The R100GS has a problem after lots of parts rebuilt/replaced

[ohiorider]

I am certain it will be an obvious problem.  Jamie, my long-time BMW tech, has volunteered to drive the 60 miles from his shop and do what needs to be done in my garage this Tuesday.

Weird.  Jamie had put over 75 miles on the bike after rebuilding carbs, tranny, replacing clutch, tranny bearings seals and gaskets, and cleaning the fuel tank and replacing the petcocks and fuel lines.  I look at the parts bill from Max BMW and can't believe all the pieces and parts it required.  Don't want to forget replacing the driveshaft.  I'd put an additional 30 or so very pleasant miles riding home. 

She was running like a new bike on my way home.  But after 25-30 miles, rolling on the throttle resulted in the engine running on one cylinder. 

Didn't seem to be rpm related, but more related to throttle position.

If I gradually rolled the throttle on, i could take the old gal up to 6000rpm, and stay there, with no issues.  I was cruising close to 90mph on one section of I 77, without an issue.

It was only if I was accelerating from lower rpm, and decided to roll it on quickly to full throttle.  Then she'd revert to one cylinder, until I slowly rolled the throttle back.  Once I'd rolled back to a lower throttle, she'd once again start running on two cylinders.

Oh, and at idle, the bike once again went into one cylinder mode, and wouldn't idle, and would stop unless I kept idle at 2000rpm or so.

I'm extremely pleased with the level of detail the tech spent on the bike.  I can only think this is a minor issue causing a major problem.

She'll be ready to cruise in 2019 on some road trips, and some shorter rides toward the tail end of our 2018 season.

Any thoughts re the problem?

Bob

[reidy]

Did you happen to put fuel in it just after picking up or was it fueled up just before you picked it up. What you describe sounds like a bit of water in the bottom of one of the float bowls. The idle jet is normally higher in the bowl than the main jet. At idle or cruise the idle jet is supplying a lot of the fuel. At full throttle the main jet is supplying almost all of the fuel. If your carbs have a drain on the bottom of the bowl I would drain the carb into a glass jar and look for signs of water in the fuel. It will be in the bottom of the jar. Even if no water appears present I would drain and take for a ride. Depending on the outcome determines the next step which may be a drain of the tank.

Steve 

[ohiorider]

Did you happen to put fuel in it just after picking up or was it fueled up just before you picked it up. What you describe sounds like a bit of water in the bottom of one of the float bowls. The idle jet is normally higher in the bowl than the main jet. At idle or cruise the idle jet is supplying a lot of the fuel. At full throttle the main jet is supplying almost all of the fuel. If your carbs have a drain on the bottom of the bowl I would drain the carb into a glass jar and look for signs of water in the fuel. It will be in the bottom of the jar. Even if no water appears present I would drain and take for a ride. Depending on the outcome determines the next step which may be a drain of the tank.

Steve

Thanks for your comments. I will run them past the tech on Tuesday morning.  FYI, I did fill the tank to the top on my way home.  The Bing CV carbs don't have a drain on the float bowl. 

I think you might be onto something.  We'll see.

Thanks,

Bob

[oldbike54]

 Sounds like an issued with vacuum Bob, check that the carbon top is tight and sealing.
If the diaphragm isn't sealed oft will act that way.

 Dusty
 

[wirespokes]

The float bowls are very easy to remove on the GS - and most of the airheads (except for the 90S with Dellortos) for that matter. They're held on with a wire bail, so all that's needed is to flip it open and allow the bowl to drop down. Be sure to close the petcocks first, of course!

And hold the bowl while removing the bail so the contents don't all spill out. Water looks like BBs, or marbles, in the bottom below the gasoline.

No need to wait till Tuesday to check this out and loose a few days of riding.

Another possibility is high resistance in the spark plug lead, plug cap or plug. There could also be something wrong with the coil. But the symptoms are really odd and resistance in the cap or lead don't seem to totally fit.

The BMW coils can exhibit symptoms of idling on one cylinder when they're going bad, so that is a possibility.

Water in the gas is possible, but doesn't make total sense either. Does it cough and spit?

The problem is strange enough it might require starting at the beginning trouble shooting step and working your way through. But my first thought was electrical. If it was carburetion as Steve suggests, then why do the revs need to come all the way back down for it to run again? And if there's water in the gas, I've never gotten past low RPMs, much less taking it easy to get the RPMs up. But who knows? Anything's possible.

I'll be watching to see what fixes this one - a good puzzler fit for the Tappet Brothers.

