1976 BMW R90/6 left cylinder not working

My left cylinder won't run. I've got new spark plug and it has a good
spark. Havn't done compression test but compression seems good when I
try to kick start it. Right cylinder runs fine and I can't seem any
diffrence between the two. Any ideas? Thanks.

 

A weak coil can show spark when the plug it out of the engine, but won't
muster enough energy to spark in the engine. I'd switch coils if
possible. Also do the usual such as check the carb for clogs, water in
the bowl, etc.

Here are speciality boards for BMW airheads. Check there too.

 

Does it start running on two cylinders when you open the throttle past
1/4 turn? Clean your carburetor out. Especially the idle jet...

 

You ought to be able to swap the spark leads. Both cylinders fire
on each upstroke, so it doesn't matter which lead you connect
to a plug. Leave them plugged into the coils, swap at the plugs
and see what happens.

You probably could do the same for the carbs as well.

I think you can be fairly certain that it's an ignition,
carburation or compression problem.

A couple of candidates on my suspects list would be plug wires,
punctured carb diaphragm, clogged jets or stripped crankcase
threads (cylinder studs). What work was been done to the
bike most recently before the problem turned up ?

 

Hey guys thanks for the advice. I tried switchin spark plug cables, no
diffences. I then took off the bottom of the carb float bowl and it
was prety disgusting. had lots of white junk in it (aluminum
oxidation?) . Well, i cleaned it out, and gave it another try. It
started up, and i suddentl noticed it was running on both cylinders.
Revs up fine but idling is rough. left cylender randomly shuts down and
comes back to life while idling.
I guess my next step is gonna be to just try to clean the carb. I've
had this bike since '76 and never touched the carbs. Anyone recomend a
place to find good isntructions for cleaning the carbs?

 

Here's a link to a generally good site. http://www.dansmc.com/carbs.htm.

Charles

 

Well guys, I got it working! I took the carb off and found more white
junk. So i sprayed the whole thing with carb cleaner and wd-40.
Sprayed wd-40 right throught the idle and main jets, took a tooth brush
and scubbed all the white junk out of the main passagway. Put it back
together and the bike runs great. I while ago my center stand broke
off so i have been using the side kick stand. I think that leaving
that bike tipped at an angle for so long caused the left carb to clog
up with all that white junk. Well, thanks for all the help.

 

I expect that the left carb rested lower than the right but that they
were both tipped at the same angle.

 

http://www.dansmc.com/MC_repaircourse.htm

Hey, there's a lot of good stuff at that site. E.g. see the photos of
spark plugs at http://www.dansmc.com/spark_plugs/spark_plugs_catalog.html

Thanks!

 

http://www.bingcarburetor.com/
http://www.airheads.org/old/index.php?Technical Tips
http://ibmwr.org/r-tech/airheads/index.shtml

 

Hi everyone. Well, it looks like I have another problem, but it's
different i think. I started the bike up today and it seemed ok.
However, when I started riding it, it was having trouble. The engine
would kind of kick in and out sometimes. As if the clutch was being
let in and out. It then stalled. Couldnt get it to start up unless
the choke was in and it would idle for mabey a few seconds, then stall.
It sounded as if it was out of gas, but the tanks got more than a
gallon in it. I also noticed that the carb float bowls had only about
1 cm of fuel in them. They usually have more. Any ideas? Thanks

 

There is probably a lot of crud in the gas tank. Drain the tank and
remove the petcock and flush the gas tank and petcock out and flush the
fuel hoses out.

This time, when you clean the carburetors, use an aerosol carburetor
cleaner containing xylene and acetone. WD40 isn't much of a solvent,
you need something that goes after gum and varnish.

Berryman's B-12 ChemTool, STP, and GumOut are all strong carburetor
cleaners.

You probably have gum and varnish on the sides of the float valve and
in the idle jets and idle passages.

As I recall, the idle mixture screw is on the underside of the carbs.
Screw it all the way IN, counting the full turns and fractions of turns
and write it down.

Then remove the idle mixture screw so so you squirt carb cleaner
through that hole.

When you reinstall the idle mixture screw, screw it all the way IN and
then unscrew it the same number of turns and fractions of turns that
you wrote down and you're back where you started.

 

First repeat your test with the gas cap loose to make sure
your tank vent isn't plugged. If it gets better, clean the
the vent. If not, continue.

Above your fuel petcock there are most likely some fine mesh
screens which are clogged with red tank liner and/or other crud.

Drain the tank, carefully unscrew the petcocks, clean the screens
and while you're at it, check for water in the tank.

If the tank is really badly crapped up and the liner's in bad
shape, consider giving it a POR-15 treatment soon. In the meantime
though, just clean the screens and feed it some clean gas.

 

Hey Everyone. I cleaned out both carbs with gumout carb cleaner and
then took a look at the fuel tank. When i took the petcocks out, there
was lots of red gunk surronding the mesh. I cleaned everything: tank,
petcocks, and fuel lines. Started it up toady and it was running
perfectly. Road about 3 miles when it suddnely stopped. Couldnt start
it up again. I am starting to think that there is some type of
electrical problem. Maybe the coils are going? Anyone have any ideas.
Thanks.

 

wrote in @g14g2000cwa.googlegroups.com:

That, my friend, is rust. You need to clean the rust out of the tank and
seal it. (See: www.por15.com. Kreem sucks.)

You'll also have to clean the carbs again. And this time install a fuel
filter between the tank and the carbs, to catch the rust coming out of the
tank.

 

The red stuff is tank liner. You might want to look into a POR-15
treatment if there's a lot of it.

You sure the petcocks and kill switch were on ? (Not sure if a '76
has a kill switch, but good to check if it does).

Assuming it cranked but wouldn't fire, it's most likely another
fuel problem, but could also be compression or ignition.

Open the gas cap slightly and see if it'll start. If so,
your tank vent is plugged and needs to be opened.

When you push the bike in gear, can you feel the engine
compression ?

When you've cranked it and pull the plugs, are they wet or dry ?

When you pull the float bowl, is it empty or full ?

When you pull a plug and ground it to the engine (be careful not
to shock yourself), do you see a good spark ?

If it won't crank

Check the battery, the kill switch and the neutral switch.

Try holding in the clutch and see if it'll crank over.

Do you get a click from the starter relay ? If not, consider
replacing it.

Consider buying some manuals (Haynes, Clymer and/or the BMW manual).
It's good to have more than one so you can compare them.

 

It's most likely tank liner, not rust. Agree with everything else
you said.

 

Hey guys, i feel really dumb. Turned out it was just low on gas. It
was dark out and I didn't think to check the float bowls or even turn
the petcocks to reserve. It's running fine now. Thanks for all the
advice!

 

After I posted, it occurred to me to ask about that.
I thought about it for a minute and said Nahhh !!

Glad it was that simple.

 

Back with one more question: I took it on a ride for abour 20 miles.
Runs ok except every once in a while while I am slowing down by simply
releasing the throttle, it backfires. This usually happens while
slowing down for a curve. It doesn't seem to be a big problem, just
curios what might be causing it. Thanks.