Induction Bias observation.

[LJ.]

Interesting observation... I had not been using the Blue A10 over the winter months and besides that it was waiting for me to buy a new battery.

Came to start it up and although had not wet sumped, the left hand cylinder just was not firing. Now I Know the bike was running perfectly last Autumn. Checked the spark which was perfect as expected... even after a few mins running the left pot was still cold, some wicked back firing pops coming out of the left exhaust occasionally. What is needed is a GOOD run and then will all be well. Seems this is a common problem with A10s and usually the left driving side.

Anyone have similar experiences?

[orabanda]

I have reached the conclusion that A10 induction bias is a lot like STD's; there is a lot more of it out there than you realise!

Three out of four A10's I have dyno'd, showed induction bias (revealed by measuring the mixture ratio in each pot, via a probe up the [exhaust] pipe).

Two of these bikes had more discolouration on the LH pipe, than the RH.

I have just received half a dozen bias spacers from Cakes Classic Services, so will let everyone know how the mixtures change once installed.

The other thing I learnt (thanks to Pat at Lyford Classic Services), is that all A10's came out with a 1/8" thick bakelite spacer (factory fitted) between the head inlet manifold, and the carby drip tray.

Only one of my bikes (SR) had this fitted when I got it. Pat says the spacer is a heat sink, there to prevent the fuel boiling in hot conditions.

I pointed out it is not shown on the engine parts lists, but Pat says BSA simply neglected to include the item on the drawing.

Let the debate begin!

[tombeau]

When I was trying to get an induction bias gasket for mine.
The bloke I spoke to at SRM said they were nonsense "carbs just don't work that way mate" I was told.
I tracked some down at Cake St Classics.
Maybe you should send your dyno results to SRM, Orabanda
Cheers,
Iain

[bsa- bill]

I have a biased gasket on my A10 ( made it myself - thick gasket and rasp), undecided as to whether it makes a difference or not.
Thinking about it more - I can see it working if both cylinders sucked at the same time but as they don't then assuming each cylinder sucks with they same gusto they should help themselves to the same amount of mixture. ONTOH perhaps the time between the sucks is short enough to negate this argument.

As you can see I have more questions than answers, just thought I would add this to the debate

All the best - Bill

[orabanda]

Mate,
I have a healthy respect for SRM, and spend enough money on their high quality products (just received a confirming email that my latest order is leaving today; yippee!!).

However, some of the things they come out with I just cannot agree with. I.e. their views on oil type and in-line filtration.

I have worked in the fluid power industry since 1977 (that's before you were an itch in your father's scrotum, Josh!), and manage a hydraulics company respected for it's technical expertise, so feel my opinions on these issues have some credibility. I no longer debate these issues, just do what I have found will work.

However, variety is the spice of life. We all have views, and opinions from our life experiences, and they are frequently different.

Perhaps the head patterns were made by a right-handed person, and the internal cv factor (frictional resistance to flow) is slightly different because of this?

It might be worth taking a head to a porting specialits and have it flow tested, to measure the resistance to flow, between both sides.

[bsa- bill]

I see where your coming from orabanda, another factor - the tract from carb to valve ( this had slipped past me - many things do these days) so I'm thinking now sorting this would have much more effect than just pointing the carb around a degree or two, I would also think this is not a job for anyone, the scope to make matters worse seems very high.

[MG]

I fitted a 30mm Concentric to my A10 ('56 iron head) and therefore treated the inlet tract with an air grinder  to match
What attracted my attention was a rather large cast pip inside the LHS inlet tract. Maybe there are more heads around like that, due to a faulty casting mould/core?

As time permits, I will try the colour-tune plug in the A10 and report how my modified head (now without cast pips and burrs) behaves.

Cheers, Markus

[tombeau]

Orabanda,
I must say, I have a lot of respect for SRM and was rather surprised by the response of the person I spoke to.
I had always assumed the gasket was a quick fix for dealers, saving the time and expense of improving the porting.
Cheers,
iain

[LJ.]

Well... This is embarrassing! Took the bike out for a good hard ride expecting the induction bias to correct itself but found that the whole journey of five miles consisted of a completely dead left hand pot. Now I knew the sparks were good as I had a screw driver in the plug cap to frame but did  not try plug cap with spark plug in - to frame... Ahem!... turns out to be a complete dead plug donnit! Soon found a new BP6HS NGK plug and took for another spin, damn bike now flies like never before.... Seems that plug was slowly on its way out and having had a cold winter with no use finally put the plug to permanent rest. Sorry Mr Induction bias!