Won't start, at a loss as to cause.

[k_enn]

I have an 1982 model.  A little over a year ago, it did not want to start.  Battery is good, starter is only a few years old, as is the Dyna Brown coil, spark plug wires and plugs.  The engine would crank strong, but not start.  It appears that it was getting both fuel and spark because after cranking the engine to try to start, when I would turn the key off and then back on shortly after the attempt, I would get s solid "backfire" in the exhaust system.  I eventually put the bike away for the season (and rode my other bike).  Winter storage included a complete draining of the gas tank an carborators.

Turn to this season.  About three weeks ago, I took it out of the garage.  Gave it fresh gas, checked and cleaned the carborator bowls (including the pinhole to the enricher), and put in a new air filter (there was some debris that got in over the winter).  With about 5 to 10 seconds of cranking (and working the enricher lever), it fired right up.  I rode it for about a 75 mile ride with no problem other than the idle being low and an occasional stalling.  Even then, it fired right up.  When I got home, I ran the carborators dry and shut off the petcock.

This past weekend, I went to start it up again.  It went back to its old problem of not wanting to start and backfiring.  I am at a loss as to where to start to troubleshoot this problem. 

As I see it, it is getting fuel as it must be passing through the cylinder and into the exhaust for the backfire to occur.  Likewise, it seems that it must be getting spark because the spark when I turn on the key is igniting the backfire.  I had thought that the airflow might be blocked or reduced might be causing the backfire where there might be more residual air in the exhaust system, but the filter is new and the bike ran just a couple of weeks ago. 

And thoughts on what to look at would be appreciated.  For what it is worth, the carborators still have the original floats, but I am not sure if this is problem since other than a short ride, they have been dry for over a year.  The last valve check and carborator adjustment was about 3 years ago, but I don't think it can be that far off since it started easily a few weeks ago. 

Where should I start? 

k_enn

[Bob_Roller]

Well, you have to start somewhere !!
I would say check the fuel delivery system, do you have the Karcoma fuel tap, it has a barbed fitting for the fuel line at the the o'clock position and should have a fine metal mesh screen over the intake tubes for the fuel tap, the Germa fuel tap has the fuel line at the 6 o'clock position, with a nut to remove the line from the tank, it has a fine plastic filter in there .
Any inline fuel filter ??

[mrclubike]

remove the carburetors and do a compression test

[Barry]

For what it is worth, the carborators still have the original floats, but I am not sure if this is problem since other than a short ride, they have been dry for over a year.

If you have access to an accurate scale you could weigh the floats. If they get heavy they won't necessarily dry out to the original weight of 12 grams even after a year. I have some that have been drying 4 years  on the shelf that are still heavy.

[k_enn]

Thanks for the suggestions.  It is definitely getting fuel to the carbs, it is in bowls.  Compression test is a possibility, but I will need to borrow a tester.  I like the suggestion about weighing the floats, just not sure if they would overwhelm my reloading scale (probably would).  Will check the range of my scale tonight. 

I was thinking a little more last night.   Could the Hall unit be malfunctioning?  Would setting up a testing spark plug and looking for spark or using a timing light and looking for flashes when cranking provide the answer?  If so, that is a very easy place to start, and I have what I need to do that. 

k_enn

[tunnelrider]

Would setting up a testing spark plug and looking for spark

Hi k_enn, rather than testing spark with testing plugs I'd test the spark with the ones you're trying to start with.  IMHO the only time I've seen a bike not start and backfire was valve clearances (not a BMW) so fixed that, started up ok, went for a ride but had the same problem the next day from a fouled plug overnight or trying to start it.  (Still haven't got around to looking into it for my mate).  Could be a poorly set up carb or oil or fuel getting into the chamber while not running? Have you tried cleaning the spark plugs before cranking or getting new ones?

The slow idle and stalling when it was running both point to it needing a basic tune up - could be time to go through it starting with torquing the head nuts, checking the valve lash then you've either got the ignition or carbs.  Make sure the carb/head rubber manifolds aren't loose. 
I think the ignition is worth checking but I wouldn't bother with a timing light while turning over the starter over unless the plugs are out and grounded to the cylinders with tape or something tight. Then perhaps you'll see the advanced timing mark?  (Sorry I can't back that up except I do know they turn over pretty fast without the plugs.)