The Amal Mk1 and 1,1/2  Strip down, and Rebuild Page.

(And all Carburettor related Stuff).

Mikuni's further down the page...

Including  "Dirt-Doc's Carb tuning tips...

After a urgent phone call and a missed trial after paying the entry fee.

I thought not only One of you had found and had this problem, after a machine had been left for a while with fuel left in the system.

And even if the system had been drained you may still have the problem with the white corrosion.

Ethanol just takes in its own weight of water so this is the cause, water does not like zinc. Or alloy to that matter...

Remove  your carburettor and drain.

Lay an old table cloth or piece of carpet onto your bench.

This is mainly to stop you from loosing parts, by letting them roll off of a smooth surface. 

Yo have probably left the slide on the throttle cable of the bike?

Leave that for a while.

Remove the two screws holding the float chamber in place.

Gently remove the chamber, the gasket may be stuck.

Watch for the float and needle falling out, and the bits scattering.

See what I mean about the piece of carpet!

Leave the bowl for now, prop it upright and drop the two screws back into the holes the wrong way up.

You now have the body of the carb with the jets in, you can now remove and check the main jet and holder, be careful with the holder threads, as you know what zinc is like. Cheese...

The carb will either have a pilot jet or bush, if it has a brass jet the carb is old, or a two-stroke version, as these were changed in 1967-8 to a pilot bush, for four-strokes.

This bush I have found is the main problem with this carb, and is probably blocked with crud...

Unscrew the air screw and remove, behind this is the dreaded bush.

Photo Amal carburettors.

I have found the best way of cleaning this bush out, is with a length of old guitar string. (wire)...

And wearing a pair of magnifiers, to make sure you get through the pilot hole.

There are two more airways that need to be checked on this pilot system, two small holes either side of one side wall of the slide.

The left photo from Amal shows them clearly.

Photo "Otterman"

You can see here that the carb on the right is a "Bushed" pilot jet, because there is no pilot jet fitted, some of the carbs with a bush fitted did however have the threads taped into where the pilot jet was positioned.

And don't forget that the pilot system is handed to use the side where the air screw is...

The left photo you can see the pilot jet in the top right hand side, this is an old four-stroke carb, but the two-stroke versions still have pilot jets so I am told.

Before we go any further here is what Amal say about this part of the carb.

Pilot Circuit...

Mark 1 Concentric is equipped with two types of pilot jets. 2 Strokes use a removable pilot jet that should be replaced if it is damaged or shows signs of oxidization. 4 Strokes generally use a pressed in bush pilot jet in the gallery behind the pilot air-screw.

Fuel residues and oxidization can cause problems with the pilot circuit, particularly in bikes that have been left standing with fuel in the carburettors for several months or more.

It may be sufficient to clean the pilot circuit with an aerosol carburettor cleaner, stopping the various outlets selectively to ensure the entire system is clear.

Do not forget to inspect the two pilot circuit outlet holes either side of the slide location slot in the bottom of the carburettor bore.

If necessary the pilot bush can be cleaned by removing the pilot air screw and using a No 78 or 0.016"drill, held securely in a suitable extension, rotated lightly in the bush to remove encrusted deposits.

Pilot Jets...

When first introduced all carburettors were fitted with a detachable pilot jet screwed into a threaded hole in the bottom of the carburettor body.

This arrangement is retained for 2 strokes but from 1968 was replaced for 4 strokes by a bush pressed into the gallery behind the pilot air-screw.

The detachable pilot jet caused problems with some 4 Stroke engines.

The remoteness of the jet from the two pilot circuit outlet holes caused starting and idling problems on bikes with lower manifold vacuums, and could cause stalling during deceleration due to a weak mixture.

The introduction of the pilot bush moved the pilot jet nearer to the outlet holes to overcome these problems. The bush has a flow rate of equivalent to 20cc/min. Carburettors fitted with a bush retain the threads originally used to fit the removable pilot jet.

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OK... we will continue later with some more photos when I have taken them.

The Float Chamber...

If you have checked the main body of the carb, and main jet, and they seem OK for you, and there was no over scoring on the side of the body where the slide runs, we can now go to the float chamber. If the slide was sticking in the chamber, the body of the carb may be distorted, and this could be a problem. There is a cure further down the page to try and rectify this...

Photo "Otterman"

The two chambers above clearly show the age gap of manufacture, the one on the right is from the original Amal factory in Birmingham, with a nylon float needle fitted.

The one on the left has a later float needle, and is brass with Viton rubber tip.

There are now available later stay up floats that are solid material, and don't rely on the float not puncturing as in the earlier versions, and you can adjust the height of these by bending the metal tab.

