One for the group...looking for suggestions for where to start. Lot's of info on the site but nothing quite matches my situation so I hope someone can offer some tips on the most likely cause(es).

Just fixed the oil/water pump. Was running fine and doing about 50 miles a day. Some dual-carriageway (highway) and some back roads.

On my way out, engine warmed up and about 5 miles in, briefly hit "good morning judge" speeds on dual carriageway. Just another day. Total of 15 miles up on journey, had slowed to 70-ish mph and rolling along in traffic when it started mis-firing. Fuel gauge was flicking into the red but low fuel light wasn't on. Stopped for fuel (E5 super as ever) but that wasn't it. Very, very lumpy pull away like it's lost half a cylinder but will roll at 70-ish on light thottle - bit rough and no get-up-and-go but sort of drive-able and I was able to get to my destination and home again. Won't really tick-over without some throttle and is very. very rough. All four exhaust pipes are hot. No water in oil or oil in water. Squirted  WD40 around the throttle bodies and intakes and no obvious signs of leaks.

In last 2000 miles it has had:
- new plugs and leads (pulled plugs and sparking on all 4. All plugs look fine and same.)
- new fuel filter and in-tank pipes (pulled pipe to injector rail and fuel pumps out - although it stops after a few seconds. And fuel returning to tank when connected.)

So guys, what goes bang, all of a sudden while rolling along, one moment fine, next moment terrible? And stays terrible but doesn't get any worse (although that would be difficult).

Two things.  First, check the battery connections and the grounds, including the one under the tank  Are they all tight and shiny?

Second, Check the four pin connector for the fuel tank.  Give the connections a shot of Deoxit.

And thirdly the infamous crankcase breather might just have picked that time to split. Had that hacking around the Welsh mountains about six years ago - one moment I had a full set of grunt, the next I was running on 2.5 cylinders with all the symptoms you describe.

While going thru cleaning and re-seating all the likely looking connectors, I found that I could leave #2 injector unplugged and it made no difference. Unplugging other injectors made things worse. I swapped the plug and lead on #2 for old parts - no difference and good spark on all cylinders. I pulled the injectors out, still attached to the rail and fuel pipes - when cranking on starter all injectors appear to be firing as expected. Only shooting an inch or so onto the cylinder head so no idea about spray pattern but definitely a good jet of fuel coming out of all of them. O-rings look ok. Measured the resistance on #2 injector at 16.8 Ohms (which seems to be ok) and voltage on injector supply plug is, as near as my DMM can register, 11.5V. Put back together - no change.

Maybe it's dropped a valve? Nope, all clearances are where they should be. I don't have a suitable adapter for my compression tester but the head gasket seems an unlikely candidate.

Examined the crankcase breather pipe (Z-pipe) in situ as best I can. Seems good and flexible with no obvious damage. I should take it off to be sure but getting at the hose clamp is going to be a pain. I don't understand how the Z-pipe can affect a 16V 'cos the Motronic doesn't have an air flow sensor - although I don't know how it does its magic without.

So what do people think?
Could an injector be superficially ok while causing a bad mis-fire thoughout the rev range? Motorworks have injectors at £123, a set of four "EV14"s at £130 or a second-hand one for £35. Are there better choices?

Is there anything else that just goes bad, in an instant, like this wiithout stopping the engine completely?

Instead of replacing The plug and lead for cylinder two, swap them with the plug and lead for cylinder one. If the problem moves to cylinder one, the plug and/or lead might be bad.

Check that the lead connections are clean, and the leads for resistance.

Swap the injectors from the bad cylinder to a good cylinder and see what happens.
Regards Martin.

A compression test would help it may have dropped a valve I had one let go about 10 years ago. A piece about 4mm wide about 90° of the exhaust valve.

stanthomas wrote:
Examined the crankcase breather pipe (Z-pipe) in situ as best I can. Seems good and flexible with no obvious damage. I should take it off to be sure but getting at the hose clamp is going to be a pain. I don't understand how the Z-pipe can affect a 16V 'cos the Motronic doesn't have an air flow sensor - although I don't know how it does its magic without.

You're right, a leaky crankcase breather has no effect on fuel/air. Air is metered with the Throttle Position Sensor on the throttle bodies which replaced the Throttle Position SWITCH and AFM used on the 2v.

***CHECK FOR WATER IN THE TANK***

Could be low fuel pressure. A quick check that doesn't need a pressure gauge is to take the fuel line off the rail and poke it into an empty 2 lit coke bottle. When cranking the fuel should gush out fairly well. Do the same with the fuel return line. Fuel should just bubble out.

