that only leaves compression
maybe a valve is not seating well or the rings are not sealing perfectly and slow speed operation is where it would first reveal itself
have you done a compression test and even a leak down?
my guess if the engine mechanicals are not the problem.......
I'd go back to looking at ignition and then carburetion
I suppose you could always start by checking your ignition reserve.... this is done by lowering the supply voltage to the coil, while running....... I'll see if I can find the Bosch spec for that
you can test secondary output with a 10 buck calibrated spark tester which will simulate the ignition sparking under full compression load....... but in open air with both a visual and audible confirmation
off any decent tool truck like SnapOn or Mac
you could gap the spark plug tighter to tax the ignition less
15 thou is about as close as you can go without almost assuredly creating a misfire
if you try to gap the plug closer and it alleviates or even the stalling...... you are on your way to finding your ignition problem
would have to be
1)resistor cap high resistance (if it has one and throw it away running it direct thru a non-resistor cap instead.... there is no logical reason to run a resistor cap on a copper wired antique bike)
3)low supply voltage or ignition circuit dropping voltage across ground
or maybe you simply have a vacuum leak or the carb really isn't adjusted perfectly
hard to say but good luck
may be helpful to know if the "misfire" ever occurs while the engine is idling AND loaded in gear........ if so is it a hard or soft miss?
soft would indicate leaness
hard would be richness or ignition
your post regarding my inquiry has revealed your expertise in these matters and has given me several avenues to pursue the solution to my problem. in other words...thank you very much !
i'll post a reply with the results...
a viton tipped float needle is an excellent upgrade for any bing carb but not easy to do on the ones with the tiny float needles that aren't much bigger than a sewing needle
also agreed that a spigot mounted and rubber isolated carb adaptation of a more refined metering device is a good idea
but I know a BMW single can be made to idle and run smoothly even with comparatively cruder, and less than optimally fitted, stock carburetor
... erratic idling was a comon problem on the R26-27. Most attributed it to the vibration at idle that would unseat the gas shut off valve. A common fix was to replace the carb. with one from a jap bike. or better an Italian...
... a BMW single can be made to idle and run smoothly even with comparatively cruder, and less than optimally fitted, stock carburetor
I have had two R27s, a new 1964 on which I put 18K miles and my present 1966 restoration on which I have put 10K miles. At low idle, a miss is likely to occur within 30 seconds of idling, and would stop the engine unless one was quick to give some throttle. With my first R27 the miss was pretty frequent, and I thought it was a normal effect of the engine shaking the float, interrupting the fuel, and leaning out the mixture to cause the miss. I just accepted it.
My second R27 does not do it as much - mainly just when cold. After the engine is hot, it will idle unattended for as long as I need it to. That may be because I now fiddle with the carb, which I did not touch 40 years ago, and have probably got the idle mixture richer. You might try screwing in the idle air screw a 1/4 turn to richen the idle mix.
i just read your 8/25 post... since my original post, i have checked a few things suggested by "euroiron", and i have replaced the original spark plug wire with NKG copper core wire. the miss which will still occur if i set the idle where i like it ( which i guess is too low ), but it has improved significantly. i like your idea of enrichening the idle mixture. i am going to try that approach today.
does anyone know what the factory reccomended rpm for idle was ? are there any clues as to what the rpms are without using a tachometer, like just below or above the point where the shaking or vibration is most evident ?
it's not a wildly cammed machine
and make sure the advance unit for the ignition is functioning smoothly and not trying to stick
it is an often neglected service item which needs removal, disassembly, cleaning/polish. lubing, and back in the bike
the issue that plagues many vintage bike of supply voltage, which can vary wildly depending on battery state of charge and output of the alternator( which will be next to nothing or even a loss at idle) is a pesky one but very real
if you noted improvement with a zero resistance secondary wire...... this is an indicator your idle voltage is very likely giving you some grief
sometimes....... if all components are good...... you can fudge a bit and close (edited) the points gap a tiny bit to increase coil saturation/dwell
also, you want to put a good eye on the points contacts and carefully observe for metal transfer....... even the slightest hint
you can determine if the culprit is the condensor or coil by noting which contact the metal is moving from and too
this is a basic electrical circuit referred to, among technicians, as an LCR..... and it must be properly balanced to avoid other pesky problems associated with metal transfer on the contact points
glad you are making headway and good luck, Sir
for many of them........ due to marginal output of the charging systems...... along with OEM batteries just too small
the cure for many has been
1)go with the best battery that money will buy....... glass matt and as large as can be fitted in place
2)if you don't ride it often, buy a good battery tender
3)ride the bike like hans intended it to be ridden
many trailer queens and show weapons......... not to mention the small handful owned and "ridden" by overly reverent and timid riders
keep all the symptoms of bikes with erratic or low supply voltage
I recall working on a bike for one guy........ kid from the Bronx and he dressed the part and talked a good game
got his bike done
it was back in less than a month with a dead battery
he said he had been riding it but neighbors nearby had told me he was terrified of it and it hadn't seen 30mph
checked the charging system....... all good
recharged battery and tested........ solid
then he and I went for a bike ride
he was just terrified to ride........ all the leathers and t-shirts didn't get him past his fear and lack of skill
Not making any such presumptions about you, I don't know you from Adam, but these bikes were made to be ridden and they don't suffer being put on a pedstal and only given what they want on special occassions
Thanks for tolerating my rants
thanks, will anderson firstname.lastname@example.org