Suddenly won't start!

[MikeFossl]

I got back yesterday from a long ride.  Bike was working flawlessly other then the brake light switch (see below).  This morning I couldn't start her.  Turning over fine, just no fire.  Plugs were dryish but when I gave each cylinder a shot of carb cleaner, no change.  Both carb bowls were full.   I though I could see a spark (yellowish?) but it's tough to make out in the sun.

Any suggestions as to where to go from here?  :-/

Thanks!

[montmil]

With spark, even slightly weak, and compression, the only other element missing is fuel. You did say the plugs were dry when inspected which would seem to show no fuel reaching the combustion chamber.

Try a small dribble of gas into each spark plug hole and see if you get a short run.

[Bob_Roller]

With a bit of carb cleaner in the cylinders, it should have fired at least briefly .

What ignition coil do you have on the bike ??

[thrang]

With a dry plug it sounds like fuel is the likely culprit, but that yellow spark also seems to indicate another problem with the ignition.

So how old are your plugs, leads and coils and can you see any signs of damage or wear, also make sure that the coil feed wires are sitting correctly and are not showing any signs of damage.

Have you noticed her misfiring or struggling to start recently?

[MikeFossl]

It has a Bosch coil, plugs are brand new and the leads are older., probably due for replacement.  The connections have all been double checked and cleaned.  I'm getting 12v to the coil.

No recent problems starting.  She was running perfectly yesterday.  We went for a long ride, several hours, and only this morning did I have a problem.

I still have dry plugs after lots of cranking with full choke.  Carb cleaner doesn't seem to want to light up.  I'll try some gas.  How much should I put in each cylinder?

[Hammer65]

I second the ignition issue. That being said, make sure the float needles arn't stuck shut too.  Pop off the bottom of the carbs and make sure fuel is flowing by jiggleing the floats a bit.  Could have a blockage in the fuel line too.

[Tony Smith]

Basically it sounds like you don't know if fuel or spark is your problem.

My suggestion is as follows.

Crank the engine for 10 secs or so then stop, turn the ignition off, then back on again, if there is fuel and spark there will be a "pop" from the exhaust as the mixture ignited, while the ignition is still on flick the kill switch, you should get at least one more "pop".

If no pop, then you can try one of two things.

Option 1
Spray some starting fluid into the airbox (lift the topcover and aircleaner out and spray down the intake trunks of both carbs for preference. Crank the engine again - of you get a pop, you problem is more likely fuel than electrics, if no "pop", leave the ignition on and flick the kill switch - if you get a "pop" ICU is OK, but suspect hall unit.

No "pop", well at least you had added to the list of things you know it isn't.

Option two, pull a plug and lay the plug, connected to the cap, along the fins of the head and use a lump of timber to hold it in firm contact metal to metal. Crank the engine - do you see sparks? If no spark, leave ignition on and flick kill switch. If you get spark then, ICU OK but suspect Hall unit.

Let's assume you have a nice strong spark.

Have a look at what is in the bowls of the carbs - is it petrol or water?
Tap working? Filter blocked?


If you do not have any spark, you need to have the coil tested which any auto electrican can do (after first checking all power connectors to the coil of course). If the coil is gettign power and the coil is good and you get no spark under any circumstances - ICU has failed. If you get a spark when you turn the ignition on, or flick the kill switch with ignition on, you have a failed Hall unit.

Probably.....


Check the kill switch too, more than one has failed and resulted ina  lot of needless expense.

[MikeFossl]

I went back in the dark and with the plug well grounded on the Cyl head I see nice sparks...so fuel it is.  The carb cleaner didn't light up but in any case I'll have to track down why (suddenly) I'm getting no fuel.

I'll try a squirt of gas in each cyl tomorrow and see if it lights.

I'm impressed with the AGM battery.  Lots of cranking.

[soulmachine]

Maybe it's just me, but in my years of experience, I've never gotten a motor to fire off of carb cleaner. Starting fluid, yes. Carb cleaner, no. So, I would definitely be using gasoline as an aid to help whiddle down a starting problem.

[k_enn]

Not to hijack a thread, but . . . .

Basically it sounds like you don't know if fuel or spark is your problem.