[reidy]

If it is water, which is only a suggestion, what the reason for having to roll the throttle back is to stop the vacuum trying to pull a water/fuel mix up through the main jet. What I have never been able to work out is why the water does not just get sucked through the jet and clear itself. It appears to just block the jet. If I read the description properly ohiorider just said he rolled the throttle back not the revs dropped.

Of course water is just one suggestion. I hope ohiorider lets us know the final outcome as I have often seen a question asked and heaps of excellent suggestions given with no final outcome. I would not have thought of a leaking diaphragm but can see how it would cause the issue by causing an over rich mixture.

Steve       

[ohiorider]

If it is water, which is only a suggestion, what the reason for having to roll the throttle back is to stop the vacuum trying to pull a water/fuel mix up through the main jet. What I have never been able to work out is why the water does not just get sucked through the jet and clear itself. It appears to just block the jet. If I read the description properly ohiorider just said he rolled the throttle back not the revs dropped.

Of course water is just one suggestion. I hope ohiorider lets us know the final outcome as I have often seen a question asked and heaps of excellent suggestions given with no final outcome. I would not have thought of a leaking diaphragm but can see how it would cause the issue by causing an over rich mixture.

Steve       

Steve, that is correct.  I held the revs the same, but reduced the amount of throttle I was applying.

Thanks to all for your suggestions.

Bob

[Kiwi_Roy]

A break in the coil lead makes it harder to jump at higher throttle openings, does the R100GS have one coil and a distributor or 2 coils?
if it has 2 you can compare the resistance from side to side, it should measure the same from each cap to chassis
A Guzzi with 2 coils will measure about 8,000 Ohms, the beamer won't be the same but it will be the same on each side.
If it has a distributor you won't be able to measure to chassis because there's a gap but you will still get a circuit from each plug
to the contact inside.

Every time you fix more than one thing you are bound to get the odd glitch, I'm sure your mechanic will soon have it purring again.

[PeteS]

My first thought was diaphrams but that would not account for the problem at idle. Pulling the float bowls should allow you to check for water or maybe a loose jet. Then on to ignition.

Pete

[Vince in Milwaukee]

Not sure if this model has points, but could it be a bad condenser?  My LeMans III was cutting out, and then hitting, and then only running on one cylinder.  My mechanic buddy down in Chicago found the problem - one of my 2 condensers was bad.   

[wirespokes]

The BMW airhead ignitions are very different from the Guzzis - the only real common thing is one set of points driven by a points cam (with one lobe) with a mechanical advance. Or, in the case of electronic ignition which came along in 1981, a mechanical advance driving a Hall Sensor signal to an Amplifier (ICU as it's known).

All airheads run wasted spark - both cylinders get zapped at the same time. One cylinder always gets a spark TDC the exhaust stroke.

Because of that there's no distributor. On the early airheads, there were two oil filled coils, but after 1984 they all had one epoxy coil with dual outputs. The R65 and R80G/S got that system earlier in 1981. And the coil wasn't simple as you'd expect but constructed like two coils in one package. You'd think it would be easiest to construct the things so both outputs would be the same, but that's not the way they did it. There are lots of stories of starting up cold on one cylinder having to do with the coil.

The beauty of a flat twin is the simplicity of the ignition system - no distributor needed.

So, to be clear, the 100 GS in question has an electronic ignition driven by a hall sensor delivering one signal to the ICU (which isn't just a simple amplifier) which then tells the coil to store a charge and then spark both cylinders at the same time.

Anyway, back to the problem at hand - yeah, it could be a diaphram issue, but like everything else here, it doesn't match the normal issues faced with a leaky diaphram. 

I enjoy puzzles like this and all the other views I might not have considered. Agreed - the OP better fill us in on the resolution!

[Tusayan]

FWIW I had a similar problem with my US spec R100GS that ended up being the flapper in the filler neck.  Much to my surprise it was sealing so well when closed that it restricted air flow into the tank.  A slight tweak to open up a crack fixed the issue.

[wirespokes]

Wow - who woulda thought the flapper could seal that well? I've always performed flapper-ectomies earliest opportunity. Get rid of those suckers! What a pain in the ass!

[tetarabra]

Already got the same symptoms on my GS.   Check your floaters levels .  Inexpensive 

[Bob Mehmen]

I had the same problem with my R100RS after carb rebuild, float level too low. If you replaced the head gasket might be a tight valve so it doesn't want idle.

