T140v Amal Carbs Mk1

The bikes have been in bed for the winter. I drain all the fuel from the tanks and allow the float bowls to empty. I've always done this - even before Ethanol fuel - but it's probably more important these days due to the petrol now absorbing water and therefore the dreaded rust inside tanks.

I'm sure they've always rusted, probably due to condensation but it makes me feel better.

So, refuelled a couple of them with nice fresh E5 and did the starting heat up oil return check and all that. The red one wasn't running well......so suspected pilot circuit blockage due to the 'varnish' left behind by the drying fuel remnants.

Thought I'd give those, who may be interested, a quick look at the pilot circuit in a Mk1 Amal Carb.



Taken the carbs off. This is a bit of an arse if the studs are not secure enough in the head as they will want to come out rather than the nuts coming off. There's not enough room between the flange on the carb and the tickler to remove the nut unless it screws off of the stud. So if the stud moves then the whole lot has to come off.
Some actually file flats onto the studs so they can be spannered into the head more securely than the nut, I use threadlock (blue not red) which is normally enough to hold them while the nut is removed.



Here's both the carbs off. I unscrew the lids and allow the slides to come out with cables attached and leave those on the bike.
Note the 'fat' o rings.

 

Remove the two screws and carefully remove the float chamber. Hold it upright or the float, bar and needle will fall out



It's fairly easy to figure out how the fuel gets into the float bowl. It's much the same as a toilet cistern. The fuel level falls causing the float to fall which opens the needle and allows fuel to flow in. The filling bowl pushes the float back up causing the needle to close.



The brass bit on the bottom is the main jet. The needle that has stayed with the slide still on the bike slides into this jet. The more the throttle is twisted the more the needle is pulled out of this jet allowing more fuel through. If the throttle is not touched (sat at the lights waiting for the green light drag race) the needle is all the way down into the jet effectively shutting it off. Jets are available in different sizes, the bigger they are, the more fuel is allowed through.
In this case you can just make out the stamps in the brass - 190. A T140 haynes manual agrees with this, as does the workshop manual. However, the parts catalogue states it should be a 210, pfffft



The carb will need to enable the bike to run on tickover so it uses a different way to get the fuel to the engine. This is called the pilot circuit. This can be seen if you turn the carb over and look at it from underneath

 

Here's the pilot cicuit as a sketch



Thanks to Jim Bush for the drawing. It's fairly easy to follow on the diagram how the fuel gets to the engine.



Here's the carb sawn up to show where the fuel gets in, where the air gets in and how it's amount is controlled through the jet and also by the air/fuel mixture screw.
This is where blockages occur, usually the miniscule jet gets blocked or gummed up. So this is where it will probably need cleaning. It can block anywhere around the circuit so check it all.

 

To gain access to the pilot circuit the screws are removed from the carb.



On the other side to the tickler, tick over adjustment screw and air adjustment screw is a cross headed screw that blocks off the pilot circuit. Take it out and keep it safe.
Note that in older carbs this is blocked of with a plug that will need to be drilled out and replaced with one of these screws (unlucky if you have an older T120 etc )



No need to take out the tickover adjustment screw but I do, just because I can. Note the number of turns it takes to get out - that'll give you a clue as to where it needs to be when you put it back in.
Take out the air/fuel mixtrure screw. First, screw it in till it stops and note how many turns. Again make a note of that (it should be about 1 and a half turns).



Here's what they look like, don't confuse them. Left is the pilot circuit blocking off cross headed screw. Centre is the air/fuel adjustment screw (with the pointy bit). Right is the tickover speed adjustment screw with the flat end that pushes on the slide to raise or lower it.

 

With all screws out the passageways can be checked to ensure they are clear.



You can just make out the pilot circuit jet. If you refer to the diagram/sketch above you can actually blow into the air intake and cover each hole in turn to ensure there is air flow each time.
Also hold the carb up to some light and check that you can just about see through the pinhole in the jet.
If there's a blockage it will need to be cleaned out CAREFULLY. The jet size must remain the same DO NOT force anything through the jet to clear it. If I have to, as it won't clear with compressed air or carb cleaner spray I'll use one thread of copper wire taken from an electrical wire.
If it's really not budging then ask your friendly neighbourhood garage to use their ultrasonic cleaner to shift it.



Also check the float is adjusted correctly at this stage. There's a couple of types of floats (again the older versions are trickier so maybe we'll get into that on another thread at some point). The float should be closing (do this by pressing lightly on the tangs, not the needle) with the float at 60 to 80 thousands of an inch below the rim of the bowl. That's just under 2mm.


Reassemble the carbs. Make sure you get the bowls the right way round, they can be screwed together the wrong way round.
The bowls can be tightened up too much. They are made from some shitty type of zinc/alluminium/pewter/tin/plasticine stuff that can be bent out of shape. If you have a leak at the float bowl it's worth checking it to make sure it's flat on a plate of glass. It can be corrected by using fine wet and dry on a flat surface but go look that up if you are going to try it.

 

Once all back together they can go back on the bike.



The 'fat' o rings should be used. The 'thin' o rings are ok but they allow the carb to be bolted ridgedly onto the flange of the inlet manifold. This then doesn't allow any 'flex' to the carb and transmits vibrations and, more importantly, allows heat to be transmitted to the carbs. This can cause issues on hot days in bad traffic to vapourise the fuel in the bowls. The bike then just stops at the traffic lights and will not start - it's a right pain - and you feel like a bit of a knob as you push the bike to the side of the road just when you thought you was lookin cool too.

As the 'fat' o rings don't fit into the slot they need to be stuck on with some grease.



Carefully slide the carbs on while watching that the o rings stay where you want them.



Whack it all back together. Adjust the screws to where they were before (remember, you made a note of that). The air mixture screw should be about 1 and a half turns out, start with it in that position if it wasn't there to start with.
Make a cup of tea and admire. Then start it up and allow to warm up and adjust as per setting up carbs.
Although I didn't find any blockages (I could see light through the jets) I blew everything through to make sure it was all clear. It started right up and once warmed it ticked over like a teacher doing maths homework (what?) ....over...

Edit* others please add tips, advice or amendments as you see fit. It'll help me out I'm sure as I'm always learning and trying to use the force.

 

Amal do hard anodised slides but probably better to get them in while the bodies are ok. If they are sloppy the bodies are probably a bit worse for wear. Having someone who can sleeve your slide and bore your body is well handy. Rebuilding carbs with new parts sometimes just doesn't help. So yes, new carbs. (If you can get them - Amal always seem to be out of stock because the dealers have them on continous order) So go Mikuni. Or you can go Wassell. I've got a pair of Wassells on the shelf - they seem to be made of even more inferior light grey plasticine - I may try them one day. Longest way up, shortest way down......over.....