I'd be taking the injectors right out so you can get a good look at how they're spraying. A good indication is that they spraying all the same. Worth going to the trouble of cleaning them. I've always had good success with a can of carby cleaner and a special adapter you can get off ebay. Replace the kit. They are cheap. The little filter basket on top of the injector could be blocked.

Just to be sure you can check spark plugs with a DMM. Info here... https://www.k100-forum.com/t10954-spark-plugs#130448

Make sure you've got the nuts on the top of the spark plug and there is no corrosion either end.

Hadn't thought of swapping injectors to see whether the problem moves with them. It doesn't.

#2 is runnning, kinda. Being careful not to move the throttle (have to hold it open otherwise it stalls), unplugging/replugging #2 injector does make a small difference to engine speed but not to the uneven running. Unplugging any of the other 3 makes things considerably worse.

Fuel comes out of the injector rail supply pipe like turning on the mains cold water tap. Haven't tried the return end but I had the rail off to swap injectors and it blows thru with an airline.

Finding an adapter for my compression gauge and doing that test seems like the next step...


Thanks for the tips.

stanthomas wrote:. . . unplugging/replugging #2 injector does make a small difference to engine speed but not to the uneven running. Unplugging any of the other 3 makes things considerably worse.

If there is sufficient voltage supplied by #2's coil and its spark plug is in good condition, it seems to me like #2's injector is faulty.

Following MartinW's suggestion, I swapped injectors 2 & 4 but the problem didn't move with them. Sorry I didn't make that clear. Seems to be something going on with cylinder #2. Compression test next.

Given that its an easy check I should swap the coils witrh each other and see what happens.

When was the last time you checked the valve clearances?

Point-Seven-five wrote:When was the last time you checked the valve clearances?

Friday. Checked in case valve bent. All ok, a couple right on the lower limit but nothing opened up like might happen if a valve had kissed a piston.

Olaf:
I'll try swapping the coils, easy to do as you say. Have assumed that, being a shared coil, a bad one would take out two cylinders.

For anyone who might be interested, 'cos I'm not sure I am anymore, the compression test readings were (bar):

1. 5.5
   
2. 0
   
3. 6.75
   
4. 7.0
   

So there we have it, no compression on #2. Valve clearances OK which suggests blown head gasket although no oil in water or water in oil. Head will have to come off to confirm. £36 for a head gasket but I'm not sure I can be bothered. It's been utterly unreliable and I don't much like it anyway. A year ago a guy came up to me in Tesco's car park and offered me £1,000 for it; should have bitten his hand off.

Always being the semi-eternal mechanical optimist, I wonder what the valve clearances are on cylinder #2.

Then I would do a leakdown test on that cylinder to confirm that the problem isn't rings or a holed piston.

Then, since I am a hopeless optimist, I would dump a can of Seafoam in a couple gallons of fuel in the tank and run the engine until it warms up.  Let it sit overnight and repeat again the next day.  Do this five or six times. 

Unless your engine has a couple hundred thousand miles on it, or has been badly abused, I would trust the mechanical integrity and hope that the problem is just a stuck valve.

Point-Seven-five wrote:Always being the semi-eternal mechanical optimist, I wonder what the valve clearances are on cylinder #2.

There's glass half-full, glass half-empty and, where I am with this bike, glass lying broken in the road ready to puncture a tyre ...

#2 valve clearances are: inlet .006 & .007, exhaust .010 & .010 . Rest are also in spec. Could be gasket, valve, valve seat, piston rings, piston or something else. Whatever, head will need to come off. I'll qualify that the compression test was done with warm engine, closed throttle, plugs out, cranked for count of 10.

88k miles. Don't know the history but MOT records show it doing 8k miles/year from 2005 to 2010 then used regularly (2k/year) up to 2015. Guessing that the clutch started slipping in 2015 and it was dumped in a corner where it languished for 5 years. Likely that basic maintenance was carried out up to 2015. I bought it in 2020. Fixed faults as they arose but it's been broken so often I've only had 3k miles out of it. Engine seemed strong until it went pop at the beginning of this year since when it has resumed its role as garden ornament.

Rick G wrote:A compression test would help it may have dropped a valve I had one let go about 10 years ago. A piece about 4mm wide about 90° of the exhaust valve.

