Very Intermittent Stumble/Bogging, R75/5 1973

[bayleyhazen]

Intermittent Stumble/Bogging, R75/5 1973

Hi All

I have spent the winter doing some routine maintenance on an LWB R75/5 which I bought last fall, so we're still getting acquainted with each other. Mostly it's running very well - nice and smooth, starts right up when cold, etc. I have done three long test rides of about 100 miles each, in addition to several smaller rides for carb sync purposes.

HOWEVER. I have a problem, which is that on each of my three big test rides there has been an instance of a bogging down which comes out of nowhere, seems to only happen after an hour or so of riding, and then doesn't happen again. It feels like fuel starvation. The bogging lasts anywhere from a few seconds to a minute or two, but it's pretty pronounced, and adding throttle either powers the bike into a rev band where the bogging seems to go away, or increases/induces further bogging. The bogging feels like loss of power for a second or two, then fine, then loss of power, then fine - seemingly random. Sort of a brap, brrrrr, brap, brrrrrrrr. I've tried to induce the bog intentionally, but can't do it - tried going up and down steep hills at various speeds, tried tight, slow maneuvering, tried hard braking and accelerating, going from wide open to closed and back again, and even going down washboard dirt roads, but no dice. After the bogging starts (and goes), the next couple of full stops on the bike result in an idle that is so low the engine lopes and cuts out. Afterwards the bike is totally back to normal.

I'm going grey trying to figure out what it is. I've disassembled and cleaned the carbs completely twice, and replaced all of the O-rings. Diaphragms and needles look fine, as far as I can tell, though one needle has some scuffs like someone used pliers to change the clip position (clip positions and jets are correct according to the Bing book). Shooting starter fluid at the intake hoses while running causes no change in RPM, and compression looks good. Carbs are well synced using the shorting method, though I know I need to adjust the cable free play just off idle. Bike also has new coils, and new plugs. Petcock screens were also cleaned, but petcocks were not totally disassembled. Not much gunk in there, but the gunk was on both sides of the screens. Ignition is controlled by an old Boyer Bransden electronic unit which appears to be working fine and setup properly, confirmed by dynamic testing with a timing light.

So, any ideas? My best theory at this stage is still intermittent fuel loss, so I'm going to double-check the floats and fit some bigger sintered in-line fuel filters. My other theory is an intermittent electrical short, but I've checked and cleaned every connection I can easily get to, and nothing looks pinched or frayed. I'm hoping to not have to get into the headlight pod and check everything there, but that's probably next. I'm really just guessing at this point. Could it be sticking carb slides?

Thanks, appreciate the help. I love finding and fixing problems on these bikes, but I'm stumped!

Other issues I'm having but may be unrelated - carb on the right side has a particularly unresponsive idle mix screw. It responds slower and has a comparatively wide band of no-effect adjustment. Other issue is an occasional starter solenoid clicking and not cranking, but fully charged battery seems to solve that. Also, one time I turned the ignition on and the GEN bulb didn't light up. Next time it did, and that symptom has never repeated since. I also spliced a pigtail into the wiring harness so that the parking light would work, but that has since stopped working - another sign I need to get back into the pod, but not sure if anything in there could even cause this bogging.

Thanks again.

 

[20774]

Welcome to the forum! I like your discussion of what's going on and what you've looked into. You didn't mention valve clearances, although I don't think that's the issue here. Tight valves tend to show up more around idle.

My inclination leads me to think electrical...you have a few annoying issues beyond the loss of power. I'd be sure that the Boyer has good grounds...the Dyna III electronic ignition can be affect in a similar fashion of the replacement points plated doesn't have a good ground. Is there anyway you could revert to condenser/points and see if the problem goes away? You should also find the starter relay under the tank and be sure that the terminals are clean of corrosion.

 

[bayleyhazen]

Welcome to the forum! I like your discussion of what's going on and what you've looked into. You didn't mention valve clearances, although I don't think that's the issue here. Tight valves tend to show up more around idle.

