Author Topic: Anti Bias Spacer (gasket)  (Read 3094 times)

Offline Clive54bsa

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Anti Bias Spacer (gasket)
« on: 08.11. 2013 16:44 »
 Hello everyone, I recently purchased the "Anti-biased Gasket" from Cake Street Classics  for my '54 Flash with the 276 Amal, to try to rectify a lean right cylinder problem, when I called them, I was asked "how much bias I wanted ", well err umm, I don't know, what do you have?  was my reply. They can make a little bit of bias or a lot, so I ordered one of each, as I have no idea as to how much I would need for my situation. They are 13 pounds each and they arrived yesterday so I thought I'd post a picture or two of them for the forum to see as I've never seen one before. I tried to measure the angle on them and it appears to be 1 and 2 degrees.
I'll let you know how it turns out.
Clive


'54 GF,  '61 SR,  '71 B50MX

Offline tombeau

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Re: Anti Bias Spacer (gasket)
« Reply #1 on: 08.11. 2013 17:00 »
When I was looking for one of these, I called SRM and asked them if they had them. They were surprisingly dismissive, as if I'd asked for a bottle of smoke to replace what had escaped from my wiring harness.
Got mine from Cake St. Classics. ;)
Iain

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Re: Anti Bias Spacer (gasket)
« Reply #2 on: 11.11. 2013 10:12 »
I must admit I find it hard to believe that an A10 would be sensitive enough to require this kind of fiddling - which would put me in the SRM camp I suppose. But then Roger at Cake St put my motor back together after it last got sniffy and I'd most certainly have respect for whatever he goes along with.
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Offline a101960

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Re: Anti Bias Spacer (gasket)
« Reply #3 on: 11.11. 2013 14:40 »
This is what D W Munro (one of the A10/A7 design team) in his book BSA Twin Motorcycles, has to say on the subject of induction bias. “The two induction ports may be mathematically symmetrical, as far as the best inspection methods can ascertain, and yet there may still be induction bias” Munro then goes on to say that the problem was not universal, and that the A10 seemed to be more prone to induction bias than the A7, and although there were customer complaints they were relatively few.  He continues “Never the less the makers (BSA)  have made a serious investigation of the trouble, and as a palliative for persistent cases have produced a special anti - biasing gasket , part  number 67 - 359 for insertion between the carburettor flange and the head.” It is most interesting I think, that Munro chooses to use the word palliative (derived from the Latin palliare, to cloak) or as the dictionary says to relieve the symptoms without effecting a cure. While I am quite confident that the anti - biasing gasket might work in some circumstances for some people, I am of the opinion that it was a cop out on the part of BSA. It is much more likely that the problem is caused by a combination of factors related to the magneto that would have cost BSA much expense and inconvenience to rectify. Inaccurate cam ring lobe machining, and poor alignment of the armature shaft due to the bearing housings  being  off centre for example. The magnetos were built down to a price and considerable extra cost would have been incurred by BSA if they were all inspected to make sure that all magnetos complied with the designed tolerances not to mention the extra cost that Lucas would have added to do the same thing. It was the custom in Midlands engineering companies in those days to pay their employees on a piece work basis, and piece work is not exactly conducive to producing  consistent guaranteed quality and precision. The number of units produced was the overriding factor. Because my engine appeared to be running richer on one cylinder I tried an anti - biasing gasket and I noticed no improvement. I then had the magneto rebuilt with particular attention being paid to the cam ring (it did indeed require rectification) and it was also discovered that the bearing housing was in need of re - alignment. It transpired that the armature shaft was turning eccentrically. Since this work was carried out my engine runs beautifully and the plug colour is the same on both cylinders. Of course another palliative in bygone days for this problem (and it is still advocated by some people) is to split the difference in any point gap errors that are observed. Probably today with CNC technology this problem would no longer arise. Do you remember the work round on car distributers? The solution to eliminate the problem of point gap errors was to set the points up with a dwell meter.

John

Online morris

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Re: Anti Bias Spacer (gasket)
« Reply #4 on: 11.11. 2013 21:20 »
I back that up John.
The people I spoke with that know a thing or two about BSA are all a bit awkward about the gasket. On the forum there's not much support neither
I think Munro hit the nail on the head with the word "palliative solution"
Mine is also running rich on the right cylinder. That plug is always sooty, compared to the left one which has a nice brownish colour.
Some time ago I found out that, if I adjusted the points one side to .012, the other gap (corresponding to the right pot) has .015!
I never got round checking both sides, as I assumed when adjusting one side, the other side was automatically correct.
It was only after coming across a thread from Orabanda, the penny dropped....
Interesting read;
http://www.a7a10.net/forum/index.php?topic=1610.0
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Re: Anti Bias Spacer (gasket)
« Reply #5 on: 11.11. 2013 22:13 »
Rightly or wrongly I set the gaps and opening position to the same on each cylinder when I time the engine with the degree disc.
A little judicious grinding with a fine stone does the job nicely.

Trev.

