1976 BMW R90/6 left cylinder not working

It is a big problem. This popping can turn into stalling in dangerous
situations, like when you roll off the gas to approach a stop sign, and
the idiot behind you in a car expects you to immediately move away from
the stop and he hits you from behind. Or, you could be in a real slow
tight turn and be depending on the engine to keep running to keep the
bike upright, and it will stall and you'll tip over.

Your idle jets and idle mixture passages have dirt in them. That's
because you did the carb cleaning half-assed with WD40.

Find the idle mixture screw underneath the carb. Turn it all the way IN
until it's lightly seated. Count the number of turns and parts of a
turn. Write it down. Unscrew the idle mixture screw. Pay attention to
the assembly of springs, washers, and little rubber o-rings if there
are any. Don't lose the o-rings up in the hole.

Once the screws are out, you can squirt real aerosol carb cleaner like
Berryman's B-12, STP, or Gumout up through the idle mixture hole.
Forget about WD40, it won't do the job.

Also squirt it through the pilot air jet hole which may be in the
intake bell or under the diaphragm. There are online drawings for Bing
carburetors if you need to look at diagrams to find the parts I'm
talking about.

Squirting carb cleaner through the pilot air jet hole should cause carb
cleaner to squirt freely out through the idle jet in the float bowl, as
well as some transition holes downstream of the throttle butterflies.
If you take the carb off for a proper cleaning with aerosol carb
cleaner you can remove the float bowls and squirt carb cleaner directly
through the idle jet.

When you go to reinstall the idle mixture screws, springs, washers,
and o-rings, make sure it's in the exact order them came out. And screw
the *same* idle mixture screw back into the same hole it came out of
until it's lightly seated. Then screw it back out the exact same number
of turns you counted when you removed it.

When you ride the bike the exhaust popping will be gone...

 

Might be running slightly lean. What color are the plugs ?
Both the same ? Might also want to take a good look for
leaks in the intake and exhaust. Does it still have all
the emissions control crud on it ?

 

Hey guys. I'm back with another question. First of all, did what you
said and did a THOURGH carb cleaning, unscrewing mixture screws, etc.
and it now runs perfectly, no backfireing. But, I think i now have
some sort of electrical problem. A while ago, i was riding and the
engine suddenly died. Tried to start, wouldnt crank. So i left it and
came back later and was able to kick start it. When i as just about
home, i turned on the headlight and the engine suddleny diead. Left it
for night and put the charger on the battery. Now, it sometimes starts
fine, and other times the solenoid will click and then nothing happens
and the indicator lights all day. Seems like it's shorting out
somewhere, Other times, it will crank and start up fine. Any ideas?

 

Dead battery?

Does it start and run Ok with jump leads from another decent battery? If
so, that's your problem.

 

wrote in @z14g2000cwz.googlegroups.com:

Change the battery.

Did you balance the carbs too? If not, do it. You will be amazed how much
bettter it will run.

r

 

Batteries do not last forever. I got two years MAXIMUM out of original
equipment Yuasa batteries before the invention of sulf-stop, which is
their anti-sulfation technology. Yuasa seems to have bought up the US
battery manufacturers, at least those who make motorcycle batteries...

If you discharge a battery (maybe because your charging system isn't
working right, or maybe you rode the motorcycle in hot weather and a
lot of water evaporated out of the electrolyte) and you don't
immediately recharge it, lead sulfate forms on the plates, killing the
battery.

You might as well figure on getting a new battery if you inspect all
the wiring and don't find any loose connections.

The best service life I got out of a Yuasa battery with sulf-stop
technology was five years. Then they came along with maintenance-free
batteries that never need water. The first one of those lasted eight
years.

Even a maintenance-free battery will eventually evaporate all of its
water. Then it will sulfate the plates.

Battery plates used in small starting batteries are made of spongy lead
so the electrolyte can penetrate deep into the plate for more cranking
amp capacity out of a small battery. That's how car and motorcycle
batteries package a lot of power in a small space. If they didn't use
spongy lead, your car battery would be as big as your engine.

But, if the spongy lead plates are allowed to sulfate, the electrolyte
cannot get deep into the plates and, when you do charge the battery,
you only get a charge on the surface of the plates.

