The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: lawnmowerman on 15.06. 2010 22:40

Title: Running on one
Post by: lawnmowerman on 15.06. 2010 22:40
Here is an ineresting one for you all.
Just taken the bike out for its longest run yet, about 25m round trip at a steady 45mph (only done about 50m before that since rebuild).
I have noticed problems when starting - starts on the left hand cylinder while the manual A/R is set to half retard then the right cuts in when set to full advance. When the bike is warm, retarding the ignition on tickover causes it to run just on the left.
Has anyone experienced this before? Don't want to start pulling things apart if it is something simple. Not sure what to check first.

Title: Re: Running on one
Post by: BSA_54A10 on 16.06. 2010 09:59
Unequal plug gaps can cause this type of behavour ( or mismatched plugs )
Title: Re: Running on one
Post by: A10Boy on 16.06. 2010 13:33
Perhaps an un-equal cam ring throwing the points gap / timing out on that cylinder??
Title: Re: Running on one
Post by: lawnmowerman on 16.06. 2010 14:18
Have tried new plugs to no avail. Could the cam ring have come loose or slipped its location? Did not seem to do it when I first got the bike so the cam ring is probably machined ok, although I do remember the A/R lever tightened up when I used it once but I forced it and it seemed to free it up. Can't remember if this was when the problem started.
I guess the first step is to have a look in the mag end cap and check the operation, then swap the leads at both ends to see if it moves to the other side.
Title: Re: Running on one
Post by: lawnmowerman on 16.06. 2010 15:07
Had a look inside the mag end cap and nothing seems out of place. The leads are not long enough to swap so I will swap the mag pickups later.
This is a strange one because if I have a weak spark on one side I would think it would fail on max compression, ie fully advanced, not when I retard ignition after TDC. *dunno*

Title: Re: Running on one
Post by: MG on 16.06. 2010 15:32

I had a similar problem on the A7 SS (manual adv. also) recently. I fitted a new end cap when rebuilding the mag (repro part). There are 3 small dogs on the inside that hold the cam ring in place. These were too shallow, allowing the cam ring to wander out of the housing by almost 3mm. I first noticed that when it jammed the advance lever, soon afterwards it came loose completely (the plunger on the cable jumped out of the groove in the cam ring) and the timing was way off. When taking the end cap off, I noticed that the ring had come out of the housing. When comparing the end cap with an original one off another maggy, I saw that the dogs on the original one would just leave a play of approx. 1mm, compared to more than 3mm on the pattern part.
Had no problem ever since after fitting the original cap.

Cheers, Markus
Title: Re: Running on one
Post by: a10sausage on 16.06. 2010 15:54
my old a10 did this and that one  was using lecky turned out it was induction bias
Title: Re: Running on one
Post by: groily on 16.06. 2010 17:35
The lever getting momentarily stuck and then freeing up could have been due to the problem Markus describes - I've had the same thing with manual mags and not-the-right-end cap. But if all appears well and the cam ring and cable are properly located and working smoothly, I'd have a look at those HT pick-ups and brushes asap. Could be a tracking problem on the pick-up, or a worn brush, or a bad connection to the HT lead itself, etc. Swapping bits round will tell you what's dud - if the problem swaps sides as well. If the problem persists on the same side, then things get more fuzzy, with fuel and valve issues starting to come into it.
Sparks are in fact weaker when the mag is other than fully advanced (the ATD was invented to avoid this by maintaining a constant relationship between armature position and points opening - but 'sporty' bikes were deemed to need lots of levers, just like people in cars wanted loads of gauges!), so an ignition fault will truly often show up with a bit of retard.
Title: Re: Running on one
Post by: wilko on 16.06. 2010 20:29
Check your points gap in both full retard and then advance to see if something's changing.
Title: Re: Running on one
Post by: dpaddock on 24.06. 2010 21:47
Grolly - perhaps you need to review the ATD reference in the last sentence in your posting.

Title: Re: Running on one
Post by: groily on 24.06. 2010 23:08
scratching my head a bit here david. Point was simply that a mag with manual cam ring sparks at point of max flux at full advance; with an ATD and fixed cam ring the points will always open at said point of max flux as the A/R is applied at the drive end and doesn't disturb the relationship between the armature, the body and the cam ring/contact breaker. Ergo, ignition faults may conceivably show up first when a manual mag is running other than fully advanced as the spark is weaker.
Sorry for any confusion.
Title: Re: Running on one
Post by: alanp on 25.06. 2010 07:51
Groily, this reminds me of a weak spark problem I had once on a Gold Star I restored which turned out to be an incorrect points cam firing the plugs at a weaker flux position. The difference was clearly visible in a much brighter spark when the correct cam was fitted.