Harv's AMAL thread

[Harv]

The postie dropped off a package today:

Monoblocs.jpg (892.87 KiB) Viewed 2117 times

Many thanks to Darryl for raiding the spare parts department. If a couple of big burly British bikers show up looking for their carbs, tell them you haven't seen me .

Lots of learning here for me. I've got some good tuning literature from the US, but there seems to be precious little out there on the basic carb circuit operations, and absolutely nothing on running them on a car engine.

Will share what I learn as I go. If I start talking about buying a Triumph or Beezer, someone smack me and point me back in the right direction (I get distracted easy).

Cheers,
Harv

[gpi]

the grin youll get is different. wind assisted

[In the Shed]

Good stuff Harv
A distraction towards two wheels is a good thing . I have some AMAL carby info on the version prior to the monobloc. The theory/principle is the same just the fuel bowl is separate to the body. I doubt I will be able to upload here it as the file exceeds 1 MB. If it is of any use perhaps I can get it to you some other way?

Regards
Stephen

[Harv]

Many thanks... PM sent

Cheers,
Harv

[In the Shed]

Harv,
This may also assist

http://amalcarb.co.uk/downloadfiles/ama ... d_Tips.pdf

Regards
Stephen

[In the Shed]

And another

http://archives.jampot.dk/technical/Car ... d_tips.pdf

[Harv]

So you want to run Amals on a grey? Hopefully the information below helps you to get there. I am going to draw from a heap of information that is available online, but also from the book Super-Tuning Amal Monobloc, Jeffrey Diamond, Victory Library (http://victorylibrary.com/brit.htm). Jeffrey’s book is damn good – lots of technical info and practical hints. I’m not going to upload it here (as it is copyrighted), but will draw info from it as I go. If you want to play with Monoblocs, this booklet is worth buying (his booklet on Concentrics is similarly good value).

There are various types of Amals, but I am going to focus solely on the Monoblocs. Amal made a few different types of carb, but the two most common are the Monobloc (which has an integral float chamber off to one side) and the Concentric (where the float chamber is under the carb). The Monobloc was available from 1955, and hence was around for the grey motor’s hey-day. The Concentric didn’t appear until 1967. I’ve seen a few Concentrics on grey motors, but realistically by the late 60’s/early 70’s the grey had been ousted by the red motor. There are some pre-Monobloc Amals that could do the job, and some fancier Amals (the GP and TT carbs) that were probably only available to blokes with a bigger budget. The Monobloc feels more period correct though – right era, and readily available to the general enthusiast.

So you bought some greasy, dirty carbs off eBay… just what are they?

Monoblocks came in four series: 363, 375, 376 and 389 series. The easiest way to tell which series carb you have is to check the specification number stamped onto the manifold flange, as per the red circle in the photo below:

Specification number.jpg (544.92 KiB) Viewed 2003 times

The stamped number will start the series, followed by a few numbers (e.g. 376/303). You can then use the online Amal catalogue (https://issuu.com/burlenltd/docs/amal_catalogue_2013-8) to find more information on your carb.

Each series also came in a variety of bore diameters:
363: 0.475", 5/8", 11/16", 25/32" and 13/16".
375: 21/32”, 23/32”, 25/32”, 13/16” and 7/8”.
376: 15/16”, 1” and 1 1/16”.
389: 1 3/32”, 1 1/8”, 1 5/32” and 1 3/16”.

It can be difficult to measure the bore size, so the easiest way to confirm is to strip the carb down and read the numbers cast into the jet block. These are the bore size, in the nearest 1/1000”, For example in the photo below, the number cast in is 1.062 (11/16”).

jet block casting.jpg (1.55 MiB) Viewed 2003 times

The bore size should line up to the info in the Amal catalogue too.

Now that you know what carbs you have, it’s time to pull them down. The Amals are pretty simple to strip down, with no fancy tools required and no special clips that go sprong! and fly across the shed at the speed of light. Take your time, and it should all unscrew. I found that the main body/tower bore can be manky with coke, and sometimes the throttle valve and jet block need some gentle persuasion to get them to come out. Go easy… this is all zinc alloy, and will bruise easily if you are heavy handed with a BFH. Once apart,give everything a birthday with some petrol and a toothbrush. Go easy with the probing tools… no oxy jet tip cleaners. There are a few very fine tolerance holes and machined surfaces that need gentle love, not flogging out until brass shavings appear.

