1986 K75… 12,800 miles

I am in the process of getting this old buddy back on the road. It’s my 12th bike but only my 2nd BMW. Had a K100 about 10yrs ago. The vacuum hose that is in the wire loom on the third throttle body had a break in it so I trimmed it and put it back on and it runs way better…. But still not great. I was excited when I found a bad vacuum hose but it’s still not right. Some sets of shifts were great and she pulled great others not so much.
I did change the plugs and went thru the coils and made sure they were not corroded and they looked good.
Anything else to try. I know once I get it ironed out they are very reliable. PO did install new fuel pump and hoses along with fuel filter. Other ideas? TIA
BTW I’m from Missouri The KC area. Let me know if there any other K-Bike owners by me

jhestandnsrscout said:

I am in the process of getting this old buddy back on the road. It’s my 12th bike but only my 2nd BMW. Had a K100 about 10yrs ago. The vacuum hose that is in the wire loom on the third throttle body had a break in it so I trimmed it and put it back on and it runs way better…. But still not great. I was excited when I found a bad vacuum hose but it’s still not right. Some sets of shifts were great and she pulled great others not so much.
I did change the plugs and went thru the coils and made sure they were not corroded and they looked good.
Anything else to try. I know once I get it ironed out they are very reliable. PO did install new fuel pump and hoses along with fuel filter. Other ideas? TIA
BTW I’m from Missouri The KC area. Let me know if there any other K-Bike owners by me

Click to expand...

First, it would help me if you could expand some on describing the nature of the "not so much" bad running. Does it bog, cut out, just sluggish, etc.? You said the PO replaced the fuel pump, etc. so that suggests to me that the bike sat long enough with fuel in the tank to ruin the fuel pump. If so you could have dirty/sicky fuel injectors. More on them in minute.

You most likely have either an air leak or a fuel supply problem. I would do the checks below first to eliminate air leaks as your problem. Those usually cause more constant problems whereas you are experiencing intermittent problems, but with a bike this age, I would still check for air leaks first.


I would check these areas for air leaks:
1. Engine crank case vent hose for cracks. It's on the left side above the back of the engine and just behind the air box. It's recessed and you will need a flash light to see if there are any cracks as you flex it. These are a common replacement (service) item.

2. Check for air leaks around the fuel injectors and at the top and bottom of the throttle bodies. I use a moderate spray of starting fluid (carefully) at the base of the injectors (closest to the sealing O-rings) and then at the bottom of the rubber mounts holding the throttle bodies to the engine (manifolds). And finally at the top of the throttle bodies where the rubber couplers attach the throttle bodies to the air box.

3. Air filter. If this is really an 18k miles bike then the air filter should be in great condition. In normal conditions they can last 30,000 or 40,000 miles, but you don't know with a used bike until you check.


If those items all check out, then how long ago did the PO change the fuel filter in miles? These filters will not pass water, so even one fill-up of gas with a lot of water will partially or completely block the fuel filter. The complete blockage is fairly easy to figure out, but a partial blockage can cause fuel starvation under different conditions which seem like problems in other areas. Don't ask me how I know.

4. So I would pull out the filter and see if you can blow through it.

If it's good, then I would fill the tank (regular fuel - not premium) and dose it with 6 or 7 ounces of Techron Complete Fuel System Cleaner (NOT the Fuel Injector Cleaner), then go ride the heck out of it. If you have dirty injectors you will see either improvement or elimination of your problems in that first tank. If your problems are better but not gone, do another tank. Keep this up until it's running right and then do one more tank for good measure. If you have to do more than three tanks I would change the oil once you are done with this sequence.

You want the Complete Fuel System Cleaner as it cleans the back of the intake valves and the combustion chamber as well as the injectors - essentially, it's a stronger formulation than the Fuel Injector Cleaner. I use Chevron fuel on a regular basis because it contains a maintenance level of Techron (about 1/10 of the bottle treatment level according to Chevron). Then, before each oil change I run a tank treated with an ounce+ per gallon of the Fuel System Cleaner. I've done this for decades in both bikes and cars and have never had any dirty carb or FI problems.

Hope this helps.

Greg- great advice! THank you! Rode her 50 miles tonight the first 25 she ran pretty well. Took throttle and pulled well. One way home I was in 5th at 4000 rpm and it just feels like it’s bogging down and almost likes it’s going to die when u apply throttle. Obviously there is something else going on besides a split in the vacuum hose.
I will check the tank vent and the air filter. Also getting the Techron cleaner. I had a feeling that I could have a partially blocked fuel filter so I am getting one of those as well.
Really appreciate the help!
Jeff Hestand
Missouri

jhestandnsrscout said:

Greg- great advice! THank you! Rode her 50 miles tonight the first 25 she ran pretty well. Took throttle and pulled well. One way home I was in 5th at 4000 rpm and it just feels like it’s bogging down and almost likes it’s going to die when u apply throttle. Obviously there is something else going on besides a split in the vacuum hose.
I will check the tank vent and the air filter. Also getting the Techron cleaner. I had a feeling that I could have a partially blocked fuel filter so I am getting one of those as well.
Really appreciate the help!
Jeff Hestand
Missouri

Click to expand...

With patience you'll get this bike sorted out, and it will be worth it! Come back with questions at any time. Here's a link for an OEM fuel filter: https://www.amazon.com/gp/product/B005GDJSXQ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Crank case vent hose

Hi Greg- is the crank case vent hose a small hose, about the size of a regular vacuum line?

jhestandnsrscout said:

Hi Greg- is the crank case vent hose a small hose, about the size of a regular vacuum line?

Click to expand...

No, it's about the diameter of your forefinger and is behind the air box on the left side of the bike. Get down on the left side with your line if sight just above the back throttle body and shine a flashlight into that recessed area. You should see a curved hose coming up from the engine block and making about a 90 degree bend to attach to the back of the air box.

This link is to the MAX BMW parts fiche, and I've attached a screen shot showing this hose. It's #11.
https://shop.maxbmw.com/fiche/DiagramsMain.aspx?vid=51739&rnd=09082020

Thanks for diagram!!!
Yes, I did have a crack in the crankcase breather tube. Temporary repaired it and taking a ride tomorrow. Also got some Techron system cleaner and ran it thru.
I’ll do another update tomorrow.
Thx Again!

Greg- rode 87 miles today. First half of trip bike was running iffy. Off and on. Needed some gas. Opened gas cap and there was a pool of fluid. Stuck finger in it. It was NOT gas! Left cover off last night and we got a hard rain. Turns out I’ve been fighting water in the gas for sure. But I also think in need a gas cap rebuild. Where’s the best place to get a rebuild kit?
Well I soaked up the water, filled her with gas went and got some heet iso and more Techron put in and ran great on way home.
Thx again!
Jeff

Probably Engle Motors would have watcha need... https://www.englemotors.com/