Offline cotterpinkid

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Re: Anti Bias Spacer (gasket)
« Reply #6 on: 11.11. 2013 22:31 »
I nearly went down the bias gasket route due to apparent lean running on one cylinder, but was advised to check the timing with a strobe. I purchased all the kit and borrowed a strobe and found that the 'new' cam ring I had fitted was at fault. You could actually see the difference in the lobes when compared to an original cam ring. I replaced the cam and re-timed the ignition which resulted in both cylinders firing at the correct time.
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Offline duTch

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Re: Anti Bias Spacer (gasket)
« Reply #7 on: 11.11. 2013 22:53 »
I'm having trouble understanding 'eccentric bearings'..??,
   I also agree John, particularly as I have an extended inlet tract offset to left @~40˚ (to accommodate air filter)
 Morris- like you I noticed the variation (.012/.017), before I saw Orabandas thread... but just last week opened the .012 gap to about .014 which made the other about .019 (can't see how 'splitting the difference' would work?), and I think it made a bit of difference before I changed the needle jet, which gave cleaner plugs and now variation is easier to see.
 As I suggested elsewhere, wondered if the plug being darker at the the rear(inlet) or front (exhaust)side (y' can maybe just see on photo attached?) would indicate if the timing is retarded or advanced??
 I've colour coded the plugs L-green/R-red, with an orientation mark (to outside) for how they sit in the hole.

nb- just saw Trevs and Cotters post come in while I was typing
Started building in about 1977/8 a on average '52 A10 -built from bits 'n pieces never resto intended -maybe 'personalised'
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Offline Clive54bsa

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Re: Anti Bias Spacer (gasket)
« Reply #8 on: 12.11. 2013 17:30 »
Yes, I'm not totally convinced of the Anti bias gasket yet, but I too have carefully followed Orabanda's timing methods, and have the 2 cylinders firing within  one degree of each other, and reduced the BTDC point to 31 degrees, and did a little Dremel grinding in the head manifold to fix a very obvious port obstruction problem, and yet the darn thing still spits back through the carb and the right cylinder runs about 40 -50 degrees F hotter than the left. So I'll be trying the gaskets soon, if not just to eliminate that solution.
Clive


'54 GF,  '61 SR,  '71 B50MX

Online RichardL

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Re: Anti Bias Spacer (gasket)
« Reply #9 on: 12.11. 2013 19:35 »
I'm having trouble understanding 'eccentric bearings'..??,
 

Dutch,

Take a look at Erling's magneto project for insight to the eccentric bearing.

http://www.a7a10.net/forum/index.php?topic=1375.msg9819#msg9819

Richard L.
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Online Angus

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Re: Anti Bias Spacer (gasket)
« Reply #10 on: 12.11. 2013 20:09 »
When I first saw the A7 after being rebuilt by Roger (Cake Street Classic) it spited back and had a lag when opening the throttle, Roger fitted a new carb which made no difference. He then fitted a bias gasket and it is now sweet as anything. It may not be curing an underlying problem, but it sure cured the symptoms and allows the bike to run nice and be very rideable  *yeah* *yeah*


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1960 A10 Gold Flash Super Profile Bike
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Offline duTch

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Re: Anti Bias Spacer (gasket)
« Reply #11 on: 12.11. 2013 21:48 »


Quote
Take a look at Erling's magneto project for insight to the eccentric bearing

 Thanks Richard, I read it again, but couldn't see an explanation how a bearing gets to be eccentric, unless I missed something-not the best reader these days....!
no place to discuss here though, so will add a note on where appropriate, cheers
Started building in about 1977/8 a on average '52 A10 -built from bits 'n pieces never resto intended -maybe 'personalised'
Have a '74 850T Moto Guzzi since '92-best thing I ever bought doesn't need a kickstart 'cos it bump starts sooooooooo(mostly) easy
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Offline a10gf

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Re: Anti Bias Spacer (gasket)
« Reply #12 on: 12.11. 2013 22:18 »
Quote
how a bearing gets to be eccentric
See http://www.a7a10.net/forum/index.php?topic=1375.msg9862#msg9862 for some explanation about what I did to get a decent L\R point opening. Problem was misalignement in the casing (cam vs bearing), applied compensation by slightly nudging the bearing cup position. To not hijack the original Anti Bias Spacer topic, I'll be happy to explain further if needed, please post in the magneto repair thread.

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Offline Clive54bsa

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Re: Anti Bias Spacer (gasket)
« Reply #13 on: 12.11. 2013 22:28 »
Thanks Angus, after reading your post, I'm a bit more confident the spacers will work.
Clive


'54 GF,  '61 SR,  '71 B50MX

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Re: Anti Bias Spacer (gasket)
« Reply #14 on: 13.11. 2013 11:55 »
I'll go along with Angus, I have an anti bias washer in my Flash (made it myself, thick fibre washer and big file) and it works.
Why it worked I didn't know , seems illogical but I did ask that question and got a informative answer from t20racerman
here http://www.a7a10.net/forum/index.php?topic=4920.msg33396#msg33396
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco