Help debugging a hard-to-start and choppy engine performance problem

[rbm]

A friend is asking me for help with debugging a '86 K75 engine performance problem.  I wanted to see if anyone has an idea where to look.  Here is a movie of the problem:
https://youtube.com/shorts/7c3mrHZp9uY?feature=share

For sure, I know that the water temperature sensor connected to the ECU is correct and working, TB bushings have been recently replaced, gas is fresh, and injectors are clear and fine.  The bike was working properly before this situation appeared recently and suddenly.

[daveson]

Spark plugs wet and black?

Smoke or smell of fuel at the exhaust, or fuel dripping at headers, especially cylinder four?

All leads operate timing gun?

Another quick test is to try his fuel injection control unit on your brick, if it's a k75.

[Laitch]

The extract below is from BMW's Testing for K Models downloadable from this site. The instructions in the right side column pertain to attachment of BMW's proprietary diagnostic tool. You should be able to work around that for your testing. Your description and the manual's troubleshooting flow chart brought me to these steps.

[rbm]

... especially cylinder four?

It's going to be impossible to determine the colour of plug #4 because its a K75.  Forgot to mention that tidbit in the original post but I went back and corrected my mistake.

[rbm]

Spark plugs wet and black?

Very black but dry looking.  Looks very much like over rich.  These are pictures of Cylinders 1 and 2.  I guess he forgot to pull #3.

Smoke or smell of fuel at the exhaust, or fuel dripping at headers, especially cylinder four?

No sign of dripping fuel but it smokes somewhat when cold.

All leads operate timing gun?

I haven't a timing gun to test that.  I do have a IR thermometer so I can test exhaust manifold temperatures to look for a cold cylinder.

Another quick test is to try his fuel injection control unit on your brick, if it's a k75.

He has a parts bike with a spare ECU we can swap out.

....  Your description and the manual's troubleshooting flow chart brought me to these steps.

Thanks Laitch.  I'll check the AFM with a multimeter to verify its operation.

This is one of those guys who strips the bike clean for that "cafe racer" lookout worse because he takes a rattle can black paint and sprays everything including rubber and plastic parts for that "rat bike" look.    It's hard to debug when bolts and fixtures are covered in paint.  Picture of the bike I'm trying to get going is below:

[frankenduck]

K75 plugs can get dirty like that if you let the bike idle too long.

[rbm]

Thanks Duck.  This guy drives the bike like he stole it.  I doubt he idles it very much.

[daveson]

It seems to be running rich. If not from too much fuel, maybe not enough air. Maybe the snorkel is blocked and it runs better if removed from the air filter box. Maybe the filter is blocked.

The smoke might be normal over there at this time, if white, but not from start up, and then disappears. The exhaust might sound wheezy if blocked, or smell of fuel, or too much soot if swiped with your finger.

Might have to check the plugs for spark.

If he lives near you it might be better to try his computer in your brick, in case his brick cooked his computer, then it would cook his spare.

Um, just had a closer look at the photo, man that exhaust is rough as guts. Hopefully the exhaust valves aren't going bad. Hopefully he still has the original exhaust, which just might make it run a bit better. I wonder if some other work was done on it recently before the problem started.

[Laitch]

Very black but dry looking.  Looks very much like over rich.  These are pictures of Cylinders 1 and 2.  I guess he forgot to pull #3.

What a piece of stylish junk yet, it's running like it looks!    Did you check the electrode gap of those plugs, rb?

[rbm]

@Laitch:  He messaged me that he changed the plugs this week after seeing the old ones.  I'll check the gap on the new ones when I get up to his place next.

@Daveson: Yes, he took an angle grinder to the muffler; that's why its so rough. Does that to all his bikes. He's a loud exhausts saves lives kind of guy. I've mentioned the lack of back pressure as a possible cause for his troubles but he refuses to fit a proper muffler.  Yes hopefully the valves aren't burning up.

He messaged me that he put the bike in storage now so maybe its a project for next year.

[The Mighty Gryphon]

Rob, is this guy your brother or something?  Do you owe him a huge favor?  Is he offering you massive amounts of money? 

I just can't imagine you helping a clown like this guy appears to be get such a pile of crap running and annoying the public at large.

[rbm]

Jim,  He's not family or anything.  Just a guy who I met and he saw that I have skills with K-bikes.  He has about four K's in various states of parts-only, not-running or not-running-well.  I'm a nice person and a patient person.  This situation started out as "Yeah, I'll do you a favour". but now its really intrusive and frustrating.  As soon as I clear one problem, he's got another one.  He did pay me for my time for the last K-bike I tried to get going but it was so far gone, that I could never resurrect it.  That bike was the last straw.  I put in more than 40 hours trying everything and right now  I'm through with this.  I have better things to do with my spare time than fix K-bikes for free.

On the one above, I've fixed about 10 problems over the last couple of years and, like @Daveson, I think he's killing the engine over time with the chopped exhaust.

[The Mighty Gryphon]

I hope you can ditch this guy.  Your time and  talents are way too valuable to be pissing them away on this knucklehead.  From what you say, it looks like he may be contributing to the rapid disappearance of classic K bikes in the past 10 years I've been riding them. 

I agree, back pressure is nice to have unless you are willing to spend the time retuning the mixture to prevent burning up the pistons and exhaust valves.  If it were me, as a public service to keep that POS bike from annoying the neighbors at early morning hours, I would modify the injector ground wire so it was direct to ground.  I'm sure you could pull it off!

[daveson]

Maybe fire a cannon next to him, he might end up like me, with tinnitus (ringing ears) then a quiet muffler sounds loud and he might end up a born again fan of quiet mufflers.

A possible gentle way, just offer suggestions, maybe the barn door on the air flow meter needs a look, or something simple, or he might break it down.

[The Mighty Gryphon]

Maybe fire a cannon next to him,

Not an option, Rob lives in Canada. 

[daveson]

I used to have a 3030, with single shots, sometimes I used to fire it just to hear it, that might do it. Maybe that's why I gotts tinnitus, but I likes to thinks so not.

[Laitch]

I used to have a 3030, with single shots, sometimes I used to fire it just to hear it, . . .

The up-side, of course, is that you didn't shoot a man in Reno just to watch him die. 

[daveson]

One of his many brilliant songs. Still remember the first time I heard it, and remember thinking "that's a bit of a  disturbing part of the lyrics idn't it" wish he just kept it sad instead of going disturbing too. Still it's a good one.

[frankenduck]

I used to have a 3030, with single shots, sometimes I used to fire it just to hear it, that might do it. Maybe that's why I gotts tinnitus, but I likes to thinks so not.

You don't even need ears to verify that a 150+ dB firearm is working. Even Helen Keller would know if that firearm is working.

[daveson]

I even liked the kick I got from it.