Coughing and farting at slow speed

[John Roberts]

The bike, '82 R100RS, not me, that is.

Ok when cold. Otherwise intermittent. Ok at above 2,000rpm. It's been like this for months

Carbs overhauled, ie stripped & cleaned, diaphragms, needles & jets and all seals/O rings renewed and rebalanced as per handbook instructions, i.e. NOT using gauges, but when the fault isn't on it's smooth and pulls well from low speed. I've changed the HT leads and plugs.

It's that it's intermittent that pisses me off, once I get home and get stuck in it invariably clears itself.

Any suggestions, PLEASE?

 

[Big Jets]

My R80 1985 was the same intermitently running rough turned out to be the coil pack .Not by any means saying this is your problem but worth a look make sure there are no cracks on the bakelite where the leads go in .

 

[John Roberts]

Done that, I'll do it again just in case I missed something.

 

[Kenny]

btw the cracks are very hard to see !!! If you know anybody with an airhead, then beg / ask to swap them over...bets way of chacking the coils

 

[Steptoe]

The bike, '82 R100RS, not me, that is.

?

My R80 1985 was the same intermitently running rough turned out to be the coil pack .Not by any means saying this is your problem but worth a look make sure there are no cracks on the bakelite where the leads go in .

btw the cracks are very hard to see !!! 2

The 1982 models should have two single output metal bodied coils fitted, not the troublesome cracking dual output coil.

 

[Big Jets]

When did they bring the Pack in Steptoe was it with the monos ,what was in the 80 81 G/S was it twin seprate coils ?

 

[John Roberts]

Yes, it has two coils. I checked one last night with a jeweler's eyeglass, and the other this morning in a bit of a hurry, hence my decision to have another, more thorough look at that one.

Steppers: I was just emphasizing the point that it is my bike that was coughing and farting intermittently at low speed. Myself I can do so at most speeds, and with a gratifying fluency.

One thing still worries me, while doing a search in the Airheads section I read that a passage to/from the pilot jet is nearly impossible to clean thoroughly, surely that couldn't be the cause of my trouble if the fault comes and goes? I believe that the pilot jet arrangement does control the lower throttle opening range, but TBH I think I'm clutching at straws there.

I was checking the primary and Secondary resistance of one of the coil last night when the ohmeter packed in (leaky cells, corroded spring terminals ) I hope I can fix it, so what should the readings be, anyone know? It doesn't say in the official BMW manual.

 

[DrFarkoff]

Does it run like it's gone off a cylinder?

Does it more just when you're clearing traffic i.e. have been sitting idling in traffic for a wee while and the motor is hot?

Possibly the leads breaking down or the "bean can" i.e. ignition timing mechanism

The little sensors insulation can break down and they fail to send the pulse when it should

There's no real test for them as you have to be on the road and it has to be messin around cos the BM pluggy in thing will only flash a lamp for ya ti tell you it's working but you can't work out the frequency when ya plug it in!

Also are the carb to head intake rubbers fresh and a neat fit ???? i.e. no residual gaps that fuel can condense into and leave pockets of wet stuff ready to leap into the airflow and bollix your mixture up!

If you're in any doubt use tin snips and trim yer old ones and fit the carbs up as tight as ya can to the stubbs and see if that helps??

 

[John Roberts]

Yes sometimes one cylinder, but only when it's really bad, rest of the time it's just rough (when the fault is on, you understand) When it's idling in traffic it could be ticking over at, say 1,000rpm then it can start running rough for no apparent reason and nearly, though not always, stall. Blipping the throttle upsets it too. (When the fault is on)

"...intake rubbers fresh and a neat fit" They look ok, but are original. They are pushed into place snugly at both ends and the clips are as tight as I can get them. In view of you mentioning them I'll try pulling/pushing them to see if that has an effect. As for fuel collecting in them, the idling is ok most of the time and I'm wondering, if that was happening, which would be the chicken and which the egg. Sorry, that wasn't very clear, I mean, if it is capable of idling happily, would there be excess fuel to pool?

"bean can" While trawling "Airheads" Section I did come across an article quoted, American, I think, that showed how to overhaul this, as it can stick. Yes, since you mention it, I think it would be well worth a rummage down there.

"the leads breaking down": leads replaced.

"the BM pluggy in thing" erm.. not heard of that one, is that a technical term??

As Kenny said, Substitution would be a great help in cases like this, but I don't have any spares to swop. I'm dreading finding myself in a corner with no option but to start buying coils,triggers and amplifiers.

 

[DrFarkoff]

Sorry, that wasn't very clear, I mean, if it is capable of idling happily, would there be excess fuel to pool?.

Fuel lines the side of the carb throat and runs down the walls If your carb and inlet stubb don't touch, the fuel has a 1 or 2 mm gap to run into!

So it idles fine but as you increase the rpm the airspeed/vacuum effect lifts the raw fuel out of the gap and screws your mixture up dramatically

It's an easy fix! check, if they butt together if not?
Just get the tin snips out and shave an 1/8 of a wotsit orf them and reassemble!

 

[DrFarkoff]

Well Any update on the patient???

 

[Steptoe]

Well Any update on the patient???

I doubt it judging by whats been suggested so far

 

[John Roberts]

Update.

Thank you for your concern, good of you. Incidentally, Jaythro, when I said that something wasn't clear in my last post I was refering to what I had just said, not what you had said.