My inclination leads me to think electrical...you have a few annoying issues beyond the loss of power. I'd be sure that the Boyer has good grounds...the Dyna III electronic ignition can be affect in a similar fashion of the replacement points plated doesn't have a good ground. Is there anyway you could revert to condenser/points and see if the problem goes away? You should also find the starter relay under the tank and be sure that the terminals are clean of corrosion.

Howdy Kurt - Valve clearances have been set and checked a couple of times, and they look good. One job I messed with but had never done before was setting the end-play on the rocker arm for the left hand exhaust valve. It was moving close to 1mm up and down, so I gently squeezed those pillow blocks until the play was just enough to slightly displace the oil film at each end of the arm shaft, then retorqued those studs to spec. I think I did it right, but let me know.

The Boyer was my first stop in hunting for an earlier ignition problem, least of all because it's an iteration of the BB that I've been unable to identify. At the time I had some weak spark issues that I think I managed to trace to incorrect plugs and a failing coil. The unit's grounds at the battery are clean and bright - I'm not sure how to check the ground on the backing plate aside from using a test light on the screw heads that hold it on. The seal strip around the breaker housing is totally shot though, and there's usually a tiny quantity of oil collected at the very bottom of that little compartment. PO has left in place the little metal clasp that ordinarily would retain the points wire, and it's sometimes dipping slightly into that oil.

Funnily enough, a while back I impulse-bought on eBay a complete points assembly, including the rubber hose, condenser, points, advance unit, backing plate, and even the original screws, for $50! That may mean they're buggered up, but seller claimed they're good, and they look fine. The reason it's not just running on points now is because when I installed them I was getting a double image in the timing hole at idle. Because I'm a perfectionist with this bike, mechanically speaking, I got a slight pain in my teeth not knowing what that was or how to fix it - a mystery points assembly with what (according to Snowbum) is a Bosch advance unit from a /6 or /7, or a bent cam shaft tip, or timing chain wear? Is it something simpler? The rotor from the Boyer unit was a total ballache to work off the end of the shaft, so it's entirely possible that I bent it slightly. Perhaps I need to get the dial indicators out and check that shaft for straightness, but at that stage (chasing a weak spark) I was hesitant to open up another can of worms of issues to diagnose. I'd love to toss the electronic unit, as an aside.

At one point I sat up late with a photocopy of the wiring diagram from the BMW Reparaturanleitung and a jar of colored pencils, and came up with the relay as a possibly culprit too. I took it off the bike and brightened up those contacts, then brightened the female ends on the bike. I saw that someone had uncrimped the edges of the relay at one stage, so I got in there too. Everything looks super clean, and I sanded the little contacts in there with a couple passes of 800 grit. Put everything back together; no change.

Thanks a lot, this really helps!

 

[20774]

Any special requirements for cooling of the Boyer "brains"? On my Dyna III, I was able to mount the box on the frame cross-tube in front of the bike...something that didn't exist for pre /7 models AFAIK. That puts my box out in direct air flow.

 

[lmo1131]

My 2 Pfennigs... install the points/condenser/advance unit. Replace the coils. Throw the Boyer in the trash... don't mind me, I'm a purist. It sounds, to me, like the only possibility for mal-function IS the Boyer.

The click-ity-click starter issue? Search "crickets". The OEM starter relay is a notorious weakness in the /5 starter circuit. A call to Motorrad Elektric might be a good place to start for a new relay.

Read> http://bmwmotorcycletech.info/startingprobs.htm

In part ( about half way down the page).

