Fat Blue Spark, In Pursuit Of.

[bobbydyabb]

Hi all, am looking for your experiences in actually achieving a Fat Blue Spark on your ignition system.

Bike: '83 R65, stock, 12/83 production date.

Background:  While pursuing an intermittent bog down at idle which later became consistent, I found out that I'm not getting a Fat Blue Spark while doing spark plug tests.  I went through the recommended steps: starting with valve clearance check, ignition system check, and dropping the carb bowls.  Everything, except coil output and resistances, seem to have checked out. In putting it all back together, I replaced the spark plugs, cleaned coil tower contacts with DeOxit and checked for the proverbial Fat Blue Spark. No luck, Sparks still yellow and not fat.

Fired it up, went for a test ride, and the bike now consistently idles even after only 2 miles (750 rpm, 1100 when fully warmed up).  TWA?

But knowing that I'm not getting a Fat Blue Spark just bugs me and takes away a little from fully enjoying my ride.

So, is my ignition system (coil perhaps) on the way to being marginal? Has anyone attained this Fat Blue Spark nirvana?

Ignition system resistances check ok except a couple of iffy readings: coil primary is 0.6 ohms, it seems original (black, single, dual tower, no apparent cracks), and tower to tower is 12+K ohms (too high?).

Ideas, comments?

Bob D
'83 R65

[Barry]

Ignition system resistances check ok except a couple of iffy readings: coil primary is 0.6 ohms, it seems original (black, single, dual tower, no apparent cracks), and tower to tower is 12+K ohms (too high?).
 

O.6 ohms sounds low. A dual output coil on an 83 model should be 1.5 ohms although 0.6 ohm coils were used on much later airheads.  Maybe the module and coil have been changed by a PO.  If the module hasn't been changed then the it could be at risk of overheating unless there is some in built current limiter.  Is there a part number on the coil that might help determine whether it's a faulty 1.5 ohm coil or a later O.6 ohm coil.

I don't know what the secondary resistance should be but 12K Ohms sounds to be in the right ball park.

[Bob_Roller]

The primary circuit too low, as Barry has said, 1.5 ohms is what it should be .

Also, how old are the spark plug leads ??

I had a second generation BMW coil, the red and black version, have the same low primary circuit and it manifested itself as hard cold starting below around 55 F .

I've been on this site for 15 years and the OEM black and gray coil has been a problematic part since I've been a member here .

I would replace it, a popular aftermarket coil, is the DYNA ' brown ' coil .

From what I've heard from businesses that sell them, they rarely get a confirmed failed coil  returned back to them ..

[bobbydyabb]

Thanks for your replies.

Taking a closer look at my configuration:
Coil is not all black as I reported but has a black main body with Red Towers. Part number is Bosch 0 221 500 203, which according to 'bum's site, gray problem coil was "replaced by Bosch 02210500203, BMW 12-13-1-244-426". This ties in very well with Bob R's second generation black red coil which his experience showed to also be problematic.

The Ignition Conrol Unit paired with this coil is a Bosch 0 227 100 103 screwed (not riveted) onto heat sink 1214-1244-328.  I've had no overheating control unit issues at all to confirm an abnormally lowered primary coil resistance, so it could be that this ICU supports the .5 - .7 ohm coil primary.

Can you tell from any of the above part numbers? If the ICU supports low ohm primaries and I install a 1.5 ohm Dyna, would that work without aggravating it (the ICU)?  Or maybe I should just bite the bullet and get an ICU AND a matching Coil from such places as Motorrad Electric?

Sorry for all the questions, but thank you for all your responses.

[Barry]

Your Ignition module Bosch 0 227 100 103 is the original one not intended for the later 0.6 ohm coils. Although it functions the overheating risk may arise form an ignition on engine not started senario if the primary current continues to flow. Later Ignition modules intended for the O.6 ohm coil had a timed cut off.

The ignition module regardless of what version it is would work fine with a replacement 1.5 ohm coil like the Dyna so I see no reason to replace both.

[bobbydyabb]

Ok, my plan now is to simply replace the coil with a Brown Dyna  (1.5 ohm) and to not over think this issue.

Many thanks to Barry and Bob_R for their responses (and patience).