My suggestion is as follows.

Crank the engine for 10 secs or so then stop, turn the ignition off, then back on again, if there is fuel and spark there will be a "pop" from the exhaust as the mixture ignited, while the ignition is still on flick the kill switch, you should get at least one more "pop".

I have a possibly similar starting problem on my 1982.  It just will not start, but I get a loud backfire when I stop cranking and turn the ignition off and then on.  So that should mean I am getting both fuel and spark.  What's left to be the problem, other than air intake?

FWIW, I have had few isolated instances where, when riding, it just went dead (like everything was shut off) only to come back on in 5 or 10 seconds.  But currently, it seems to be getting both spark and fuel.  


k_enn

[Luca]

Some cleaning products don't burn, such as non-chlorinated brake cleaner.  I use CRC carb cleaner at work as a starting fluid, although every now and then you'll get an engine that just doesn't want to run on the stuff.

As far as the spark goes, you need to show a really good spark if your testing with the plug out.  When the spark plug is in the cylinder it will take more energy to fire through compressed air.  An in-line spark tester is not too expensive and can help you figure out if you have a weak spark.

Bosch coils tend to fail when the housing cracks at the end, so a quick, free inspection might help you get started in your search.

My instinct says this is an ignition problem.  There should be enough gas in the carbs from the other day to fire the engine up.  These engines can run on one cylinder, too, so it would be mighty impressive if both carbs got screwed up overnight.  You only have one ICU, trigger unit (bean can), and coil, however.

[mrbuck]

Very interesting analysis here from several sources.  For my personal info please explain what is causing the engine to pop when the ignition and kill switches are turned off and on.  Also how to  check for healthy bean can and or control unit.

[thrang]

Since you said you have a good spark, I'll say its fuel. Whip the float bowls off and make sure that you've not got a stuck needle and that your fuel is not contaminated with water. If you've got an air line give the jets your carbs a blow through also make sure that your air filter is not contaminated. I'd whip the filter out and try firing her up with some easy start but wd40 will do if you don't have any of the real stuff.

[Tony Smith]

Very interesting analysis here from several sources.  For my personal info please explain what is causing the engine to pop when the ignition and kill switches are turned off and on.  Also how to  check for healthy bean can and or control unit.

A"benefit" of the way BMW implemented the Hall effect ignition is that they left you a way to determine which of the ghastly, failure prone, expensive pieces of cr@p has failed this time.

Every time you either first power up or briefly interrupt power to the ICU it will cause a single spark. If there is combustible material in the cylinder(s) it will "pop". So - we can use that.

If you get a spark on switch-on or on flicking the kill switch, there is a better than average chance that the ICU is alive. If you then crank the engine and DO NOT get spark you have a hall effect fault (or corrosion on one of the connectors which does happen).

To then independently test the hall device you need a 9V "box" battery, three leads with crocodile clips on them and a cheap LED (cheaper the better as it may not survive this test).
 
Connect 9V +ve to the +Ve terminal of the beancan and the -ve terminal to the one at the other end, connect the "base" of the LED to the negative terminal of the battery and the input to the middle pin on the 3 pin beancan plug.

Spin the bean drive. If the LED blinks on and off, the hall effect is alive and doing what it is supposed to do. EME want $39 for a device that does what I've just described. You relal should have a pull-down resistor in series connection with the LED, but for th ebrief periods of time you are going to be powering it, I wouldn't bother. Last time I bought a "lolly bag assortment" of LEDs they were $2.50 for 50 of them.

[MikeFossl]

I just spent the day cleaning the carbs.  Complete tear-down, cleaning and back together.  The carbs were definitely dirty, the left quite a bit worse with lots of gunk around the atomizer of the main jet.

Once back on the bike, with the battery back from a fresh charge, no joy.

Cranked with full choke, heard a tiny stumble, then lonely cranking with no fire.

Immediately checked the plugs and found them bone dry.  Carb float bowls both full.

There is no question that the spark I'm seeing is pretty weak.  I had to go out at 9 PM to see anything at all.  A new coil is on the now urgent list.  

But why am I getting no gas in the cylinders with full float bowls and clean carbs?