To adjust the height without this float, you will need an appropriate small drift to move the brass needle seat up or down.

Although there is nothing wrong with the nylon needle if it works, replacing it with a Viton tipped one, will probably be more reliable.

The only reason to discard the float as I see it, is if it is distorted and gives a false float reading and level. Or it gets eaten by the Ethanol fuel.

Photo "Otterman"

Clean the bowl out and make sure there is no crud left in the bottom.

If you have access to a ultra-sonic cleaner, the best way to clean any part of your carburettor is with one of these.

I have one on the way, so story later.

One tip on reassembly of the float chamber unit is, to gently centre-pop the body to hold in the float pin, but I am in two minds to use it.

Pic later

OK... lets go back to the body for a minute.

If you think this is distorted you can check with a straight-edge across the middle of the two fixing holes, with the rubber O ring removed.

Hold it up to a light with the straight-edge across, and if you can see a distinct ark of light, the body has been over tightened onto the fibre or plastic heat sandwich, by an ham-fisted owner, or mechanic.......

PIC Later

Cure!.

Don't even think about lapping it with valve grinding paste on a flat piece of plate glass or marble work top, ETC.

This is not going to cure the problem, just slim the flanges of the carb body.

You may find if you put a vernier gauge across the choke of the carb on this side, that it will be oval, and narrower across it's width.

The body has distorted with the over tightening, and this is what is making your slide stick.

The only real way unless you replace the body, is to make up a mandrill the same size has the choke of the carburettor, the spray tube will have to be gently drifted out for this operation.

This mandrill will have to be drilled for a bolt to run through the centre, and a thickish piece of flat machined plate that the carb mounting can sit on with more than an inch extra all around.

The mandrill is lowered into the choke of the carb, and when it comes to a stop on the distorted part of choke the bolt is inserted through this mandrill and the plate and gently tightened, until the mandrill sits onto the plate.

This should have persuaded the carb body to return to its original shape, a tip is to warm the body of the carburretor slightly before this operation.

And then leave it on the mandrill and plate until it has returned to normal room temperature. (normalising).

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You may have now salvaged the body for reuse.

I will do this operation later and take pictures for you.

Slide...

Look I am not going to over complicate matters by going into the different slides that you can get, the one that works for you is the one you need, and there is more than enough information on-line with this information, and also about jetting etc.

One of the big mistakes with these carburettors is, that they were made down to a minimum price for the in deep trouble British motorcycle industry, so Jeff Binks and Barry Johnston were asked to design a carburettor that was has cheap to produce as possible.

The Concentric was loosely based on a Mikuni design as the two companies had a loose working relationship with other projects, at the time of design..

Photo "Otterman".

You can see that the influence of Mikuni has rubbed off on most of the Japanese and Chinese manufacturers.

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"The Mk1 Concentric" that Jeff and Barry came up with for Amal was said to sell to the trade for has little as two pounds per unit...

So we should not expect too much, from the item should we...

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Similar metals...

Zamak or Mazak as it was named in Britain, was used for the slide because it was cheap, but being the same material as the body of the carb there was bound to be trouble, golden rule is, you don't have the same metals rubbing together.

Even the Amal Monobloc had a coated brass slide.

So Problems with this Slide.  "Well they do wear out sir", and has we now know the body of the carburettor does distort, so with the two parts trying to change there shape we get galling, (scoring) of the body and the slide, which leads to the slide sticking, which is not good even on a trials bike.

The old cure for a sticking slide on a Monobloc was to clean it up and then rub it with candle wax, but as I say this was brass.

But this would not solve the situation on a concentric, with both parts being of the same material.

With the gap getting wider when wear sets in this causes more air to pass between the two surfaces, and just adds to the running issues with the carb.

Besides scraping the carb and buying a new one, there is only one way or perhaps two, but if you start boring out the body to fit an oversized slide you are probably wasting your time, and money.

So the last ditch attempt to salvage the carb is to make another mandrill similar to the one above, this can be turned from alloy, then it can be gently pressed into the body of the slide chamber, hopefully returning the chambers roundness...

There are slides made from other material, Brass ETC on the market, that will now be able to be turned to fit, but is it really worth throwing good money after bad?

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After researching the Amal Concentric carburettor for this  page.

There is only one I would consider buying.

And that is this one below,

It  may be more money to buy, but I am sure that it is a far better bet, and must work a lot better than the Mazak bodied versions, and is easier to service.