[wirespokes]

If it's starving for fuel, it can be a vacuum building up in the gas tank. An easy check when it happens is to loosen the cap. Those newer caps don't vent - totally sealed. The tank vents through a tube positioned near the top of the inside of the tank just in front of the filler neck. It exits on the bottom of the tank, but forget which side. There are two lines exiting side by side, the other is the gas cap gutter overflow. 

[ohiorider]

Jamie, one of the best BMW wrenches I've encountered over 32 years of BMW ownership, immediately rose to the occasion.  He now runs an independent shop, and as soon as we talked, he volunteered to drive 60 miles to my place to check out the bike.  Now that's service like you don't often get!

I am confident that a small problem is manifesting itself as a major running issue.  Jamie has bailed me out before, when other shops failed.  He'll do it again!

Bob

PS - for a bunch of Guzzi guys, sounds like there's considerable background in BMW Airheads.  Love it!  And thanks to all of you for your comments and suggestions.

[Chuck in Indiana]

PS - for a bunch of Guzzi guys, sounds like there's considerable background in BMW Airheads.  Love it!  And thanks to all of you for your comments and suggestions.

Guzzi guys are motorcycle guys. Many if not most of us have had airheads, too. 

[wirespokes]

I was headed in the Guzzi direction when a friend sidetracked me with a swap for his R90/6 in 1990. Then I was totally submersed in airheads till a couple years ago - the yearning for a Guzzi wouldn't go away. I've had a number of different GSs and G/Ss since 2002 and currently have a 1990 red/white PD and a 93 PD, unridden since the LM gets all the saddle time these days.   

I've rebuilt engines, transmissions, but no final drives (only the splines), currently repair gauges for everyone, and most everything else. And I've ridden them across the country several times. I've had just about every model airhead so am pretty familiar with the whole lineup.

[reidy]

Ohiorider

Did you get the problem on your BMW sorted? I have not been able to sleep at night wondering what the fault was. 

Steve

[jas67]

Guzzi guys are motorcycle guys. Many if not most of us have had airheads, too. 

Or still do!     I've got two (R90S, R75/5), three if you count my R100S project sitting in the corner.

A friend of mine who's been riding for a very long time got interested in Guzzis, because he wanted an R90S back when they were new.
The R90S was $6k.   At the time a 1000SP was like $3,800.     He actually ended up buying a V50 cheap, and then owned a whole series of Guzzis, including, but, not limited to a Mk1 LeMans, Mk3 LeMans, T3, Quota, and who knows what else.   The Quota was a horrendous ownership experience that experienced all sorts of failures.   He then switched to BMW's and has put over 600k miles on them.

[ohiorider]

Ohiorider

Did you get the problem on your BMW sorted? I have not been able to sleep at night wondering what the fault was. 

Steve

Closer to resolving.  Took a lot of measurements (resistance across coil primary terminals, across secondary terminals, plug wires and caps.  Found when it ran on one cylinder, say the right side, we could switch to left side by switching plug wires on the coil.  And ran a few other tests too.  Send this info to Rick Jones, Motorrad Elektrik, today, to give him a chance to opine.

Probably end up being a failed DynaTech coil or the Alpha ignition system.  Hope to hear back from Rick tomorrow.

Pretty much ruled out carbs, after the BMW tech  who has rebuilt a bunch of these things, came to the house and took both apart and reassembled just to confirm everything was ok with he work he'd done.

Bob

[wirespokes]

It's a dual output coil (one high tension lead to each cylinder) so if swapping leads at the coil got the problem to swap sides, that's a pretty good indication the coil is at fault.

[wrbix]

It’s ALWAYS the coil - 

[ohiorider]

I'm hoping its the coil!  One is on the way.  I'll let you know if that was the solution.

EDIT:  What a freak thing to happen! (if it is the coil.)  The ignition was nearly the only thing that wasn't touched during the recent refurbishment.  I'd already installed the Alpha ignition system over 3 years ago, as well as a DynaTech coil at that time.  So to me, it was logical to think the problem might have been carburetor-related.  That'll teach me to doubt the work of one of the best BMW wrenches I've known.

Bob

[wirespokes]

There's a slight possibility it's the ICU, but most likely it's the coil. One output failing isn't an uncommon problem.

[ohiorider]

It was the coil.  After all that drama!  Now I need to borrow Mark's Carb Stix and re-balance the old gal (though I must admit that it sounds and feels like Jamie, the BMW tech, got very close by ear, when he took the carbs apart and reassembled them in my garage looking for something he may have caused.)

Bob

EDIT: Carbs balanced. Running fine!