The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Amal, Carburation => Topic started by: AdrianJ on 24.03. 2019 13:55
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My 53 A10 has the carburetter mounted on two studs screwed into the head.
My understanding was that this should be two screws.
Does any one have any idea why these are studs and why I shouldn't replace them with screws?
They make getting the carb on and off difficult.
Thanks in advance.
Adrian
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Studs have a fine thread on the nut-end which are more resistant to vibration and easier to control the torque applied.....also repetitive screwing and torquing into Ally can destroy the threads prematurely. If you need to remove the studs, just use lock-nuts to undo and replace them. I don't think they need undoing very often when everything is sorted....
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Plunger with iron head: I've got bolts
screws. Allows the carb to be removed while the head is on.
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The parts book has screws. I understand the alloy head/stud issue, but why would you bother with studs in an iron head? It makes putting the carb on with the head in place difficult. I have to put the slides in then put the carb on, then screw the ring and slides down in a very tight space. If there is no reason not to i'll replace the studs with screws. I have a horrible feeling that when I take the studs out they will turn out to be metric on the head side or something equally daft.
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Adrian..You must remember that when these bikes were new, they were relied upon to take their owner reliably as and when required. So, if you broke a bolt on a Sunday morning, anything to hand was used or adapted, as then the whole country came to a halt on Sundays.
BSA were unusual in their use of the fine cycle thread, which was ideal for the application and suited them from their cycle manufacturing heritage. Spare cycle thread nuts and bolts were rare in the average domestic setting. However, most blokes of the time had a tin of nuts and bolts, most often coarse Whitworth, which could usually be relied upon to fit nothing. So Whitworth bolts were forced down UNC threads, sometimes the sizes matched (almost) well good enough. BSF and cycle? Almost but not quite. So, you get the picture, that's how we get these mechanical travesties.
Someone has decided that the easiest way around his problem was to use bolts, so that's what happened, and sod the consequences for the next lucky owner.
Your head should have cycle thread on the inlet tract. If it has been re-tapped metric, M8 will be the thread to try now, but you may find the standard M8 bolt head fouls on the carb body.
Swarfy.
Additional. Parts book lists 02-129 as the official bolt. Once more musky's very useful chart helps us out, identifying it as 5/16" CEI Thread, 3/4" bolt, but with a small hex tall head to aid installation. This head is the same hex as the thin walled gearbox cover nuts.
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Thanks folks.
I'm going to try to return it to bolts, if the studs that are in there haven't got daft threads.
Shoud probably be on the carburation thread, but thinking of replacing the 276 with a monobloc anyway. I haven't got a plunger air filter and have no prospect of finding one so a 376 might might be an improvement.
Regards,
Adrian
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Could always use the studs as bolts by threadlocking (or better still silver-soldering) the nuts on.
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Make the bolts with the smaller hex, 1/4" size which is a tad over 7/16" AF.
Makes life much easier.
I also make nuts for the studs on the alloy heads this size.
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I removed the carb on my 52 iron head with studs today, without problem, did have to remove the seat and tank first, obviously, like you do!!
Am I going to have a bit of bother getting it back on after cleaning the crap out of it??
Cheers
Jase
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Mine's the same. The tank has to come off first which is a bit of a faff.
Some are saying "bolts" and some are saying "screws" are better than studs (which I have). Is the meaning really "reduced hex set screws"?
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Mine's the same. The tank has to come off first which is a bit of a faff.
Some are saying "bolts" and some are saying "screws" are better than studs (which I have). Is the meaning really "reduced hex set screws"?
Yes and no. set screws will do but its best to leave some bare shank 3/4 depth of the carb flange not threaded all the way. ( reduced hex as trevinoz mentioned)
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Understood.
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I have standard head bolts for my 276 carb.
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I've just taken the studs out and the manifold is tapped 18tpi BSW.
The parts book says it should take 26tpi BSCY.
Is my head from a different model?
I've ordered some Whitworth screws but I'm unsure about repeated use and may leave the studs in even though they make it awkward to get the carb on and off.
Adrian.
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You can see in the picture that mine are not Whitworth. Is you'rs an iron head? Maybe someone in the past tapped out stripped threads to suit studs that he could get hold of.
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Adrian,
It is almost as I predicted, back in my previous post. Standard Narrow Fin Iron Plunger Head will have a casting number on the underside of the inlet manifold, to aid identification. BSCY was the original standard fixing, and a fine thread here sometimes fails over time, depending by how much it has been abused. So looks as if the bike has had some grief over the years, as indicated by G.B.
Swarfy
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Thanks Greybeard and Swarfy.
Yes, it's an iron head.
I seem to remember that my old swinging arm a10 had studs instead of screws. Many other things on the bike show that it has had plenty of grief over the years.
I can't stop the carb p*****g out petrol now. I'm thinking of throwing in the towel and fitting a monobloc. 🙂
Adrian.
