R100GS Coughing, Backfiring, Stalling

[bumblebee]

G'day all

Firstly, an intro. I'm new here, so be gentle. I was introduced by Grant - a top bloke I've met along the way (grant81rs). He heard I'd bought an airhead, and pointed me straight here.

Here's a pic of the beast in question. First registered in '92 here in Aus. All original except the exhaust which is a custom job. Came with panniers and sheepskin seat cover. I bought it unregistered in Queensland six weeks ago and rode it with a permit the 850kms to Newcastle. The PO had just rebuilt the carbs and warned me that it may require a rebalance at the end of the trip, and I suspect he was right in that it stalled on the odd occasion at idle. Apart from that, ran like a charm.


photo by spotcom, on Flickr

Now for the problems. I gave it to a mechanic, and asked for the "blue slip" (papers to register a bike). Also asked to go head to toe and check & replace as needed. Replaced throttle & choke cables, fixed the front caliper, put new sintered pads on, new tyres, new plugs, filters, new progressive springs in forks, new steering head bearings, new fluids all around. The mechanic gave it two thumbs up as being a great example of the model - all major bits like compression, gearbox, final drive all in good nick.

He did say one thing was odd: He couldn't find the timing marks when he put the strobe light on it. Keep this in mind...

I then picked it up, got it registered, loaded it up ready for a rally this weekend. I took it for a short (10k) shake-down run - all seemed fine.

On Saturday I joined a bunch of guys heading for the Thunder Rally in Nundle. We rode for about 3/4rs of an hour up the highway, then stopped at a set of lights.

The bike stalled. I started it, and it ran on one cylinder. Took off - it took about 100 metres for the second cylinder to fire. Apart from that it seemed to run okay.

We stopped 15 minutes later to wait for another rider who was joining us. When I started the bike again after about half an hour, it coughed and backfired then eventually ran. We took off but the bike was running poorly - missing under load and coughing. I pulled up - the guy behind me stopped too, and said he'd seen puffs of not only black smoke, but white as well. In his opinion it smelled hot. He suggested that the timing could be way out, causing the overheating. One of the other guys said that he'd been following me for a while and to him, the bike smelled like it was running rich (this was in the first half-hour of the journey).

So's not to risk it, I called the backup crew - she came with the van, and both bike & I vanned it back home.

I phoned the mechanic (he's in Sydney 2 hours away). He suggested it could be a dirty pilot jet. I took both bowls off, checked the fuel quality. The LHS had a *little* grit - not pristine. The jet was clear though. I cleaned it & replaced it. RHS was pristine, but cleaned & replaced it. Still runs rough.

I checked the plugs - both okay, but the LHS did smell a little "fuel-y" if that's a word. Neither was wet or oily.

I checked the oil level - fine. Checked the air filter just in case - brand new and clean & dry.

I then put a strobe light on, after first turning the engine over using the kick starter. The marks are definitely there! But, just as the mechanic said - they're not visible at all using the timing light.

So, for the gurus out there (this is going back to the mechanic on Thursday, by the way, but I'm kinda keen to get any suggestions other than "take it back to the mechanic" if you know what I mean):

* Have you ever come across this issue with the timing marks not showing when using a timing light? If so, what's caused it? Dumb question, but is it possible to put the flywheel on 180 degrees out?

* Any other suggestions as to what to look for before taking it back down on Thursday?

* I read somewhere that you shouldn't balance the carbs on these things by removing a plug lead, setting one side, then removing the other plug lead to set the other side - it can damage the electronic ignition. Is it possible that this is the cause?

Thanks in advance guys (and thank for your patience so far!)

[Jean]

Yes, to answer the easy question.
The flywheel can be put on many ways (depending on how confused you get!) and lose track of the timning marks.
Not a good thing.
We've discussed this a few weeks ago...

[Deleted User 72]

Don't know your mechanical skills, but on that model its pretty easy to loosen the bolts holding the ignition cannister to "go looking" for the timing marks with the engine running, supposing that the flywheel is on correctly.

[Airbear]

Gidday Bumblebee, and a big welcome. Grant is indeed a good bloke. That's a fine example of the breed you have there.

I'm not sure what your problem is (there's quite a range of possibilities) but your mechanic's statement about the timing marks not being visible during a dynamic timing is certainly curious. Your concern about the flywheel position can be checked easily enough:-

Remove the spark plugs and the little rubber plug to view the timing marks.
Remove the front cover to the timing chest (Disconnect the battery negative first - something to always remember - you don't want to accidentally short the diode board).
Use an Allen Key to rotate the engine clockwise while you hold a pencil or similar object in the left side spark plug hole.
With the pistons at top dead centre you should see at least some of the timing marks in the little window. Rotate the engine back and forth a few degrees to make sure. If the marks are not visible around the TDC position then the flywheel might be incorrectly positioned, but it won't be 180 degrees out. The flywheel is held on by 5 bolts, so factors of 72 degrees would be possible. If this is the case you can probably do a workaround by making temporary marks with white paint until you have the chance to pull the gearbox and fix the problem. With an accurately placed mark at TDC, you can figure out the other marks by looking at a workshop manual. To my mind the Clymer manual is the best, by the way. Let me know if you can't get access to one, and I'll scan and send the appropriate pages to you.

