Cylinder bias

[a101960]

My engine (A10 ally head) is definitely suffering from induction bias. Does anyone know what the angle of the BSA 67-359 anti induction bias spacer was? The drive side cylinder is richer than the timing side cylinder by quite a margin. If I richen the mixture to suit the weaker cylinder then the richer cylinder becomes quite black and sooty. has anyone else had this problem, and if you have how did you resolve it?

[RichardL]

Did read the posts at the following location?

http://www.a7a10.net/forum/index.php?topic=408.0

[a101960]

Yes, I have read that thread, and it is an interesting read, but what I really need to know is weather the angled gasket is still available and if it isn't, what the angle of deflection is. I certainly am unbable to eradicate this problem by adjusting the mixture.

[dpaddock]

Try finding the angle by using thin flat washers between the flanges and wrapping the joint with tape while you experiment. When you find the angle you need, make a bias adapter out of wood. A paint stirring paddle might be a good source.
I realize this sounds hoky but it could be an easy way to solve the problem.

[octane]

My engine (A10 ally head) is definitely suffering from induction bias........ The drive side cylinder is richer than the timing side cylinder by quite a margin....

Would someone care to explain this to me?
I just don't get it.

The fuel flows from the carb at a certain 'mixture' ( 'fuel to air ratio'), let's say 1:14,
then this flow splits up at the Y section into the two intakes.
How on earth can the 'fuel to air ratio' suddenly
split up into two different gasses with different air/fuel ratio ?

(It's a bit like having a hose of lets say 50 degree hot water
splitting up at a Y-section into 40 and 60 degree hot water...or?)

..and PS:
why would a different shaped intake gasket change this ?


I might be dumb, but I totally baffled.

[a101960]

There is no logical reason for this for this phenomenon, but it never the less happens. A10s are much more likely to be affected than A7s. Acting in response to customer feed back BSA spent some time and effort researching this problem and concluded that although the induction ports for all practical purposes may well be asymmetrically balanced, the fact was that the problem did arise on some models for no apparent reason. Hence the palliative solution of an anti induction bias gasket. Anyway no matter how unscientific it appears to be the gasket seemed to cure the problem. Any multi cylinder single carb engine can suffer from this strange phenomenon to a greater or lesser degree. Incidentally my head has been gas flowed but it has not eradicated the problem. Modern multis do not seem to be so prone to this because injectors meter fuel to each cylinder as required. This would be an extreme solution for an A10 which is why I am interested in part number 67-359, which according to BSA is the correct way to rectify this particular fault.

[groily]

The fuel air ratio will obviously be the same on the engine side of the Amal; the problem is that from time to time it was found each cylinder was not getting the same share, illogical though it sounds. BSA addressed this with their Induction Bias Washer, which simply angled the carb a fraction in favour of the weaker-running cylinder. You can put the thing on either way round, to tilt in either direction.  No other twin I have had has ever had the problem, other than for mechanical reasons requiring attention of a very different sort in the valve/piston dept, but there we go, another special BSA foible.

[dpaddock]

Octane's dilemma has a real-life solution, found in the Hilsch Tube. Google it.
Always presented with reality, we often times can't square it with observation and consequently tend to dismiss it. BSA created the bias adapter to solve a problem that has no apparent source, thus acknowledging reality.
By way of example, here's another conundrum: the clutch spring nuts in the 4-spring unit on my '57 Spitfire tend to loosen in service. Why should this be? It doesn't happen to my friends' clutches - why not? There's a reason, of course. I just haven't found it yet.

[LJ.]

the clutch spring nuts in the 4-spring unit on my '57 Spitfire tend to loosen in service. Why should this be?

David... You'll find that the spring cups will need a 'fresh punch' for the springs to stop up against, also maybe the nuts need a punch too... Dont do *too* good a job or you'll have a job undoing them. Hope you can see what I mean, cant find me words this evening. 

[Brian]

This cylinder bias is indeed a strange thing. For many years I thought it was one of those things that we put down to "old wives tales". I was convinced it had to be caused by one side having better compression or a degree or two difference in valve timing or some explainable cause. It definitely happens and I dont think there is a logical explanation, it just happens. But why on some engines and not others? Could it possibly have something to do with the shape of the inlet tract I wonder. I am not sure how BSA ground or machined the inlet tract but I dont think it was 100% accurate. My 61' A10 shows signs of cyl bias and I have gone to extraordinary lengths to find something else that could be causing the problem. Used a dial guage to check valve opening and closing etc. etc. Mine runs slightly richer on the right side, fortunately not a big difference but I can take the plugs out after idling or high speed test and its always that touch richer on that side. I even re-timed it so the lead/points/plug etc. that ran the left then ran the right but it made no difference. Cylinder bias does exist, BSA created the bias washer to address the problem. Why it happens is anybodys guess. If anyone knows a technical explanation I would love to hear it. I have two other BSA single carb twins, another A10 and a A7 and both of them are fine. I guess we just have to accept that there are some things in life that just cant be easily explained.     

[dpaddock]

I suspect that if you had the head "flowed", much of the induction bias would disappear. But, let's acknowledge that there are many manufacturing tolerances stacking up in any given machine; most of them tend to cancel out, but in some instances they all go one way.

Re the clutch nuts, LJ, I appreciate your comment. I suspect that the spring cups, which are free to rotate in the pressure plate, do so in a slippery environment and, unless they too are constrained, the nuts will unwind. I'll tweak the upsets and will also add some Loctite to the cup/pressure plate interface. We'll see what happens. Thanks.
David

[beezalex]

Unless you believe in magic, it seems to me there has to be a logical explanation.  One could surmise that, since fuel atomization is far from perfect, particularly at idle, that a casting that isn't perfect in the inlet, different valve openings, etc. have a significant role to play here.  My guess would be that just the geometry of the inlet is largely to blame.  Also, any step or misalignment of the carb, insulator and port could deflect more of the the fuel stream one way or the other.  I doubt any of this matters much anywhere but at idle. 

[Brian]

I wish I did believe in magic, my bank balance would be a lot healthier then !!!

Over the last week I have dug out every piece of BSA literature I have regarding cylinder bias and have had several discussions on the topic with engineers at my work. It would seem that every engine with more than one cylinder but only one carby suffers from cylinder bias. The thing is 99.9% of the time the difference between the cylinders is undetectable. For some strange reason the A10 engine seems to suffer more from this phenonenem. It may well be that the inlect tract shape or angle or something influences the gas flow but BSA themselves could not work out what was causing it.

Like a lot of things we could go to great lengths and cost to try and find the cause or we can just accept it and fit a washer that costs very little and cures the problem. I guess it comes down to what you are interested in, if you like to get involved in the technical stuff then by all means go into getting the head gas flowed etc. but if you just want to ride your bike then fit the bias washer and go riding.

Probably my only comment would be to to make sure you eliminate other probable causes first, healthy engine, good maggy etc.

[octane]

Over the last week I have dug out every piece of BSA literature I have regarding cylinder bias..

Hi Brian;
in what literature did you find anything about this phenomenon ?

[a10 gf]

hi, from The Book of the BSA Twins by Haycraft