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Adrian,
Before you condemn the existing 276 carb, where is the leak? Obvious leaks will be a simple case of new sealing washers, or attending to badly bruised/damaged parts.
The most common problem is an incorrect fuel level, due to problems with the float and needle, but here you have an easily detachable float chamber, so its easy to sort.
There are two types of chamber, top feed, with conventional pointed needle and float, valve at the top, and a bottom fed chamber, which has a valve at the bottom, requiring a needle with a reverse cone at the bottom. So, check you have the right needle for the float chamber, located correctly with a clip, and the float still floats and is not punctured.
Later bottom fed needles have two locations for the retaining clip, to cover early brass and later plastic floats. Worn needles and valve seats can be sometimes be lapped with Brasso Metal Polish to improve the seal.
A Monobloc Carb has very little clearance to the frame tube, so arranging any sort of air filter is a bit awkward. Set up correctly, there is little difference between the performance of either carb for normal road use.
If you decide to replace it, a Concentric may prove a better choice.
Swarfy.
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Swarfy,
It's pouring out around the thread at the bottom of the body. The cap has been cross threaded at some stage and while it still goes on properly, I think that this is the cause of the leak. The float is on the correct notch. I was lent a concentric, but the body was too tall to fit under the frame tube, a different model may fit though??
Regards,
Adrian.
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Adran, That is a real let down.
The seal between the bottom face of the brass jet block and the big bottom nut is just a fibre ring washer. Fuel should not reach the threads, so either the washer is absent/damaged or the poor thread is not strong enough to allow the two parts to clamp and seal it tightly enough. Any petrol proof material cut to fit, will do short term to get it going. Too thick, fuel level will be too low, too thin, higher fuel level, tendency to drip from the bell mouth.
If you go prospecting, any 276 body of the right choke size will do. The numbers stamped on the mounting flange identify the original factory internal spec. Secondhand, the critical parts are that bottom thread, top thread, then wear on the throttle slide area and seized/broken idle adjuster bolt.
For your A10, the original carb is 276ER/1DB, 1 1/16" choke diameter.
Have a look at the rest of the Forum regarding carb choices.
Swarfy.
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Thanks Swarfy,
I've just replaced Jets and washers from an Amal kit.
The seal you refer to was cork and is some synthetic material in the kit.
I'll try putting the old one back in. It will be Tuesday before I get any further.
Adrian
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Note that some sealing washers for this application are too wide, and partially/wholly obscure the air hole in the underside of the jet block.
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That's interesting Rex. My plugs are dry but I put it down to the petrol poring out of the carb rather than into the inlets. I'll see what the old seal does for me.
Adrian
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Hi Adrian and All,
A too high a fuel level will let fuel leak out at the top of the threads , there's a bleed hole there
No amount of tightening will stop the carb leaking if the fuel level is too high
https://www.draganfly.co.uk/data/pdf/Amaltype6tuning.pdf
and picture attached of 289 but they are all the same
John
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So, will it pour out whenever I tickle it?
Adrian
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Hi Adrian,
So, will it pour out whenever I tickle it?
Yes
John
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What I meant was, I know it will pour out as I tickle it, but will it continue until the fuel has found the correct level again? i guess it would. My old A10 (monobloc) squirted a bit when I tickled it but then stopped. This one just seems to keep on. The float doesn't seem to be sticking.
Adrian
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Adrian, this is getting a bit curious.
Tickling lowers the float, raises the level, fuel will then overflow wherever it can, starting at the lowest point of exit, either by the official bleed hole or the bellmouth. In practice it takes a few seconds for fuel to work its way via these exits, but the level rises quickly in the float chamber, spurts out all over your finger, job done. With the tickler plunger released, the needle valve is now jammed shut by float pressure, and there should be no further fuel flow into the carb. Is the tickler sticking down more than it should, when released, and holding the float down?
If your carb is bottom entry, take the top off the float chamber, see if fuel stops flowing. Check the needle can rise fully into the guide hole in the underside of the chamber lid. If fuel still flows past the valve, try raising the needle by hand. Re-assemble with some fuel in the chamber, lower the lid onto the raised needle to make sure it enters the guide hole.
Otherwise a continual drip would indicate a basic assembly error, cracked body, or simply a poorly performing needle valve. Valve seating, needle taper and float height are all suspect. If a metal float, check that is not leaking and there is no fuel sloshing inside to compromise its action. The old test was to dip it in hot water, the air inside expands, watch for bubbles, there's the leak.
Swarfy.
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Swarfy,
It's pouring out around the thread at the bottom of the body. The cap has been cross threaded at some stage and while it still goes on properly, I think that this is the cause of the leak. The float is on the correct notch. I was lent a concentric, but the body was too tall to fit under the frame tube, a different model may fit though??
Regards,
Adrian.
If you really want to try a concentric, try a 626 26 mm, the body is shorter. If it fits, bore it out to 27mm ( 1.0629" ) = 1 1/16" . *work*I've been running another bike with this mod for years now. Works fine.