The /5, ONLY, had a peculiarity; and, the circuitry arrangement needs understanding. The battery feeds power to the ignition switch, as in all the airheads, but there was NO handlebars-mounted emergency cutoff (KILL) switch. From the ignition switch in the /5, the battery power fed a special version metal can starter relay that has had various names, including Starter Lockout Relay. It is located furthest forward on the left of the backbone under the fuel tank. This is not just a simple relay. It incorporates a simple transistor circuit; which, when stock (that is, UNmodified), is unfortunately overly-sensitive to temperature (possibly failing at even modestly cool temperatures) and it is also sensitive to even slightly decreased battery charge. When the 'problem' occurs, the relay may chatter, or sound like a cricket, and may even power the starter solenoid that way too; making a much louder cricket or rapid clickety-clackety noise. To an old time mechanic, it sounds just exactly like what happens to cars with a quite low battery. The transistor circuit's purpose was to eliminate possible starter relay operation (as in YOU trying to start an already running engine!) as soon as the alternator is spinning at engine idle or, actually, somewhat above idle. There are low battery and low temperature problems with that starter relay circuit, and a modification is HIGHLY recommended by me, and there is an article on this website specifically dealing with that problem, and the relatively easy fix: http://bmwmotorcycletech.info/slash5cricket.htm

 

[pglaves]

Fuel tank venting?

 

[lmo1131]

Second thoughts, based on discussion with folks at SovietSteeds.com. Ref Ural (Ukrainian) sidecar rig, with similar performance issue (stumble/hesitation at full speed) ... ignition system is "electronic", ignition sensor interrupter is a fabricated 2-piece unit.; the two parts, over time, can become loose and cause exactly the same problem you are describing.

Granted... we're not talking about a Ural, but we are talking "electronic" ignition systems ... check over the Boyer unit itself for loose/broken components. I'm not familiar with it myself, but it must have similarities.


EDIT

Old discussion with link to Boyer https://forums.bmwmoa.org/showthread.php?58533-1978-R100-7-Boyer-Bransden-MkIII-Electronic-ignition

 

[bayleyhazen]

Any special requirements for cooling of the Boyer "brains"? On my Dyna III, I was able to mount the box on the frame cross-tube in front of the bike...something that didn't exist for pre /7 models AFAIK. That puts my box out in direct air flow.

Not to my knowledge, but I'm happy to be corrected on that. I would have thought that its current placement would be fairly conducive to cooling. Even though the Boyer Bransden instructions I got (in order to check the installation) specify placing the box behind the coils, PO placed it above the voltage regulator, right up at the front of the tank where the airflow is pretty good. I've considered modifying the aluminum bracket they sit in so it acts as an air scoop, if indeed temperature makes a difference.

As an update, I got into the headlight pod today and it's a real mess - I cleaned up some connections and replaced some aging aftermarket connectors that had at one point been used. Parking light works perfectly again. I also found this little beauty (photo below), which had bad burn marks on either side of the fuse (the fuse holder is a 15A blade fuse replacement, having a seemingly heavier gauge wire than the original, installed by PO). I cut out the burnt sections and reconnected the fuse to the harness.

I noticed that the wiring has been altered as compared to the workshop manual (both the Reparaturanleitung and Clymer). One of the fuses is supposed to have a single wire coming from the ignition switch into the fuse, then one cable out to the plastic junction box (which also had a slight melt mark on its bottom), and another cable to the annunciator lights in series, before getting to the flasher relay. This bike is wired so that the Gen light is hooked up to the same side of the fuse as the wire from the ignition switch, and the other three lights are hooked up in series after the fuse, as it should be.

I know the Gen light is important to the ignition and charging systems - could this unorthodox wiring have caused in any way either the scorching on the fuse wire, or the intermittent bogging? I have left it as-is for now. Will ultimately want to rewire everything correctly, but I was lacking a suitable connector in the parts bin.

I also checked the camshaft tip for eccentricity, but it's dead straight. Not sure what would be causing the double image in the mechanical ignition, so I left it with the electronic setup for now, and double checked the timing. A 20 mile test ride later, and the carbs were completely synced, including the cable free play. No issues. Would have tested for longer, but I hit the reserve and wouldn't have made it to the nearest station that stocks hi-test 0% ethanol gas.

