276 Amal

[flatdeck]

Hi guys, I am re-gasketing 2 x 276 Amals for the Star Twin. I have blown them out with compressed air and am now a bit more confident that some steady idling might be achieved. As usual doing this has raised as many questions as it has answered :-)
In the photos below you will see (just) that in the middle of the brass plug at the bottom of the mixing chamber body there is a hole. This I presume to be the pilot petrol supply. At the top of the thread for the mixing chamber union on the body there is another hole. Before the carb was cleaned this hole was fairly well blocked by an unknown substance that looked like it was there by design. Suffice to say the other carb is the same so yields no clues. In the second photo you can see that when the union is done right up the hole is visible. The 3rd photo I have blagged from a web page showing a exploded 276. The brass insert appears to have a hole in the right place for the hole in the body to match up to ... is this the pilot air supply??? I don't have the carb in front of me and now I'm confused. Any clarification would be gladly received.
Cheers,
Dave

[beezalex]

I think the hole on the side of the body is for alignment between the main carb body and the brass throttle body.  The idle fuel comes through the little hole in the bottom of the throttle body and the idle air comes from the primary air inlet.

...at least that's how I remember it.

[a10 gf]

At the top of the thread for the mixing chamber union on the body there is another hole

afaik, in case of flooding into the mixing chamber, petrol will run out there instead of into the engine.

Pilot air intake is the quite large hole at the bottom of the bore at the inlet side.

[LJ.]

in case of flooding into the mixing chamber, petrol will run out there instead of into the engine.

It's primarily to act as a drain and I think mainly to prevent petrol being spat out if a backfire through the carb should occur, this was a hazard faced by Dispatch riders on M20s and one of the reasons why fire extinguishers were carried. It's crucial that its kept clear. Sadly though with petrol draining out there it gives the impression that the gaskets are leaking, so do ensure that everything is new and put together correctly, I use ptfe tape round the threads which helps alot.

To prevent overflowing petrol from the carbs it is important to get the floats the correct hight in the float bowl by adjusting the brass float needle, easy to adjust and can be done with a full bowl of petrol. Again it appears from my own experience that it is crucial that the level is correct, too full and it overflows from that hole just above the threads and if too low the bike will always need a good tickle before starting even if engine is warm. If you can get both carbs equal then you'll find they'll both require the same length of time to tickle. Probably you'll know all of this anyway but I've found these types of carbs don't like to be left on a side stand especially with the petrol tap open it might be why the early Star Twins had no side stands because of this.

I've actually surprised myself in how well my twin carbs have performed with hardly any petrol leakage and how the bike runs evenly. But yet with my M21 it is still constantly wet with overflowing petrol, cant win can ya?

[flatdeck]

Thanks guys. One of the most frustrating parts of this hobby is not knowing how things are meant to be ... so often I read instructions like .... "if the wotsit is worn replace it". Sometimes the shapes of some of these items don't exactly give away whether they are worn or not ... makes it all interesting though.
Cheers, Dave

[chaterlea25]

Lads! Lads! Lads!!

Lots of bunkum here

The hole at the top of the threads is for the pilot air.

The large hole at the bottom of the inlet (choke) is for Atomising air for the needle jet

One thing to be VERY careful with on reassembly is that the paper washer in the big chromy nut does NOT block the little hole in the base of the jetblock (big brass bit) as this is the hole for the pilot petrol!
Squirt some wd40 or similar with a long nozzel up the hole and you should get it to spray out the two tiny holes, one on the engine side of the throttle slide and one almost under the edge of the slide,
this is where the pilot mixture enters the engine when the throttle is on tickover
HTH
John

[a10 gf]

Ahhh, a good discussion starting.

I stick to what's written in my former post. Am sitting here with a jetblock and a carb body, even digging out Amal's original 276 "how it works" service doc. Would go as far as agreeing the hole at the top of the thread would suck in a small amount of air, but the main air inlet for idle and lower throttle opening is what amal denotes as... the air inlet (big hole at inlet side). I think one could block the side hole, the carb would still function perfect. Try forcibly flooding a 276, petrol appears first at the lower side hole, in practice avoiding filling the carb body and flowing into the inlet manifold.

Ehhh... 'Atomising air'... are you sure? I'd think the air is not liquid, and needs no atomization... the petrol does.

Anyway, I'd be happy being proved wrong, always eager to learn more about the 276.

Edit: But let me rephrase the somewhat imprecise "Pilot air intake is the quite large hole at the bottom of the bore at the inlet side" to "Air to pilot comes from the quite large hole at the bottom of the bore at the inlet side".

[LJ.]