When your battery has a surface charge, it will have the correct
voltage and the specific gravity of the electrolyte will be correct.
But, when
you turn on the ignition switch, maybe the lights will come on for a
second and then the whole ignition system goes dead, leaving you
scratching your head wondering what happened.

A surface-charged battery is just like a big capacitor. It immediately
totally discharges and is done, it needs to be immediately recharged.
As soon as you turn the ignition switch on, it discharges again. It's
done for.

Another problem with spongy lead plates is that particles of the lead
will fall out of the plates and sit on the bottom of the battery case.
If enough of this lead builds up in the bottom, it can short out
adjacent plates.

If your motorcycle battery has a clear plastic case, you can look up at
it from underneath and see the silvery mass of lead particles in the
bottom of the case...

 

No, im postive its not the battery, its fully charged and brand new.
See, sometimes it wont crank but the soloneid will click. I try mabey
4, 5 times in a row and it wont crank. Then sixth, boom, cranks
prefectly and starts up fine. What i think is happending is that
somehow when its just about to crank, something shorts the whole system
out, cause all the lights go off to. I then disconect battery and try
again, sometimes it works right away, sometimes it will short again
for a few times, then crank.

 

I doubt that "something shorts the whole system out". Most
non-technical types don't know what a "short" is. They say "short" when
actually what they have is an OPEN circuit.

I think that what you have is an *intermittent open circuit" due to bad
electrical connections. If you had a real "short circuit" in any
circuit except the starter circuit, you'd be blowing fuses.

A short circuit occurs when electricity finds a shortcut around the
load that it is supposed to operate. Suppose you have a simple circuit
containing a battery, a switch, a light bulb, and wires connecting the
battery to the switch, the light bulb, and back to the battery.

Now, suppose the electricity finds a way to go directly through the
wires and back to the battery. *That's* a short. The light bulb was
supposed to use the electricity from the battery, turn that electricity
into heat, the heat was supposed to make light, and almost all the
electricity except just a tiny few volts were suposed to be used up by
the light bulb.

But, suppose the lightbulb socket had a direct connection so all the
power didn't have to go through the light bulb. There is enough power
to make the bulb filament hot, but that power doesn't all have to go
through the bulb filament so IT HEATS UP THE WIRE INSTEAD!

That's why there are fuses in all the circuits except for the starter
power circuit...

When you try to start your BMW, you might have a short inside the
starter armature. That's a very low resistance piece of electrical
equipment. it normally gets hotter than hell PDQ. Or, one of the
starter brushes might
be grounded out, short circuiting the starter, and using up all the
power.

That's another thing that confuses electrical newbies. People talk
about
electrical "ground" and the newbies figure that the frame is "ground"
and that all the elctricity just somehow *disappears* into ground when
the wire goes into the frame. No, no, no, no, no! The motorcycle's
engine and frame may be "ground", but all the engineers are doing is
using it for a return wire to the battery. The battery ground cable
usually comes off the
battery's negative terminal and it hooks up to the frame. There may
also be a second ground cable that goes from the frame to the engine,
making sure that the engine is "grounded", that it can make a complete
return path from the spark plugs and the starter and anything else that
is "grounded" to the engine.

But, even that extra "ground" cable is NO GUARANTEE that you'll get a
complete circuit from your battery, out the battery cable, through the
solenoid, through the armature in the starter, through the engine
block, back out that "ground cable", through the frame, and finally
travel through the battery negative cable to the battery. A bad
connection anywhere will stop the starter from working.

We even had a guy in here complaining his starter wasn't working and it
turned out that(after he went through all the wiring and cleaned all
the connections, including the "ground" connections on the frame), his
engine block was so corroded right where the starter bolted to the
engine, he wasn't getting a good "ground"...

So, you need to go through all the electrical connections around the
battery and especially follow out the main power bus circuit to the
fuse box. A "bus" is a main power feed that everything else is hooked
up to. Whatever power that comes out of your generator or alternator
and goes to the battery will also usually go to the main power bus.
That's where you get the main "juice" to power your ignition, lights,
turn signals, etc...

Look for dirty or loose connections everywhere on the main power bus...

Loose connections get hot, they melt plastic and discolor the
insulation on bullet type connectors.

 

<snip>

kk, with the best will in the world....