According to the Clymer manual you can't do a static timing adjustment on the post 81 bikes without a special BMW tool.

Other possible problems could be coils, plug leads, a loose 'bean can' (bottom of timing chest) and a whole lot of interesting things to do with carbs. The timing mark issue puts it pretty high on the list, though. Good luck, and keep us informed. Others might chime in with other suggestions - I have no experience with the bean can.

[Steve in Golden]

Since the bike ran fine for a while, does this indicate a timing problem? Wouldn't a timing problem cause it to run poorly right out of the gate, not run OK for a while then start acting up? I'd be suspecting fuel or ignition problems before timing. I'd be looking carefully at the items that were recently serviced, choke & throttle cables especially. Something may have come loose.

[bumblebee]

Thanks for the replies guys. Yes it's either something that happened during servicing or bad fuel or similar. I did also suspect the loose "bean can" as you called it but it's all good.
It goes to the mechanic tomorrow. He's a decent bloke and I know he'll work hard to fix it.
I will ask him how he went about balancing the carbs though (assuming that the warning I read about potential damage to the ignition system by running on one cyl is correct).
Oh and yeah, the flywheel must be on incorrectly - those marks are clearly in the wrong spot. (How big a job would it be to correct this???)
I'll let you know what transpires.

[Jean]

Check your exhaust system for airleaks, and the connections between the carbs and the cyls likewise.

And NOW:
"Oh and yeah, the flywheel must be on incorrectly - those marks are clearly in the wrong spot. (How big a job would it be to correct this???)"

Airbear said it...You have to take the transmission and the clutch assembly OFF, to get to the flywheel. ALSO, pay attention to the instructions about blocking the front end of the crankshaft to be sure you don't dislodge the thrust bearings when you undo the flywheel.
Do a search on R&R for the rear main seal (in this forum) or read other postings by snowbum, or Duane about how to do this.

That's why it's very important to be sure when you ever DO this sort of service (removing the flywheel) that you DO get it on again correctly.
I have a '77 R100RS (modified S) in the garage that had this happen before I bought it...it runs fine, but timing requires checking the valves, the TDC as described and a lot of hair-pulling. SOMEDAY, I'll pull the tranny and clutch and fix it...but that's a BIG job, for me anyhow.

Electronic HEI ignition systems do not like "pulling a SP wire" , so I just use a spare SP laid on top of the fins for that one to spark to ground (earth) when balancing on one cyl. Better safe than sorry. I actually do this for any ignition work that involves running on ONE cyl...provide a path to ground for the "dead" cyl. Since your engine ought to be warmed-up for balancing, it ought to start OK on one cyl...do the disconnecting and grounding with the engine off.

[Garnet]

It's doubtfull that your mechanic would use the pull the plug method to balance the carbs. If he did it incorrectly the bike would not be running now. Good techs can balance the carbs by ear/feel or use a manometer of some description much faster and more accuratly then pulling plug wires and looking at bouncing tach needles.

[Deleted User 62]

Another thought, my R90/6 had these exact symptoms when one of the sparkplug wires had gotten loose from the coil. It hadn't fallen completely off, but was hanging by the rubber boot. Easy enough to check...

[Airbear]

Oh and yeah, the flywheel must be on incorrectly - those marks are clearly in the wrong spot. (How big a job would it be to correct this???)
I'll let you know what transpires.

BB, I'll just add to Jean's excellent advice:
Pulling the gearbox and clutch to re-position the flywheel is not beyond the amateur mechanic. You appear to be handy enough with tools and could probably do it in a day the first time if you have everything ready. The job would be pretty expensive in labour terms if you are paying a mechanic. The battery box and air cleaner need to be removed. The swingarm needs be detached to allow the gearbox to move rearwards sufficiently. The flywheel bolts are 'stretch' types which need to be replaced, and it's probably good to replace the clutch bolts as well. You need a torque wrench and also need to make up at least one special BMW tool, to hold the flywheel. Jean's warning about blocking the crankshaft from moving forward is a critical one. The Clymer manual has very good instructions with clear illustrations. In the meantime the workaround mentioned in my above post will work comfortably enough. Talk to your mechanic about this.

Note that pulling the gearbox every couple of years is necessary maintenance - to grease the input splines. It's just one of those quirks of our lovely machines, making mechanics out of BMW riders for decades now. While you are there, you can clean and grease the swingarm bearings and check your engine rear main seal and gearbox input seal.

For any bits, Munich Motorcycles in Perth is a good mob to deal with. You can order on-line and get things in a couple of days. For bigger stuff - clutch parts etc, check Motobins in the UK. Delivery takes about a week.

NB: My experience is with a 90/6 only. Your new fangled but very pretty machine might be different in a few ways.