Once everything is clean, there are five tuning parts to check to make sure are the same across all your carbs. These are the bits that could have been swapped out by a previous owner. The photo below (from left to right) shows the jet needle, the main jet, the needle jet and the pilot jet.

needle and jets.jpg (1.63 MiB) Viewed 2003 times

The red circles in the photo show where numbers should be stamped into the parts. The numbers can be really hard to read though.

The photo below shows the throttle valve. Cast into the top of the throttle valve is the series number, followed by cutaway number. In the photo below the number is 376/3½.

throttle valve casting.jpg (1.58 MiB) Viewed 2003 times

The options on the tuneable parts are as per below:
jet needle: one needle only for 363, B for 375, C, C1 and C2 for 376, D and D2 for 389.
main jet: 55-150 in increments of 5, 160-600 in increments of 10.
needle jet: 105, 106, 1065 and 107.
pilot jet: 20, 25, 30, 35, 40, 45 and 50.
throttle valve: 2, 2½, 3, 3½, 4, 4½ and 5.

More to follow.

Cheers,
Harv

[Harv]

So how does this thing work? There is a fair amount of info out there about Monoblocs, but a lot of it describes how the parts work together, rather than how each part works. In the info below I’ll see if I can explain how they work circuit by circuit, similar to the other carb guides I have written.

Lets start with the float system. Monoblocs have a simple float, needle and seat, very similar to the grey motor Stromberg carb (see image below).

Float system with red.png (67.52 KiB) Viewed 1971 times

The floats were originally brass, though plastic versions are also available (often marketed as Stayupright floats). The original float needles were also brass, though plastic ones and Viton-tipped brass versions are also available. Fuel pressure (the red arrow in the diagram above) enters the carb via a banjo fitting, squeezes past the float needle and splashes down into the float bowl. The fuel level rises, the float pushes the float needle up into the float needle seat, and fuel flow stops. The float system is trying to maintain a level a 1 9/32” above the bottom of the float chamber housing, as shown by the blue arrow on the diagram above. On the early Monoblocs, the float chamber cover has a “dimple” cast into it at this level (see photo below).

float bowl cover with dimple marked.jpg (923.2 KiB) Viewed 1971 times

Monoblocs are set up to be simply gravity fed from a motorcycle fuel tank, with only about a foot or so of height difference. This leads to a very low fuel delivery pressure (around 0.3psi) that the float is try to fight… very much lower than the 4½psi that the grey motor pump will put out. If you try to bolt on a set of Amals and plumb in the factory fuel pump, you end up with badly flooded carbs because the float does not have enough grunt to close the needle and seat. More on this later.

Amals are a slide carburettor. The bore of the carburettor has a throttle valve (a hollow piston shape), very similar to the SU. Where the SU uses vacuum to draw the valve upwards, Amals instead use a cable (the throttle cable), connected to the motorbike’s throttle bar. The more throttle, the higher the more cable pull and the more the throttle valve lifts. In the diagram below, the throttle valve is opened (lifted) more and more going from left to right:

Throttle opening and controlling circuit.png (86.83 KiB) Viewed 1971 times

The image above also indicates that at different throttle openings, different circuits take control of fueling. From idle up to 1/8 throttle opening, the pilot jet feeds the carb. From 1/8 to ¼ open, the throttle cutaway is controlling the carb. From ¼ to ¾ opening the needle position is dominant, and from ¾ to WOT the main jet is in control. We’ll look at each of these circuits below.

To first start the vehicle, the choke system is applied. This has two parts. Firstly, an “air valve” (glorified block of metal) is lowered into the carburettor throat. This blocks off a lot of the air flow, giving extra vacuum to suck on the pilot jet, and hence a richer mixture. Just like the throttle valve, the air valve is operated by cable. In the diagram below, the air valve is marked in yellow, and is lowered in the direction of the blue arrow.

Choke operation.png (75.14 KiB) Viewed 1971 times

Secondly, some extra fuel is added by the tickler (no… I’m not making this up… the factory name really is the tickler. Love it!). The tickler is a small, spring-loaded metal tube that fits in the top of the float bowl. The tickler is pushed down, which sinks the float, letting the float bowl flood. Hold it down long enough and you fill the engine with fuel. The tickler also acts as a float bowl vent during normal operation. The diagram below shows the tickler operation. In the image to the left, the float bowl level is low, and the float needle is opened up trying to fill the bowl to 1 9/32”. In the image to the right, the tickler is pushed down (blue arrow), and the float bowl is flooding:

Tickler.png (82.16 KiB) Viewed 1971 times

On a grey motor Strommie, the choke is similar (though is a butterfly rather than a metal block), and the tickler is the equivalent of giving the Strommie a few accelerator pumps before starting.