Sorry for the David Foster Wallace-length posts, it's tough to explain this clearly sometimes.

 

[bayleyhazen]

My 2 Pfennigs... install the points/condenser/advance unit. Replace the coils. Throw the Boyer in the trash... don't mind me, I'm a purist. It sounds, to me, like the only possibility for mal-function IS the Boyer.

The click-ity-click starter issue? Search "crickets". The OEM starter relay is a notorious weakness in the /5 starter circuit. A call to Motorrad Elektric might be a good place to start for a new relay.

Plan is to go back to mechanical ignition, but I need to track down what can cause a double image - camshaft tip is A-OK. I may just install it and split the difference with the images, but I'm not sure I can get away with that.

The clicking was definitely coming from the starter motor; I think the battery was just low after repeated cranking trying to get the mechanical ignition timing set. Hasn't happened since the battery's been full. The cricket, as I understand it, is the sound of the tiny contacts in the relay clacking together, rather cricket-like. The starter sound could be more aptly called the Woodpecker, more like an impact driver. The sound ambiguity has, I'm sure, caused many a person to pay the hundo and a half for a fruitless replacement relay. Mine is pretty beat up, but I think it's working properly. Will get it replaced some day, when I'm swimming in hundos.

 

[guenner]

Bogging: Seems it still has an ignitable fuel mixture and a spark...but just at the wrong time for a short period.

So the idea to put in an OEM ignition sensor may show it is the Boyer ignition...or not. But for that you need a known-to-be-good one.

On the picture is a white wire with a burn mark. Where is this going to?

A 15A fuse? Unless it is a 12-14 Gauge wire coming straight from the battery for some 'gadget', it is too high for anything on the bike. I would replace it with a 7.5A fuse and see what happens.

Idle mixture screw: Did you check the o-ring in there?

/Guenther

 

[pglaves]

Plan is to go back to mechanical ignition, but I need to track down what can cause a double image - camshaft tip is A-OK. I may just install it and split the difference with the images, but I'm not sure I can get away with that.

The clicking was definitely coming from the starter motor; I think the battery was just low after repeated cranking trying to get the mechanical ignition timing set. Hasn't happened since the battery's been full. The cricket, as I understand it, is the sound of the tiny contacts in the relay clacking together, rather cricket-like. The starter sound could be more aptly called the Woodpecker, more like an impact driver. The sound ambiguity has, I'm sure, caused many a person to pay the hundo and a half for a fruitless replacement relay. Mine is pretty beat up, but I think it's working properly. Will get it replaced some day, when I'm swimming in hundos.

Every Airhead I ever owned had a double image in the timing window. Bent tip, uneven points cam lobes, etc can cause it. I never could completely eliminate it. I just split the difference and rode on.

 

[cycleman2]

Couple of thoughts.

Electronic ignitions can be damaged buy using the plug shorting method to adjust carbs. That method will work fine with points. I owned a /5 and the stock headlight didn't have any fuses, so somebody has been into the headlight. Your initial description of the way the bike ran didn't really indicate electrical to me.

Somewhere in your posts you mentioned that one of the adjuster screws on a carb had no real effect. That would be my first starting point. First make sure you have cable slack at the carbs, gently lift the cable and make sure you have 1-2 mm of slack, the choke is all the way off and set the mixture screw ( lower front of carb ) to the bing recommendation for your carbs. Adjust idle screw ( between carb and engine block ), engine should idle in the area of 1000 rpm. If you can get your hands on a set of sync gauges that will help you setting up the carbs. This is the first area I would deal with. If that doesn't work then taking the carbs apart, checking everything again, including floats. Clean everything really, really, really well. Most important.

Carbs on the airheads are usually 90 % of the problem. Get them setup right and the bike will run flawlessly. Always use new parts when rebuilding. If you are a newbie with airheads, try and find somebody locally that is familiar with these bikes. Good luck.

 

[20774]

Electronic ignitions can be damaged buy using the plug shorting method to adjust carbs. That method will work fine with points.