I bow in respect at your technical knowledge, but - a plea from the
heart - your posts are in danger of tripping my binary filter ;-))

Find an editor!

 

My guesses would be either

your battery is low
or
you've got a loose wire or other bad connection
or
your starter is in need of some rework
or
some combination of the above.

Incidentally, my guess is that the click you're hearing is
the starter relay rather than the solenoid. The starter
relay is what energizes the solenoid.

I agree with Krusty that it's not a short.

Get a voltmeter, check the battery voltage, then take a careful
look at all the battery cables, especially anything you may
have disconnected recently.

 

Hmm. The reason i was suspecting a short was because everything went
off, idicator lights, headlight, voltmeter went down, clock stopped. I
dont't think the starter is normally in series with the lights, clock,
etc. I was figuring that some how maybe the started relay or solenoid
was shorting out the system when it was mechanivly moved. The OTHER
reason i suspected a short is be cause the battery was dead. I've been
riding it moderatly for a while and boom! One day the battery is
totally dead. That smells like a short to me. I think we can defintly
rule out the battery as a CAUSE of the problem. After all, how would a
battery cause the bike to stop ONCE its running. Btw, I confirmed that
the alternator works fine by starting it up and then disconnecting the
battery - it kept running fine. I was thinking of loose wires, but i
couldnt find anything loose near the coils and voltage regulator under
the tank. I suspect that if there was some sort of bad connecting, it
would be near the starter.

Another idea I had (just a thought) was that something was wrong with
the points. If the points were bad, the coil could be drawing current
longer than its supposed to, draining the battery. But then again, the
alternator works fine so that shouldnt make a difference. Also, if the
points were bad, I suppose the cylinder firing would sound wierd.
These are just vauge thoughts. Any other suggestions would be great
before I start working on it. Thanks.

 

wrote in @g14g2000cwa.googlegroups.com:

Gabe,

Your chronology of events is not 100% clear to me but be that as it may, I
will try to help you with your problem.

These issues lead me to think you have more than one problem.

Your assesment about a short may be wrong. It could be an open. Check
your fuses. I am not familiar with BMW, but on many bikes there is a
rather large "main" fuse. These can develop physical cracks that will
intermittently cause a total loss of power to the bike (no lights, bike
stops, etc). You have to check this fuse by removing the fuse as cracks are
almost always invisible. This failure does not explain the battery being
dead however.

Clean your battery connections, particularly the positive terminal. Using
a suitable tool, scrape the terminals until you see shiny metal. Lead can
and will corrode and this corrosion transforms lead from a conductor to an
insulator. Thankfully this corrosion occurs at the surface and it can be
removed by scraping it off. Lead oxide can be pretty tough so you might
have to use a small file. This could account for the battery being dead.

You may be right about the short, but it may be where you least expect
it... Internal to the battery itself. Batteries can be intermittently bad.
What happens is that as a battery ages, material comes off the plates and
collects in the bottom of the battery. When the level of this conductive
junk gets high enough, it will short out the plates and the battery will
discharge quite rapidly even though it is disconnected.

Try this test. If the battery is charged and can start the bike,
disconnect it from the bike and let it sit for 8 hours or more. The longer
the better. Connect it and try and start the bike. If all it does is
click or crank the engine slowly, then your battery is bad.

Also try the following test. WIth the battery fully charged and with the
bike not running, turn the headlight on and leave it on for 3 minutes.
Then try to start the bike. If it doesn't crank or cranks the engine
slowly, the battery is weak/bad.

Do you have a voltmeter? That could prove very useful right about now.

pierce

 

Thanks for the advice but let me try to explain the problem better.
First off, the battery is brand new, about a month old. The terminals
are in great shape and it is fully charged (is able to crank the engine
fine and checked voltage with voltmeter). There is absolutly no
corrosion.

The wierd problems started about a month ago. I was riding and the
engine suddenly stopped. It wouldn't crank up so i left it. Came back
a bit later and was able to kick start it. I figured the battery was
dead. I almost got home when i switched on the headlight and imiedtly
the engine dies. I disconnect the battery and put it on the charger.

