The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: lowbrowbsa on 04.11. 2011 00:38
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not sure if this is how its spelt but a mate was talking about biasing he said there is some gasket on the manafold that can stop biasing from happenning can enyone shed some light on this subject *eek*
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spellings ok Lowbrow
the gasket or distance piece between carb and manifold is is biased, meaning it's thicker one side than the other, this is to counter any tendency for the engine to suck more on one side than the other by directing the flow to the weaker side.
One side running weaker than the other is regarded as a siqn of the engine having more suck on the richer side.
I have a biased washer on the Flash and it seems to work although I'm a bit lost as to how sucking less of the same mixture (air/petrol) would make a cylinder run weaker
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Cake St. Classics sells them. I've got one on mine. seems to work.
Cheers,
Iain
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I've recently been through this and from advice from this forum (thanks) and Cake St. Classics (Richard?) the first thing I checked was the ignition timing. This is sound advice as if there is a differnce in timing you'll not sort out the root cause of the problem. In my case the drive side was running a fair bit hotter and I initially though of induction bias. However, having spent a little time and a bit of money on one of the SRM timing disc set ups I found that the drive side piston was firing 5 degrees retarded (having timed the ignition from the timing side). You should bear in mind that the mag had been refurbished and a new cam ring fitted. You could actually see the difference in the cam ring when you compared it to an original. Anyone want a cam ring with different ignition settings as I've got a brand new one in my scrap box? Anyway, once I'd thrown the new cam ring some distance up the garden and refitted an old one I found the timing spot on on both cylinders and now running well (even temperature). The use of a strobe, timing disc, and magneto timing buzzer all helps. I know that some people prefer to use fag paper and a stick down the plug hole, but it's nice to see the the bike running and the strobe showing you exactly at what degree it's firing on both cylinders.
Brian
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Just put one (from Cake St) on my A10, along with a new carby and the bike has never been better.
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I have a biased washer on the Flash and it seems to work although I'm a bit lost as to how sucking less of the same mixture (air/petrol) would make a cylinder run weaker
I think I can answer this one! *smile*
This is due to the Bernoulli effect - ie the principle on which carbs work. The rush of air through the constricted carb choke lowers the pressure drawing up the fuel through the main jet. The effect gets stronger, the faster the air goes. At low speeds the effect is quite small and carbs need pilot jets to get enough fuel through as nothing will come through the main jet and up the tube into the air flow. You can see this in action by getting an old main jet, putting an 1/8th drill through it and running your bike. It will run exactly the same as with a normal jet at most throttle settings in normal running. Only when giving it a handful at higher revs will you notice the difference.
The single carb on a twin cylinder bike will therefore supply a different mixture depending on the air intake speed. If each side is sucking different amounts in, each will run at a slightly different mixture. By biasing the carb you are trying to slow the air down a bit on the 'quick' side and speed it up on the 'slow' side.
*smile*
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By biasing the carb you are trying to slow the air down a bit on the 'quick' side and speed it up on the 'slow' side.
Thanks for that T20racerman, a very eloquent explanation
I'm not sure I see how this works on a single carb though, the main jet being central and I would have thought far enough from the point where the inlet tract splits to negate any difference between cylinder suction at the point over the main jet, but I am not going to argue with your more expansive knowledge and experience in the matter and the fact that it seems to work.
Thanks again
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It doesn't matter where the main jet is - each cylinder 'sucks' in the air individually through the same carb, but at different times, and it is the air choke speed for each 'suck' that determines how much fuel mixes with the air. So, if each cylinder sucks in different amounts, the air choke speed will go fast-slow-fast-slow-fast-slow etc making the fuel mixture go rich-weak-rich-weak-rich-weak etc.
On my T20 race bike I changed the porting a bit and upped the carb size from 2 x 30mm to 2 x 34 mm. This enables more power, but the air speed through the choke decreases massively. I went from averaging 220 main jets (you vary a lot racing 2 strokes) with the 30 mm carbs, to averaging 460s with the 34mm ones! Quite a difference and hell of a size for an air-cooled 250 twin! *smile*
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That's really interesting T20. I was wondering if the angled gasket was just a pleasingly crude bodge which disguised a fault occurring somewhere else rather than curing it.
And you've described it in a way I could understand.
Cheers,
Iain
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I've got it now T20, sorry forgot for a moment theyr'e not both sucking together (it's an age thing, my excuse anyway)
Thanks again
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I've often wondered if this was caused by some sort of imbalance or gas leak in the exhaust system causing slightly different levels of back pressure..
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That could be one of the many causes. The out going gas also aids in the cylinder filling (intake). A little less comp on that side, or a bad valve seat, or a head gasket leak, uneven timing, etc, etc.
Cheers
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I have a biased washer on the Flash and it seems to work although I'm a bit lost as to how sucking less of the same mixture (air/petrol) would make a cylinder run weaker
I think I can answer this one! *smile*
This is due to the Bernoulli effect - ie the principle on which carbs work. The rush of air through the constricted carb choke lowers the pressure drawing up the fuel through the main jet. The effect gets stronger, the faster the air goes. At low speeds the effect is quite small and carbs need pilot jets to get enough fuel through as nothing will come through the main jet and up the tube into the air flow. You can see this in action by getting an old main jet, putting an 1/8th drill through it and running your bike. It will run exactly the same as with a normal jet at most throttle settings in normal running. Only when giving it a handful at higher revs will you notice the difference.
The single carb on a twin cylinder bike will therefore supply a different mixture depending on the air intake speed. If each side is sucking different amounts in, each will run at a slightly different mixture. By biasing the carb you are trying to slow the air down a bit on the 'quick' side and speed it up on the 'slow' side.
*smile*
Best explanation I have yet to hear for this unbalanced cylinder firing problem, it even makes sense. Between the Cake Street Classics #67-357 anti bias gasket, and having the poorly cast BSA Intake manifold, flow tested, and corrected, this problem should go away entirely. What say you Musky you raced these, please comment on this.
BILL
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I'd still like a dyno test to see the difference or some kind of colour tune setup?
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Firstly when I raced I had two carbs and yes they were slightly different but only in the float height to get the plugs equal.
With a single carb the only way to correct bias is a flow bench port job or a bias gasket, and then it's a guess as to how much bias to give it.
Over time the state of tune can change (a valve or rings might be going off) and as I recently found on my '51 I had to turn the bias gasket around to favor the other cylinder. The only way to test is to hook up an oxygen sensor to both pipes (after making sure there's no air leaks). A colour tune will help but not as good.
Cheers
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G'day all.
Well I couldn't help myself, I had to find out why I had to reverse the gasket. I have been getting the odd pop in the ex pipe so thought valve.
Left ex guide loose, and I mean LOOSE. That bad it could rock a mm fore & aft. When removed the shank was tapered 8 thou". Investigation as to the cause needed. The right side ex had a broken inner spring.
Cheers