope that helps.
Monte
85 K100RT

Thanks for the suggestions Monte, Scott, and Gary. I'll let you know what is discovered as I check into the tank vents and filter.

Be careful with aftermarket filters.

They MUST be rated for high pressure (above 100 psi). Working pressure is 38psi, but if the regulator fails or you put a gauge straight to the pump while testing, you could see 100 psi. A lot of filters that look exactly the same dimensions are only rated for around 15 psi.

98lee said:

Be careful with aftermarket filters.

They MUST be rated for high pressure (above 100 psi). Working pressure is 38psi, but if the regulator fails or you put a gauge straight to the pump while testing, you could see 100 psi. A lot of filters that look exactly the same dimensions are only rated for around 15 psi.

Click to expand...

Thanks for the heads-up!

Replacing the filter is a good idea. I would also examine the spark plugs. If they seem bright white that confirms a lean running condition. If however they seem dark and show signs of wetness that would indicate a rich condition. If this is so, then suspect the engine temperature sensor. If the sensor is faulty and the ECU thinks a hot engine is still cold the bike will run rich and can get cranky (tech term ).

PGlaves said:

Replacing the filter is a good idea. I would also examine the spark plugs. If they seem bright white that confirms a lean running condition. If however they seem dark and show signs of wetness that would indicate a rich condition. If this is so, then suspect the engine temperature sensor. If the sensor is faulty and the ECU thinks a hot engine is still cold the bike will run rich and can get cranky (tech term ).

Click to expand...

Thanks, will recheck plugs too. I did replace with recommended plugs ~500mi ago.

Hi All,

Thanks for the suggestions. I replaced the fuel filter and checked the plugs. No improvement after replacing the fuel filter. Plugs 1 and 2 (front and center) are white tipped. Plug 3 (rear) looks normal with a tan/light brown tip.

Any suggestions for next things to check would be welcome!

Thanks again.

Tim

Try replacing the engine temperature sensor as Paul suggested. I had a similar problem and a new sensor corrected that.

WalterK75 said:

Try replacing the engine temperature sensor as Paul suggested. I had a similar problem and a new sensor corrected that.

Click to expand...

Thanks, will try that.

Millivolt signal trouble? 30 year old spade connectors that were not gold plated

Old K bikes use millivolt signals to go through many spade and double pinch connectors: under seat computer K1, fuel injectors, sensors( rotation . . , ). These are not redundant meaning if one blade is corroded the millivolts can easily stop flowing and can start again in about 1/5th of second. If your "hesitation" is quick, quicker than you could flick the cut off to off and back on, you might need to have less corroded millivolt connections.

The act of unplugging and plugging is supposed to wipe corrosion off, what do you know, if you buy a new sensor X, half of the connection is new and half of the connection just got wiped (shined up). I've advocated using an eyedropper bottle of Stabilant 22 (is NAPA contact enhancer CE 1 . $75). You wet a supplied Q tip, rub both sides of the spade( you may hear a little sizzle, it's airplane stuff) and wet the entrance to the mouth of the mating connector, plug it back in, maybe several times to wipe the spade. I carry it and have used it every few years on my 1985 K100RS with 198,000 miles, on the underseat K1 computer connector especially, I rarely replace sensors although water temp and throttle pos. sensors have been replaced.

My bike has not been towed although twice my sons brought me a clutch cable (40 and 40 miles) that I keep over my workbench, a third time I rode it 10 mi home without a clutch cable. I've bump started it many times when the sprag clutch just whirred. I've used jumper cables with the"car" not running. I made a u turn one morning to rebuild the oil/water pump.

Hope this line of thinking helps others keep the old K bikes rolling with fewer "QUIRKY" "make you nervous" shut offs and back on mostly for "NO REASON."

177inthe70s
Phila PA

If you have a ohm meter you should be able to test your temp. sensor. It may save you from replacing a good unit or verify a bad one. You will however have to search for the table listing the resistances vs. various temps.

tubeless said:

If you have a ohm meter you should be able to test your temp. sensor. It may save you from replacing a good unit or verify a bad one. You will however have to search for the table listing the resistances vs. various temps.

Click to expand...

Tubeless,


It's in the Tech Library at the top of the Flying Brick page.

https://forums.bmwmoa.org/showthread.php?93214-Testing-K75-K100-temperature-sensor