Any ignition can be damaged by improper procedures done to the spark. The issue with damage is not providing a secure path for the spark energy to flow to ground. If on a points setup, you pull the spark plug cap so there's a large air gap between cap and plug, that can damage the coils. Same thing would happen with an electronic ignition. Using the shorting method properly, providing secure connections between shorted and not shorted, will not result in harm to the electronic ignition. If I recall correctly at the 2004 National at Spokane, Matt Parkhouse did this method with an electronic ignition.

When done right, no problems!

 

[bayleyhazen]

Update

It's been a while, so I thought I'd give you guys an update on what has been happening.

The bogging has basically gone away entirely, but there's more to it than that.

I replaced the coils with the newer Bosch super coils since the original ones were looking at bit long in the tooth. Also, the starter relay showed evidence of someone being into it before me, so after a rain-related starter non-function episode, I replaced that with a new unit from Max and the bike cranks perfectly every time now.

The stumbling has stopped, but now, very occasionally, and usually once on each ride, the ignition dies completely. Coming to a stop and cycling the ignition on and off seems to solve this, and the bike fires right back up.

I recently tried replacing the electronic ignition with the mechanical setup again, given that I've heard that double images in the timing hole are not uncommon. The mechanical setup was a complete set, but a used one I got on eBay or $50. For the life of me I could not get the timing set correctly. After setting the points gap correctly, and starting with the points plate in the middle of its adjustment range, I still could not rotate the points plate far enough clockwise to get the S mark to show up where it was supposed to. The bike was very hard starting, and ran like a busted lawnmower with plenty of backfiring to boot. I even tried reducing the points gap to give me a bit more travel in the points plate, but no dice. Just to make sure I wasn't being a muppet, I static-timed the bike with an ohm-meter, and nowhere in the adjustment range was correct. Snowbum writes about some cheap points that are impossible to set right, so I ordered a new set from EuroMotoElectrics, after refitting the electronic setup - just waiting on the new points to turn up. As far as I know there should be no major differences in the advance units or backing plates between airhead models (according to the Bosch part no. on the advance unit, it's the version used from the /6 onwards. The advance unit I have does have a tiny bit of slack in the springs which might be exacerbating things, so I've ordered a new set.

When setting the ignition timing for the electronic setup, I noticed that my timing light was not flashing consistently, on one side worse than the other. I lay one of the plugs on the cylinder head, and saw that I was only getting a weak, yellow spark. I then accidentally hit the turn signal, and saw the timing light flashing every time the turn signal relay went off. After cleaning and tightening the ground at the transmission, this problem went away, but the weak and intermittent spark was still an issue. Nonetheless managed to set the timing advance correctly.

In the last 500 miles since doing this work, the bike fires up immediately, settles into a nice idle, and runs well, aside from a very slight loping just off-idle at about 30-40mph at 3k RPM. Carbs have been recently synced and all seems well there, and yes, I synced the throttle cables at about 2kRPM. Nonetheless, every now and again the ignition dies completely, and a quick cycle on the ignition sorts it out.

I'm still at a loss to find this gremlin. Is it a matter of a bad ground, a short somewhere, or is the electronic ignition just toast? Could that be causing weak, inconsistent sparking, as well as the ignition quitting? Plugs are quite black too, which leads me to believe we're running over-rich despite repeated succesful carb syncs, and fuel mileage is 30-35mpg when I think I should be getting about 50 according to the BMW users manual. My gut feeling is leading me to believe that something is not well in the headlight pod, but I can't put my finger on what. Also, the Boyer instructions for a similar unit to mine specify running the ground to the old condenser mount above the alternator. My unit's ground runs directly to the negative post on the battery. Does that matter?

Thanks as always, the last round of comments were very helpful. I have ordered the Chitech manual.

 

[lmo1131]

Just so you have a visual on the "bad" points situation.

Old OEM points (NORA).