Few days later:

Battery is hooked back up to bike and i decied to see if it starts up.
Press the button and it starts up fine, no problem. Idles perfectly
too. I come back a few hours later to try it out. I press the button.
I hear a click and imiedtly all the lights, voltmeter, clock, goes
off. I imiedtly disconnect the battery and try again: same thing
happens about 4 times in a row. Finnaly, on the fith time, it cranks
and starts up perfectly. I stop the engine and try it again: no start,
click and lights go off. I then try again and it starts up perfectly.

Now, the first question is WHY did the engine STOP in the first place
while i was riding. There's no way that a bad or dead battery would
cause the engine to STOP running once its started. As i said before,
the alternator works fine because i was able to disconect the battery
and the engine kept running. Once the engine is started, the alternator
provides all the current neaded. The other question is why did the
battery suddenly go dead. One day it's fine, and the next it's dead.
That's why i suspected a short, not open circuit. I think that
whatever caused the battery to go dead while the bike was parked
overnight also caused the engine to stop while running. I mean,
imagine if the bike's 12 volt system shorted out, lets say close to the
battery, not blowing a fuse. The battery would quckly drain. A short
would also stop current from flowing through the coils and no spark,
stopping engine.

 

wrote in @g14g2000cwa.googlegroups.com:

I think you should concentrate on one issue at a time. I have seen cases
where salt and corrosion built up and caused a low resistance short to
ground. It was odd as the fixture would get warm without the lightbulb
being in it. This sort of situation could be your short.

As for your excessive drain on the battery, that is a tough one. It has
always been tough to trace out even on cars. I have fixed car problems
very similar to your situation. The short version of the most memorable
one is that the trunk lights were staying on, draining the battery
overnight. How did I find it? First I measured the current that was
being drawn with nothing turned on. Then I pulled fuses one by one until I
found the circuit affected. I actually had to get in the trunk and have
someone open and close the lid a few times. Once in a while the lights
would go off. I adjusted the switch and all was fine. One can use a
voltmeter to determine if something is drawing current. Disconnect one
battery lead and connect the voltmeter to the disconnected lead and to the
battery. If there is nothing drawing current, then the voltage will be
zero. Conversly if there is a load the voltage will rise to 12 volts.
This sort of testing will allow you to "divide and conquer."

pierce

 

I was thinking some more about your issue. It could be the starter
solenoid that is your problem. There could be some contaminant inside
causing a low resistance path to ground. One cannot discount anything
especially on something that is as old as your bike.

pierce

 

If you had a bad ground this would cause most of the symptoms you're
seeing. I don't think disconnecting the battery was such a hot idea.
A better technique would be to measure the battery voltage.

In spite of some of what you've seen, I'd still suspect a charging
system problem where you slowly discharged the battery as you
were riding. This could be intermittant I suppose so that it
sometimes worked and sometimes failed. A flakey voltage regulator
could do this.

When you first turn on the key, do you see a red "GEN" light, and
after you've started the engine and revved it to about 1500 RPM,
does the light then go out ?

You should have one fat lead running from the battery to the starter
solenoid, with another lead from the solenoid to the diode board
which rectifies the current from your alternator.

You should also have a second lead from the battery to a fusebox
with a couple of circuits. These would handle everything except
battery charging and the starter motor.

On the negative side, you should have one fat lead between the
battery and the gearbox by the speedo cable.

I recently installed a $6 Radio Shack LED charging meter, which
tells me quite a bit about my battery and charging system. You
might want to buy one of these and rig a connection so you could
watch the battery and charging system as you were riding. You'd
also want to be able to disconnect it or shut it off when you
stopped, so you'd want either a switched connection or an on/off
switch on the meter itself. For $6, it's a great onboard instrument.

 

Yes, the Gen light does exactly as you say.

I don't think it's the alternator. I've been riding it for about a
month, using the battery to start it everytime and never had a problem.
ALthough it's possible for the alternator connection or voltage
regulator to be flaky, i still doubt that the battery would discharge
so quickly, just overnight. I'll try to get my multimeter out tomorow
and check all those connections. Thanks.

 

When you're running with the headlight on, the battery can
discharge pretty quickly. If it discharged while sitting idle,
it's possible you've got a bad cell in the battery even if
it's almost new. What voltage had it discharged to ?

Other suspects might include the voltage regulator and
diode board.

Good article:

http://www.ibmwr.org/r-tech/airheads/charging.info.shtml