Once the carb has been choked and tickled (nyuck nyuck nyuck), the vehicle can be started. Starting and idling is achieved with the pilot jet. The diagram below is a cross-section of the lower part of the carb. The pilot jet is shown in green.

Pilot system - pilot outlet.png (48.79 KiB) Viewed 1971 times

Fuel is drawn from the float bowl up the pilot jet, and from there is sucked into the carburettor bore through the pilot outlet. This is a very small hole in the floor of the carb throat, downstream of the throttle valve, shown spraying fuel in the diagram above. The throttle valve is shown in purple (remember it is a hollow piston). Air to emulsify the fuel is drawn in from the pilot air inlet, which is a drilling in the carb air horn. The pilot air is shown in dark blue. The air passes the pilot adjusting screw before mixing in the pilot jet. The screw is shown in the diagram above as the round circle above the pilot jet… this is the round tip of the pilot adjusting screw, seen end-on. The screw can be adjusted to give more or less pilot air, which changes the pilot mixture. This is similar to the idle mixture screw on a Strommie, though the Strommie screw controls fuel and the Amal screw controls air. Initially, with the throttle valve closed, some air is also drawn into the pilot through the pilot bypass. This is another very small hole in the floor of the carb throat, though this time upstream of the throttle valve. Air flow into the pilot through the pilot bypass is shown in pale blue in the diagram above.

As the throttle is opened, the vacuum in the carb throat changes. The pilot continues to flow fuel through the pilot outlet, but now flow reverses in the pilot bypass. Instead of letting air into the pilot jet, the pilot bypass also starts flowing fuel. The diagram below shows the (purple) throttle valve opened a little, and both the pilot and pilot bypass flowing fuel. Air (blue arrows) is entering the carb both via the air horn and via the pilot adjusting screw.

Pilot system - pilot bypass.png (49.85 KiB) Viewed 1971 times

As the throttle is opened even more, the needle jet begins to flow. In the diagram below, I have “turned off” the pilot system for clarity, but in reality it continues to flow as the needle jet comes into play.

Needle jet system.png (46.97 KiB) Viewed 1971 times

Fuel is drawn from the float bowl up through the yellow main jet, and through the green needle jet. The needle jet is partly restricted because the tapered needle is in it. I have coloured the needle (in the centre of the diagram) in purple, as it will move up and down in concert with the purple throttle valve. Fuel flows out of the green needle jet and up into the carb throat. The diagram shows the fuel spraying away into the carb throat. Some air is drawn in via the carb throat (shown in pale blue), and some emulsifying air is drawn in through a drilling in the air horn and into the green needle jet (shown in dark blue).

[Harv]

At this stage, the throttle valve cutaway becomes important. The cutaway is a large slice taken out of the upstream side of the throttle valve, circled in orange in the diagram below.

Throttle cutaway.png (47.31 KiB) Viewed 1971 times

This slice allows more of the air-horn pressure (and less of the inlet manifold pressure) see the jets. The bigger the cutaway, the less vacuum at the jets, and the leaner the mixture. The cutaway determines how the carb transitions from the pilot jet to the needle jet.

As the throttle is opened even more, the needle continues to lift out of the needle jet. This provides less restriction, and allows more and more fuel to flow through the needle jet. Eventually, the needle becomes a minimal restriction (or is pulled all the way out of the needle jet), and the only restriction to fuel flow becomes the size of the hole in the bottom of the yellow main jet, as shown in the diagram below.

Main jet system.png (47.67 KiB) Viewed 1971 times

Cheers,
Harv

[Harv]

The idle speed on the Amals is set by a throttle adjusting screw (similar to the grey motor Strommie). Whereas the Strommie screw tilts the throttle butterfly open more, the Amal screw lifts the hollow throttle (shown in purple in the diagram below) valve higher in the carb throat.

Idle screw.png (43.4 KiB) Viewed 1964 times

The diagram above also shows the pilot adjusting screw we mentioned above, those this time around is it shown horizontally instead of end-on.

Amal also indicate that the needle jet has some fuel for “snap acceleration”. By measurement, the fuel level in the carb and needle jet is similar to the image below. The fuel is supposed to flow up the middle of the green needle jet. However, outside the needle jet (the area circled in orange) is a well that allows emulsifying air into the needle jet. This well will also have a fuel level in it – with the engine stopped, the needle jet is full of fuel, and the well is topped up by fuel flowing backwards from the centre of the needle jet through the air bleed holes and into the well.