Aftermarket Chinese-made
note the difference in the length of the rubbing blocks

Easy to understand why you ran out of adjustment

 

[20774]

I'm glad you mentioned that you changed the points gap setting in order to bring the ignition into time when you ran out of slot on the backing plate. So often I hear that people elongated the slots. This is not necessary and really shouldn't be done. Just change the points gap...it's not sacrosanct at 0.016" (from memory). A little deviation on either side of that makes a big difference in timing position. When the points are closed, the coils are being saturated with electrical current. In a two cylinder configuration, the saturation time is plenty long enough with small changes in points gap. On multi-cylinder engines, certainly 6 and 8 cylinders, the gap becomes more important.

 

[bayleyhazen]

Thanks Lew - that's definitely helpful, but my points look identical to the correct ones. Like an eedjit I ordered the wrong replacement points from Max, so I haven't gotten the correct replacement to really closely compare. My advance unit is definitely the Bosch ending in 012 which I believe was for the /6 and /7. This changed the dwell angle, but did it change to the point where the cam lobe of a /5 won't work with it?

Symptoms are still a weak spark, and occasional cutting out. It has gotten to the point now where when it cuts out, I pull in the clutch, pop the ignition off and back on again, and hit the starter. Bike fires right up, I put it back into gear and keep going. Don't even bother stopping anymore. Where is this short happening? I'm about to pull the trigger on a Dyna III unless anyone has a suggestion. I hate the idea of throwing money at the problem, but I'm running out of stuff in the ignition circuit to replace. New NGK plug wires go in tomorrow because WTFN.

Just so you have a visual on the "bad" points situation.

Old OEM points (NORA).

Aftermarket Chinese-made
note the difference in the length of the rubbing blocks

Easy to understand why you ran out of adjustment

 

[jad01]

When it cuts out, is the entire electrical system going dark, or does it just affect the engine operation (leaving the instrument lights working, etc.)?

 

[bayleyhazen]

When it cuts out, is the entire electrical system going dark, or does it just affect the engine operation (leaving the instrument lights working, etc.)?

It's just the engine operation - lights stay on, neutral light works properly, and oil pressure light comes on a second or two after cutoff. Charge light stays dark, not sure about instruments as this hasn't happened at night yet.

Engine cranks fine after cutoff but does not fire the bike up. Resetting the ignition and cranking again restarts the bike.

 

[lmo1131]

Resetting the ignition and cranking again restarts the bike.

Well, the /5 has no "kill switch", so by "resetting" the ignition do you mean cycling (opening and closing) the switch clears the problem? There isn't a whole lot that can go wrong with that switch ( it's only half a jump ahead to twisting two wires together ), but what comes to mind is the "spring" contractor being weakened/deformed by the previous owner pounding the "key" into the switch.

The "nail", as some refer to the key, often becomes difficult to insert into the nacelle switch cover and giving it a pop with the palm of the hand will help it along... for a while. But pounding the key into the barrel assembly that it rotates in can weaken the internal spring clip(s) that hold the key tightly; if weak/worn the key might be backing off of the contactor momentarily breaking the ignition circuit. It is re-buildable, but the switch needs to come out of the nacelle.

https://forums.bmwmoa.org/showthread.php?86936-75-5-ignition-repair-replacement

Duane Ausherman has some more information on this > http://w6rec.com/duane/bmw/ignkeys/index.htm

Unfortunately, the hardest part about servicing the switch on a /5 is getting it out of the headlight nacelle; it's secured by four thin metal "tangs" that are bent over the fiber board (a clever feat of 18th century German engineering). The tangs have a tendency to work harden when unbent, and are prone to breaking off (when re-bending during re-installation of the switch) leaving no way to re-install the switch. The tangs can be annealed, with a micro-torch, but obviously you'll want the nacelle off of the motorcycle for this task.

If you were referring to an electronic ignition system recovering after "resetting" the switch I'd say it was a Hall sensor going away.