Price of the AMALminium  Carberettors all sizes is £116.66 + VAT = £139.99

Order From Amal direct, If you want a 20 mm one for your Triumph Cub to ride in that Scottish trial, get your order in fast as they are not always in stock, and have to be built to order in a lot of cases.

Stay Up Floats..

There has been tales of the so called stay up floats from Burlen not working too well...

The problem seems to arise from the fact that the float needle plunger, the part that holds the needle is higher than the original, therefore making the needle seat further away from the needle when the chamber is full of fuel.

So flooding occurs, as the needle is not shutting off the fuel.

Bending the Tab does not seem to cure this problem either...

The only real answer as I see it, is to move the brass seat upward, by first heating the float chamber with boiling water, and then gently tapping the seat up slightly with an 1/8" alloy drift.

Another theory I have is that the float may be heavier than is necessary, you may think this stupid, but I have known of float bounce in the past.

This means that although the float is floating, so should be shutting off the needle valve, but it may be only just buoyant enough to just do this, and with movment it bounces...Dropping the needle...

I will purchase a "Stay-up Float" to test my theory.

The answer may be as simple as drilling holes in the upper part of the float?( Yes you have to re fill them, but with what)?

There Is an alternative, use the Nylon Floats and use ASPEN 4T fuel.

This Amal Mazak Concentric carb has had ASPEN Fuel sat in the float bowel all winter, and you can see there is NO corrosion at all, and no need for a Stay-Up float...

More insight later.

Where are Amal Carburettors Made?

 "Made In England"

Right there is some controversy to where Amal carburettors are made.

 Well in England of course, In Salisbury Wiltshire.

 No... No... There not. Thy are! Some of the castings may not be actually cast in the UK, and the Concentric bodies are actually cast in Spain, and some of the castings Amal use for other Carb's even come from China, but it is all about quality control, when the castings get back to England to be machined and then built up..........

Amal, or Burlem fuel Systems, have been making Carburettors some considerable time now, and that expertise should show up.

Amal is not the only brand of Carb they own, and assemble, but most British Carb brands from the past.

So as I say, you must use your head when you choose a Carburettor for your machine.

As there are now choices, and even Carburettors to the same design of the Amal Concentric Mk1.

I wonder if these will be OK with the Scottish Two Day Trial committee?

 I am talking about Wassell  of course.

And because they look the same as a Amal Concentric, that does not mean to say they are.

But they are not claimed to be built in England, but are another choice you can choose.

And how about the Mikuni VM series, basically the same, but with more adjustment of parts, and I wont mention the price, or even Pit-Bike parts.

 We will take a look at the Wassell and Mikuni carbs later.

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Home Made Soda Blaster.

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I am going to make one later.

We Know it is a joke Right.

 But What if it works?

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 More later we will see if it works.

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Dirt-Doc...

Tuning Tips...

Trials carburettors, hints and tips.

Is it worn?

Firstly, a carburettor is (or should be!) a very precisely made part and wear or poor-quality control during manufacture will make accurate tuning difficult/impossible and often leads to other parts being changed or replaced in an attempt to rectify carb related problems. In some cases, brand new carbs may well be faulty, so bear this in mind if your bike ran ok before fitting a new carb.

Rules?

If the rules allow it’s a very good idea to avoid using poorly made zinc/lead body and slide carbs which are not resistant to modern fuels without alternative extra cost replacement parts being fitted first. Japanese manufactured carbs do not require alternative parts, are very durable and work extremely well if properly set up for specific applications.

Right choke size?

Choke size (carb internal bore) selected will very much affect the low rpm throttle response of any engine and is something that needs to be looked at quite carefully if you are wanting the best possible performance. However, as a general rule of thumb flat slide carbs with 2mm larger choke size will provide the same sort of low rpm response as round slide types that are 2mm smaller.

Basic set up and tuning?

In terms of setting up a carb from scratch this is something which is very much specific to the carb in question and there is plenty of manufacturer data to help with this. What follows is simply a thumbnail sketch of how to fine tune a carb set up which is already working relatively well.

Tuning:

Firstly, there is a need to be confident that your motor is in good mechanical condition, exhausts on 2T bikes need to be recently serviced or new and to have the bike warmed up to normal running temperature.

Pilot jet adjustment

First thing to be adjusted is the pilot air (or fuel) screw. Set the throttle stop screw so the bike is just running. Pilot screw needs to be something like 1.5 turns out. Gradually turn the screw in until the motor just starts to falter and make a note of the position. Then turn the screw out again till the motor again starts to falter and note the position. Position of the adjustment screw needs to be between these 2 positions, with 1/8 turn toward richer setting (in if its an air screw and out if you have a fuel screw). If the bike is not responsive to pilot screw adjustments there are either other issues, or you will need to fit a smaller or larger pilot jet.