Well, not much to update really (been busy riding) but at the mo have removed the trigger can but I'm expecting difficulty in diconnecting the connector tucked up nearer the top of the recess behind the front engine cover. It's the sort with a wire clip running around the body of one half of the conector and which snaps over a couple of um... arrow shaped clips moulded on the other half of the connector. It's the same type as the connector to the Electronic ignition control unit. Surely there must be an easy way of separating connectors like this??

I'm off to continue my search for the thread on overhauling the trigger can.

Thank you all again for your interest.

P.S. Got it: http://www.deathstar.org/~flash/ig_trig.html

 

[BillN]

John

The wire is tight because it is pulled up and through from the top...and BMW always seem to make them just long enough..........but you should be able to remove it OK by pushing the clip off with a small screwdriver.........if you take your time

Are you talking about overhauling the Bean Can?............all you need to do is to make sure the rotary shutter which operates the A/R mechanism springs back........disconnect the battery....just remove the front cover, (two screws), and then remove the plate, (again two screws), you can twist the shutter with your fingers, it should spring back showing that the A/R mechanism works, (you should also hear the click as well),.......BMW said that the electronic system was maintenance free BUT........the only maintenance that the Bean can needs is to oil the spindle.........really you need to take it off to do this and maybe remove the small circlip so that the oil gets down the spindle better.......just use ordinary warm engine oil..only a few drops.........anything thinner is no good as it would evaporate

You should really set the timing once you have removed the Bean Can.....but just make sure that you put it back in exactly the same position and you should be OK........I would not take it off unless the A/R mechanism is sticking.

The electronic ignition was introduced in 1981 and I am not really up on the R100RS.......although I have just bought one.....but the 100 engine is rough anyway

 

[John Roberts]

Tried three things:

Checked the other coil with an eyeglass, looks OK.

Stripped, cleaned and oiled the bean can after having marked its position and put it back as near as poss the original setting. Timing NOT checked.

Rubber carb-to-head hoses checked. Due to their age there are two ridges formed on the inside of the hoses, one by the groove in the carb stub and the other by the one in the cylinder head stub. By looking at those it is obvious that the carb and head stub have never butted correctly, or, at least not for a long time. As the ridges are so pronounced there would be little point in trimming the ends of the hoses, the ridges would just pop back into the grooves, and back to square one. I do think that I'll get a couple of new ones, £4.11 from Moto-Bins. I do have reservations about this being the cause of the fault, but that's only because it's been on for the past months and I've had the bike from new. Having said that I must admit that your reasoning certainly makes sense to me, Jaythro.​

Anyway, I took it for a run. NO CHANGE. It idles happily at 750. Blip the throttle at idle and it stalls. Still misfires at around 2,000rpm when I open the throttle. It was ok when I started off but when I turned back after six miles it was back to its old tricks, but a bit worse perhaps. TBH I was too pissed off to go any further to see if it was still intermittent.

Two things stand out: the carbs have been balanced by ear, and the timing now needs checking after refitting the bean can.

OK then, Mr Steptoe:

Well Any update on the patient???

I doubt it judging by whats been suggested so far

Do you have any suggestions?

 

[Kenny]

Try another set of plugs if the leads/coils are OK. Plugs do fail on these bikes.

(Been on the so not thinking 100% straight btw )

 

[BillN]

Tried three things:

Checked the other coil with an eyeglass, looks OK.

​

COIL: You can check the resistance on the primary and secondary (windings), quite simply with a cheap multimeter

The ohm and K ohm resistances are in the "book"

It is best to take the coil off the Bike to do it

Disconect the earth cable on the battery​

 

[John Roberts]

COIL: You can check the resistance on the primary and secondary (windings), quite simply with a cheap multimeter

The ohm and K ohm resistances are in the "book"

It is best to take the coil off the Bike to do it

Disconect the earth cable on the battery

The readings are NOT in my official BMW workshop manual, honest.

No need to take them off to measure readings, simply disconnecting them is fine. I did take them off to inspect for cracks.

I've got a multimeter and that's ok for the secondary measurements around 10kOhms but not much cop at around an Ohm or so. I do have an expensive digital Ohmmeter but I'm having to fix that because leaking leakproof(!) batteries have corroded the battery terminal springs, two have come right off.

 

[BillN]

HT coil primary windings:
Single Coil...0.67 to 0.77 ohm
Twin Coil 1.15 to 1.32 ohm

HT coil secondary windings:
Single Coil...3.7 to 5.3 K ohm
Twin Coil 7.5 to 9.15 K ohm

from the Haynes Manual

 

[John Roberts]

Try another set of plugs if the leads/coils are OK. Plugs do fail on these bikes.

(Been on the so not thinking 100% straight btw )

Kenny, Kenny, Kenny. If I could think as straight when I am sober as you can when you are drunk it would have saved us all a lot of time. Problem solved, I changed the plugs.

The thing is I had already changed the wretched plugs months ago while chasing this very same problem, well, I mean, if you've put new plugs in then the plugs are OK. Obvious, innit?

Exactly when things went wrong I don't know, but I obviously went into tunnel vision mode at some point. Having said that, it's comforting to know now that after checking the coil resistances yesterday and, hopefully, borrowing a timing strobe and a set of gauges for the carbs tomorrow, as well as having overhauled the carbs, flushed the fuel tank, cleaned the fuel filters, overhauled the bean can plus lots of other bits and bobs over the past few months I now have a bike that is in a far better state of tune than it has been for years.

Thank you all for your help, folks.