Snap acceleration.png (47.85 KiB) Viewed 1964 times

Under snap acceleration, there is a sudden inrush of air to the carb throat (the inrush dies down pretty quick as the pressure in the carb and airhorn equalise). The fast air flow is enough to vacuum up this small well of fuel, flowing around the outide of the green needle jet. The squirt of fuel prevents dead spots in acceleration. This is similar to the accelerator pump in a Strommie, but works purely by siphon. The well will not refill very well with the engine still running, as emulsifying air is flowing into the needle jet, preventing the centre of the needle jet from topping up the well. The engine would need to come off the main jet and let the well fill again before being able to use the well for snap acceleration.

The needle is also tuneable. It is mounted into the throttle valve by a clip, which clamps onto a groove in the top of the needle. The needle has five different grooves, allowing the needle to be raised or lowered (with the engine off and the carb in bits). This is referred to as clip position. Raising the needle puts a thinner part of the taper into the needle jet, giving less restriction and making the mixture richer. Lowering the needle makes the mixture leaner.

The diagram we used above showed which system is dominant at different throttle openings:
From idle up to 1/8 throttle opening, the pilot jet is controlling,
From 1/8 to ¼ open, the throttle cutaway controls transition from pilot to needle jets,
From ¼ to ¾ opening the needle jet is controlling, and
From ¾ to WOT the main jet is in control.
In reality, there is overlap between all these circuits. The diagram below gives a good idea of the overlap, through this changes which each carb.

Circuit overlap.jpg (94.73 KiB) Viewed 1964 times

Cheers,
Harv

[Errol62]

Hail professor


FB ute fixer upper, EK van on rotisserie

[Harv]

Got the replacement parts order off to Amal UK today:

http://amalcarb.co.uk/

Cable adjusters, screwed plugs, ferrules and cable adjuster nuts for all three carbs (to connect a throttle cable and blank off the choke),
Mixing chamber cap spring and screw (one was missing),
Two float spindle bushes (floats were moving sideways),
Throttle valve springs (only had one spring, not sure if it is a light or heavy so bought three),
Pilot jet (I mauled one getting it out),
Float needles x 3 (nice Viton tipped ones),
Two needles (had one correct "C" needle, one bent "C" needle, and one alcohol needle),
Jet block, banjo seating, banjo bolt, pilot jet cover, side cover and jet holder washers,
Three O-rings for inlet manifold,
Throttle valve (had two 3½'s and one 4, so bought one to match the set up).
Two banjos
A main jet (had two 150s and a 250 so matched them up)
A 106 needle jet (couldn't read the marking on one of them).

Cheers,
Harv

[Harv]

Some Amal photos on early Holdens to lighten things up a little.

The first lot are of Des West’s FX Holden, with what looks like GP Amals. This beastie competed in the first Australian Touring Car Championships in 1960 (Gnoo Blas, near Orange, NSW), coming 11th:
https://en.wikipedia.org/wiki/1960_Aust ... ampionship

Des West GP Amals 2.jpg (89.38 KiB) Viewed 1936 times

Des West GP Amals.JPG (264.46 KiB) Viewed 1936 times

Note the plastic fuel lines for each carb, running across to the passenger side. There appears to be a fuel distribution block on the inner guard, being fed from the vertical steel bottle mounted to the firewall. My guess is that the humpy fuel pump feeds the bottle (with some sort of float), then the bottle feeds the Amals at low pressure (just like a motorboke fuel tank). The top of the carbs have pull cables, operated by a simple lever system.

The next lot are Trevor Grace’s FX racer, still doing the rounds. These look to be Monoblocks. Another tank on the firewall (this one definitely fed from the grey fuel pump), and again some simple cable pulls to actuate the carbs.

Trevor Grace Amals 1.jpg (47.76 KiB) Viewed 1936 times

Trevor Grace Amals 2.jpg (127.34 KiB) Viewed 1936 times

Cheers,
Harv

[Harv]

Speedcars is cool

#27 Speedcar Amals 2.jpg (270.13 KiB) Viewed 1928 times

#27 Speedcar Amals.jpg (278.89 KiB) Viewed 1928 times

Cheers,
Harv

[Errol62]

Gee the water pump drive on that speedway motor, underdriven or what?

The Trevor Grace humpy looks cool. Love a racing humpy.


FB ute fixer upper, EK van on rotisserie