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ironhead don't you have to deepen the seat for the slide to sit into *conf2*
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I've had a bit of this over the last year, (on a monoblock) last thing I did was to take a bit of stuff of the boss in the float chamber for the cover screw at 7 o clock, the float seemed to just touch it when rising, so far it seems to have done the trick, lots of other avenues were explored before I got to this one
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Swarfy,
It's bottom entry with a separate float bowl. I don't think its anything to do with the tickler to be honest.
I have a lot of fuel dripping off the bottom of the carb when the petrol tap is open. I'm not entirely clear where it's coming from. It looks like the thread on the bottom of the body. There is a breather hole on this thread which may be the source. The hole looks like it is below the normal fuel level, so why wouldn't fuel come out of it al the time? The valve cuts off the flow into the float chamber, but fuel already in there can surely just flow into the carb body.
I've checked that the float and valve are working and they seem to be. I'm obviously missing something about the operation of these carbs, because the normal fuel level looks high enough to flood the carb.
Tomorrow I'll get chance to get it on the bench and sort out exactly what's happening.
I'll change the internal seal and try adjusting the float level.
Thanks again for all your help.
Adrian.
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Joy unconfined!
Found a spare couple of hours. Dismantled carb again. Replaced seal below main jet and fibre washers.
Doesn't leak any more. Still a bit of a pain to get the bottom on as it is difficult to find the correct thread.
The engine has just started after a complete rebuild for the first time in 3 years.
Now all I have to do is find a toolbox (wrong one fitted, hoping to find one at Stafford) sort the electrics and make some sidecar fittings for my Steib S500.
When it's complete I'll post photos.
Many, many thanks for the invaluable assistance on here.
Regards,
Adrian
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Adrian
To answer your question, when set correctly, the float needle stabilises the fuel level to a point just below the top edge of the throttle needle jet, Item "0" on the usual AMAL Cutaway Diagrams.
With the level set too high, fuel can top over the needle housing, and come out of the carb at any convenient orifice.
You can see now how the thickness of that washer between the jet block and the big nut is critical as it governs the vertical relationship between the float chamber fuel level and the level of fuel in the carb body. Too thin a washer will raise the level too high, above the needle jet, hence the flooding.
When the engine is running, the venturi effect of airflow into the inlet tract lowers the pressure above the fuel, which draws the fuel into the carb mixing chamber and away you go.
Well done for sticking with it. Looking at your previous posts I can see a fair investment of time and money. So, always check the oil return to the tank on start up, run it in without labouring, forget the chair for a while, and just enjoy it.
Be aware that not everyone who admires you bike has your best interests at heart, the world is sadly a different place from when it first took to the road.
Congratulations on a runner.
Swarfy.
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Thanks again Swarfy.
Still have to sort the electrics.
I have oil return - new SRM pump.
The carb operation is a lot clearer to me now. It must have been the thickness of the seal.
With regard to your other point, I've stopped working with the garage door open, I don't want the wrong sort of attention.
Adrian.
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Adrian
Where are you, please can you come and fix my old leaky (276) on my Norton Model 7. I have changed everything and so did a Norton expert. He had a similar carb which he could get to stop leaking on my bike but he could not stop mine. It is better now but IF you over tickle the bike it just pores out and does not seam to stop even when you turn the petrol off. Start her and it stops. And god forbid that you pull up and put her on the side stand without turn the petrol off.
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Sorry Angus, too far away - West Yorkshire.
I think the float chamber has quite large capacity and if you over tickle it you have to wait a while for it to get back to the right level.
Good luck with it.
Adrian.
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ironhead don't you have to deepen the seat for the slide to sit into *conf2*
G'day Berger.
No, as you are only taking off .020" from each side of the hole. There's still seating area left. If & when I have to take it off I'll try & figure out how to post a picture.
PS It hasn't swallowed a slide yet. *eek*
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ironhead just wondered because dad bored out some rocket three carbs for bike mechanic when I was a young un, I can remember him having to recut the slide seats because Mr mechanic hadn't realised the slides wouldn't seat with the amount that was bored out. I don't know how much he asked him to take out though so it seems a little is ok a lot and you create another job.
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berger , yes I only took a small amount. There is still about a 1mm seat. From memory some Spanish 2 strokes from the 70's used Amal 27mm, that's what gave me the idea in the first place. maybe your dads mechanic tried to go to 28. ( the slide is only 30mm.)
I've seen tridents with Keihin's about that size. Also I'm pretty sure *eek* *eek* ( have to check) but 928 930 & 932 carb bodys are the same size. they just bore em bigger. I really only did this to see if it worked, I swapped a 350 single for a 500 & had to get the bike back on the road asap. that was about 5 years ago & still going.
Does at least 100k per week. ( probably swallow a slide tomorrow now that if said that) *shh*