When this valve let go the clearance was still OK it was a piece about 3 to 4mm wide came of the outer edge of the valve.
I could not figure what was going on until I removed the head. I have since learned that it is a known problem but not very common.
There was no damage to the head, piston or bore and I replaced the head because I could not get a valve at a reasonable price.
I since managed to get a replacement from an Ebay seller in US, he told me that the individual valves were worth more than trying to sell a complete head because of the problem with the valves breaking around the edge.

stanthomas wrote: I'll qualify that the compression test was done with warm engine, closed throttle, plugs out, cranked for count of 10.

Compression tests are done with throttle WIDE OPEN.  This may have had some effect on your readings.

Stan see if you can beg or borrow an endoscope to get inside the exhaust port from below or even put the piston at TDC on compression and blow air into the plug hole and see if it comes out the exhaust port or maybe even the inlet port.

Yes a leakdown test would help narrow down the problem, and a valve problem is high on the list. If you can't get one, modify a spark plug or something else.

Unlikely but some other things to check, bubbles in coolant or excessive smoke or water from the muffler, especially if it's been using coolant lately. Feel for external leaks around the head gasket joint. Often a leaking head gasket can be fixed without removing the head.

daveyson wrote:Often a leaking head gasket can be fixed without removing the head.

That's a new one on me. How does that work?

Point-Seven-five wrote:Compression tests are done with throttle WIDE OPEN.  This may have had some effect on your readings.

Noted. But zero is zero

Rick G wrote:When this valve let go the clearance was still OK it was a piece about 3 to 4mm wide came of the outer edge of the valve.

That's an interesting possiblity. It would be consistent with #2 having shown some limited signs of combustion. Just have to hope the missing piece went down the exhaust...

Rick G wrote:Stan see if you can beg or borrow an endoscope to get inside the exhaust port from below or even put the piston at TDC on compression and blow air into the plug hole and see if it comes out the exhaust port or maybe even the inlet port.

Good idea    Can't see it precluding the head coming off but might indicate how fubar'ed things are.


Anyway, thank you gentlemen. I think you've steered me away from dropping a match in the petrol tank - for the time being.

"zero is zero"

Why does the test specify wide open?  Because you need to take the throttle butterfly out of the equation.  Try doing a compression test with your hand over the air intake.  Even with a good cylinder, the reading will be very low with individual throttle bodies that prevent intake crossflow.

A leakdown test will pinpoint where the problem is.  Intake valve, exhaust valve, head gasket, rings.

Some examples of leaking head gaskets, could be a loose head bolt, a stretched head bolt, a fatigued head gasket. A loose bolt just needs tightening, a stretched bolt needs to be replaced. Sometimes I've stopped a head gasket leaking by torquing the bolts just 3 ft/lb over spec.

stanthomas wrote:#2 valve clearances are: inlet .006 & .007, exhaust .010 & .010 . Rest are also in spec.

Rest are also in spec? The #2 clearances are >50% tighter than their minimum clearances. That could not only create fueling and compression problems but also could lead to both exhaust valves being burned.
From BMW's K100RS/K1 repair manual, minimum clearances in red boxes:

Laitch wrote:

stanthomas wrote:#2 valve clearances are: inlet .006 & .007, exhaust .010 & .010 . Rest are also in spec.

Rest are also in spec? The #2 clearances are >50% tighter than their minimum clearances. That could not only create fueling and compression problems but also could lead to both exhaust valves being burned.
From BMW's K100RS/K1 repair manual, minimum clearances in red boxes:

.006" = 0.15mm, 0.010" = 0.25mm. On the lower limit but in spec. Sorry to have mixed unit systems but my feeler gauges are imperial (and this is a predominantly US forum).

stanthomas wrote:(and this is a predominantly US forum).

I don't believe that's accurate, either.

Laitch wrote:

stanthomas wrote:(and this is a predominantly US forum).

I don't believe that's accurate, either.

Perhaps by numbers but who is most active?  

I use both imperial and metric measurements but prefer thousands of an inch for clearances/gaps.  Most of the stuff I learnt my trade on was imperial measurements.

Laitch wrote:

stanthomas wrote:(and this is a predominantly US forum).

I don't believe that's accurate, either.

Quite so 

Quite an Eccentric group here if I may say so.

Given some of my recent K experiences a low fuel gauge and no warning light on and a sudden misfire....check your fuel pump and filter basket etc inside the tank.

A wrongly positioned pump will run 'dry' while you still have fuel in the tank. A blocked filter basket will also do that. Had it on 2 Ks.....it runs lean and hot when it happens.