The hole at the top of the threads is for the pilot air.

It probably serves two purposes as I know full well when I wrongly blocked it off the bike still ran and spat back lots of petrol when over flooded. I think also that the previous carb the 275 does not have this hole. Maybe Brian who has a pair of them on his Star Twin can confirm?

[chaterlea25]

Hi A10 GF,
Air is needed to atomise the petrol hence "atomising air"
If you push out the jet block, follow all the little holes drilled in it, the one from the hole at the top of the threads
goes towards the "pilot AIR screw"
The air being drawn in picks up the petrol and the ensueing mix feeds the engine.
At tickover there is no air rushing in the big hole to the needle jet, there is no air rushing over the needle jet top
to draw petrol mix into the engine!
The tickover air petrol mix is provided by the pilot circuit,
petrol flow is regulated by the small drilling in the jet block and air flow is regulated by the adjusting screw,
open the screw, more air leaner mixture!
close the screw, less air richer mixture!

HTH
John

[a10 gf]

I think beezalex got it right in his post above.

chaterlea25 wrote

At tickover there is no air rushing in the big hole to the needle jet, there is no air rushing over the needle jet top to draw petrol mix into the engine!

(In my belief) the tickover\idle\pilot is fed trough it's own petrol supply, with it's own circuit and jet in the jetblock, and the air the for the pilot air control passes trough what Amal denotes as the 'primary air passage', which is the continuity of the air inlet at the bell\airfilter side. Yes, no rushing over the needle top, but at the bottom of the mixing chamber, fed by the primary air passage.

the one from the hole at the top of the threads goes towards the "pilot AIR screw"

Nope, on the rh float carb body I am handling right now the hole at the top of the threads is nearer the opposite side farthest away from the pilot air intake. On the lh float body depicted in the 1st post the hole is closer to the air inlet, but (unless we are talking about different 276 carb models...! ) still not directly feeding the pilot air inlet, although 'towards' could apply for that carb. Flatdeck may check his jetblock out.

The pilot air gets it's air from the air inlet > primary air passage, am even checking with Amal's 'How it works' diagram depicting internal air & petrol flow. But (as mentioned in earlier post) a quite small amount of air will surely be sucked trough the overflow hole).

I was 99.9% sure before, here's the missing 0,1%: quoting Amal's 276 original service doc: ...the overflow... will not run into the engine but out of the carburetter through a hole at the base of the mixing chamber.

btw, I really like the 276 design, intricacies and discussions, so thanks for this opportunity for investigations and learning. Edit, to add to the megapost, picture of string trough airfeed to pilot adj., passing needle jet, exiting trough primary air inlet (overflow hole far away right side).

[flatdeck]

This is interesting ... http://velobanjogent.blogspot.com/2009/08/276-amal-carburettor-from-early-1930s.html

[a10 gf]

Edit: checking the 276 in practice, at tickover, blocking the overflow hole has no influence whatsoever, blocking the 'primary air passage' (as Amal calls it) at the bell\filter side stops the engine.

Thanks for the good link. A parts list there shows the early type (76?), with air intakes at the bottom of the body. John, is this the one you were referring to?

[beezalex]

All right, I wanted to go back and see whose bunk was right and whose bunk was wrong...had both a 276 and 289 laying around and they showed the same thing.  Turns out everybody is sorta right.  The hole (that I'd forgotten about), which I'll call the secondary air inlet/drain behaves as just that, a secondary inlet for both idle air inlet and as a drain for the emulsion chamber.


So, just to sum it up, idle air mostly enters the main air inlet at the bellmouth of the carburettor.  The air enters a chamber surrounding the needle jet and emulsifies the fuel for normal running.  At idle, it also supplies the idle mixture screw through the idle air supply hole.  The idle mixture screw controls the air entering the mixing chamber, where fuel from the fuel inlet is mixed after passing through the pilot jet and drawn through the idle spray tube into the intake.

Hope that clears everything up.

[RichardL]

Alex,

Did you draw the picture? If so, wow, great job. Nevertheless, a great lesson.

Richard L. 

[a10 gf]

Good 3d artwork!

A few comments, idle jet output is in the carb body, at the other side of the throttle. Amal calls your idle spry tube the pilot by-pass, which afaik is coming into action after some throttle opening. And I still stick to my earlier post, the bottom hole is there solely for overflow control, and cannot be labeled as any air intake. As mentioned earlier (Amal service sheets): overflow...\...out of the carburetter trough a hole at the base...\..., it's function as air intake is 100% negligible, and it can be blocked with no interfering to the air supply or behaviour of the engine.

ps: please excuse my tenacity in the matter