Boyer Bransden not retarding!

[Dean Southall]

Luckily the MKIV Boyer fits straight onto the Sunbeam scooter twin engine. To make life even easier they have an external flywheel so I was able the scribe ignition timining marks so strobing.
When I initially set up the igniton and timed it all worked out well; 5deg at tickover and about 20/25 at higher revs.

Subsequently I've had to strip the engine to attend to an oil  leak and on putting it all back together the ignition no longer retards! (you set these at full advance and little creatures inside the black box turn it back to retard at slow engine revs). That is , it is permanently at full advance with about 5deg variation with engine revs.

I have triple checked the timing set up and checked the voltage to the black box which is 12.4v (these systems will go full advance at low voltage).

Any suggestions what might be causing this or other tests would be most welcome.
....one day I might finally get this thing rideable!

[Spaceman]

My understanding is that the B-B system relies on the amplitude of the trigger signal it gets to change the internal bias voltage at which the ignition module triggers the primary current for the coil and therefore the amount of advance. The amplitude of the trigger signal naturally increases with revs as a result of the rotor magnets passing the pickup coils at a faster speed (Faradays's Law).

It's difficult to see how a fault with the trigger coils could cause the ignition module to generate full advance most of the time - to do this, the coils would have to generate a much larger trigger signal than is normally to case. I can't remember the setup details but it is worth checking the gap between the magnets and the coils is within the specified limits.

Assuming the ignition module has not developed a fault, the only thing I can think of that might cause this problem is a reduced supply voltage to the ignition module that might cause it to generate a larger ignition advance for a given trigger signal level. I would definitely check what voltage is being generated across the ignition module supply leads and that one side of the coil is properly earthed.

[Mike Farmer]

 

Any time, not often, I've had a problem with BB I have called/emailed their tech dept and spoken with them. Always helpful.

I think you can still get most answers on their Home Page.

Mike

[Mike Farmer]

 

Just checked Boyer Bransden has a fault finding page on their site with contact details.

Mike

[Dean Southall]

Many thanks for your replies guys.
I'll recheck supply voltage again(it was over 12v though) and coil connections to earth.
I have the boyer tech stuff and have found them helpful over the phone in the past.
FYI there is a really in depth tech article on Boyer Bransden fault finding in Vintage Bike https://vintagebikemagazine.com/technical-articles/boyer-trouble-shooting/

As you can see, I held off bothering you lot as long as I could

I'll have another look tomorrow and get back to you.
Thanks again

[Spaceman]

I inherited a B-B Mk IV that has sat in a box for the last 10 years but I've just connected it up and got it running on my bench with the rotor fixed to a variable drill. I have got both the trigger signal and the output current/voltage from the ignition module displayed on a dual trace oscilloscope so that I can determine the amount of advance. Not surprisingly, it does what it says on the tin and gives about 30 degrees of advance above 4-5 k rpm.

As said, it's hard to see how a problem on the trigger side can give you more advance than it should - a weaker signal should produce less advance. If I can think of a way of reducing the supply voltage (I'm presently using a 12V battery), I'll see what effect this has on the amount of advance.

[Dean Southall]

Well that is a bit embarrasing.
I decided to simply retard the timing and it seems to have fixe the issue?! 
Its not dead accurate (when strobed) and moves around a little. But then so do they all! I've never understood the need or reality of timing to within a degree or two!
I need to sort out another issue (he says mysteriously) before I do another road test and will then tweek ign timing if road performance indciate the need. I'  try and remember to giev you feedback.

Major Tom, many thanks for your further researches and infomation.Whe I have a quiet moment I'll and try to get my cognitive valve gear running properly and study it.

cheers
Dean