Blowing Bulbs

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nomadwarmachine

nomadwarmachine

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My '74 850 Roadster is consistently blowing taillight bulbs (in some cases, all I find on the inside of the lens is broken glass). I also recently lost my main running light (though the high beam still works). Any idea what this could be, or how to diagnose/fix it? I have a multimeter and am happy get down into this -- perhaps add a resistor somewhere in the circuit?

Also, any leads on ordering a new bulb for my Commando? I have a lamp with a removable bulb, rather than a single sealed unit, but it does not look like something I could easily find at an auto parts store. OldBritts also does not appear to carry this bulb. Thanks!
 
check the voltage at the battery at idle, then rev it up to 3000 and see what happens.
 
Check the connection to the Zenner diode, if this is loose or disconnected there will be no control over the charging rate.
 
nomadwarmachine said:
My '74 850 Roadster is consistently blowing taillight bulbs (in some cases, all I find on the inside of the lens is broken glass). I also recently lost my main running light (though the high beam still works). Any idea what this could be, or how to diagnose/fix it? I have a multimeter and am happy get down into this -- perhaps add a resistor somewhere in the circuit?
Click to expand...


The main causes of regular bulb blowing are excess voltage, bad earths/grounds, or more likely; vibration (watch how your Commando's tail lamp assembly dances about at low revs)! When I first got my Commando, it would regularly blow its tail filament, and I came to the conclusion that the bulb was blowing because it was vibrating and knocking against the inside of its holder. I looped an elastic band (three times) around the bulb so it could no longer rattle about, and I can't remember it blowing since then.

Blowing Bulbs




nomadwarmachine said:
Also, any leads on ordering a new bulb for my Commando? I have a lamp with a removable bulb, rather than a single sealed unit, but it does not look like something I could easily find at an auto parts store.
Click to expand...


Can you identify the bulb from this list? http://www.bitzforbikes.co.uk/generalPr ... dTypeID=91

-or check for any markings on the bulb?
 
Love the rubber band idea for the taillight. WIll definitely give that a try on the new one I've got in there.

Everything on the electrical system has behaved well so far, and the bike DOES vibrate excessively, so I am inclined to think that the Iso adjustment will fix the blown light problem. Does anybody have an opinion -- and I know you do! -- on whether the rear Iso is more likely to cause vibration in the rear of the bike? I have only adjusted the front Iso, and have always been intimidated by the rear.

It is very difficult to get anything approximating an accurate reading with the feeler gauges, but am I correct that high frequency vibrations suggest a too-tight Iso?
 
but am I correct that high frequency vibrations suggest a too-tight Iso?
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I would say yes.
 
Operation vibration cancellation.....on a British bike or even worse on a Commando!!! Let me know how you get on.....I am all ears.
I stopped my tail light assy dancing about at idle by fitting a rack with stays down to the exaust plates.
I think that idea with the rubber band round the bulb neck is ace. That is just the sort of solution to these issues I like!
Stu.
 
Pulled the bulb out of my headlight -- it is a Blick 12V50/40W bulb, which appears to be common in British vehicles. Does anybody have a recommended vendor in the US for such a bulb? I checked eBay and they appear to be selling a lot of cheap imports at significant markups. I would like to get a couple, so I have spares. Thanks!
 
I blew a number of commando tailights when the bike was near new. I used foam rubber to mount the socket sideways inside the original housing and that worked pretty well.
 
L.A.B, my bulb looks a lot more like 392-446 , which is the 50/40W bulb in the picture. Should I just order that?
 
nomadwarmachine said:
L.A.B, my bulb looks a lot more like 392-446 , which is the 50/40W bulb in the picture. Should I just order that?
Click to expand...

Apparently both 370 & 446 bulbs are "BPF" "P36d cap" type?: http://www.mgaroadster.co.uk/Lucas%20Bulbs.pdf

The 370 is 45/40w "Duplo" filament, and the 446 is 50/40w "transverse" filament.

Maybe both types are suitable, I don't know?
 
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