BMW K75 Low end mis-fire

My recently acquired 1987 BMW K75 has a slow speed misfire around 2000
revs. Makes filtering a bit of pain. It's ok if it's revving. In fact
it runs along quite nicely.

It's been stood for a while and I suspect the HT leads - for no
discernible reason other than it has original BMW ones and I cant find a
reference to them being changed in the receipts it came with.

It has had new plugs. Can the injectors clog and cause similar symptoms?

Ideas?

 

Could be a dodgy plug if they are new?

--


Nige,

BMW K1200S
Range Rover Vogue

 

I suppose injectors can clog, like carbs. Try some of that snake oil
cleaner, but don't be surprised if it doesn't have any effect.

Fuel filter(s)?

Does it misfire on all pots? OK, hard to tell, but I mean, does itfeel
like one pot's dropping or all three?

It'll have the old LE-Jetronic management system. This can be regulated
and set by a BW dealership. Worth trying.

 

It was doing it before and still did it with new plugs.

 

It's only a couple of thousand miles old according to the receipts.

It's not bad enough for all three - it's like a bad flat spot, but it
isn't, IYSWIM.

Sounds pricey...

 

Tight ****! I'll bet it's ignition related, have you tried running it late
at night to see if there's any spark leakage?

--


Nige,

BMW K1200S
Range Rover Vogue

 

Agreed. I know your K is older than my bike and has more cylinders but from
everything I've read about BMW Fi it's 1) check TPS then 2) balance
throttle bodies.

 

In my experience, the vast majority of misfires are ignition-related.

As for cost, a decent BMW dealership will simply plug it into their
diagnostic equipment and give you an answer. Might be cheap, might not,
but for sure, it'll be much cheaper and easier than spending hours
faffing with it yourself.

Sweller, contact Brian Giles in Pembury. 07850 438405. He's the
independent specialist Timo and I use, and he's good VFM.

 

Email Burnt and arrange a weekend at the farm to play with bikes.
He'll almost certainly be able to sort out any issues with the bike.

 

We've discussed balance - I popped in for a tea on my way back from
picking it up. So, TPS and a balance then see what next.

Although, I tend to agree with TOG, misfires are ignition.

 

I may be quoting Champ here, but: " Misfires are /always/ ignition -
except when they're fuel-related".

Just like when the front end of the bike feels like it's about to fall
off ... and it turns out to be because the rear tyre's flat.

 

I think a trip to a diagnostic computer could save a lot of parts
swapping. At least you will find out whether it's in the spark/timing.
Having made sure the Fuel Cutoff switch is set correctly.

If it is running fine at other speeds then the fuel filter and fuel
pressure regulator *should* be ok (they both fail occasionally). As
should the mass flow box. A set of vacuum gauges will tell you if
something is wildly out on the butterflies.

Ignition is seperate from injection. The injection ECU lives over the
battery. The other is under the tank and needs a good connection to the
frame as a heatsink etc.

 

Yeah, I did this with the LT when I first bought it. There were some
gaps in the service history, and it had been off the road for a while.

Chucked it at Brian and told him: "Do everything. Absolutely
everything."

"Everything" included replacing all the disc bobbins, a complete full
service (all fluids, valve shims etc), fit a new gear position indicator
switch, grease the splines while he was faffing about at that end of the
bike, adjust head races, change brake fluid, you name it.

Came to well over £600, but money well spent.