Slide

If after you are happy with the pilot jet adjustment there is very slight hesitation or stumble at about 1/8 throttle position its possible that the slide cutaway is making the mixture either slightly too rich or slightly too lean. Slides though are in some cases quite costly, so we would suggest checking whether the slide you have fitted already is the one which is generally used for your bike, before buying a replacement with a different cutaway.

Needle clip position

Next thing to look at is the needle clip position, this affecting fuelling from approximately 1/8 to 1/4 throttle positions. If the bike feels a bit dead and wont rev cleanly in that throttle range try weakening the needle setting. This means moving the clip to the next highest position on the needle. If it feels as though its running out of fuel and maybe even detonating slightly, then you will need to richen the mixture by moving the clip to the next lowest position on the needle.

Main jet

You will see much mention of reading the plug colour to help with determining main jet size. Ethanol bearing fuels though mean that relying on plug colour readings is a very hit and miss process and can no longer be relied upon! If you have a selection of jets, its easy enough to test different sizes and work out relatively easily if running improves with an alternative jet fitted. In general, most trials bikes are jetted too rich and running will improve with a smaller jet fitted. However, if very oily 2T mixtures are being used there may be a need for a bigger jet due to the viscosity of the fuel mixture.

New carbs and parts:

If you are not restricted by the rules to British badged zinc/lead carbs then the best possible choice is a Mikuni or Keihin specifically set up for your particular application. A wide range of these carbs set up perfectly for many different makes and model of bike is available from Allens Performance in Nottingham, who also provide full spares and technical support services.

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We now have bought a Wassell  Amal copy carb to test and see if they work has well, or better, than the Amal Concentric...

 Here are the photos of it taken apart to inspect...

Photo's Courtesy "Otterman"

The Wassell looks just as good a build or perhaps better than the standard Amal.

Bell-mouth is alloy on this carb...

One thing I found is that the float bowl is a lot more chunky... and this added up to a heavier carb by 10 grams at total 778 grams.

The slide being made of aluminium seems a better job, as it is not the same material as the carb body, so should cure the galling problem that occurs with the Amal sometimes ...

I must say, if it performs OK, I can not see why it is much different than the Amal Mk1 concentric.

 And the only disadvantage is the slight increase in weight...

 But then you get more for your buck, with the price at only £89....

 So watch this space to see if it performs as well....

06/03/2024...

Tufnol Spacers...

Photo Courtesy Hitchcocks Motorcycles...

Since I had made years ago the alloy 1"manifold spacers, which to work correctly needed the use of a small Tufnol spacer carb side to help dissipate the heat from the engine, I have been playing with some thicker Tufnol spacers when I found a source for the material, but the machining without a CNC mill was difficult... I now find that our old friends up the road from us, Hitchcock's Motorcycles have had some made and at a good price for the amount of work, so here is the link...

Slow action twist-grip.. do we need one?

With the current crop of new trials bikes with uncontrollable engines for the novice rider, or even club-man.. These Throttles or "twist grip add ons" to a Domino unit could have helped with engine pick up, that is a slower initial response.

 But it looks now that they may be out of production by the company that made them Omega Design...in late 2019...

10/12/2020...

Mikuni VM 22, 26mm Pit Bike Carb...

I have been using these carbs for years as you know and Brian Bedford in the IOM has tested them more than most for me...

 When ever we build a Triumph Cub or even now BSA C15. We say are we going to fit a Amal or bung on a VM22?

  Thing is I can always get a Triumph Cub to run better with one of these cheap carbs fitted...

But the game has got a bit more interesting of late as there is now a bunch of these Mikuni Copy carbs appeared on the scene, they look the same but have not got Mikuni or T/A Japan on the carb body ,in fact there is nothing...

One was purchased to try on the BSA Bantam that was going to France but I said after it had arrived,"What you got that for the choke size is to large"...

So I now have the carburettor spare. And after Graham asked me this week about fitting one to a BSA C15, I thought I would take this Copy-Carb apart and see what it looked like inside... I needed another for Mini-Otter Seven, so have ordered a what is said to be a genuine Mikuni VM 22 made in Japan, from Red Circles as I buy a lot of parts from them, along with Pit-Bike Parts from Redditch...

So when this Mikuni carb arrives I will strip that down the same and see what difference there is, if any?

 I know the Genuine ones? have 95 main jets fitted when they arrive.

So lets take a look at this Copy-Carb...

This is the carburettor with Mikuni T/A Japan cast into it...

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Note:  One thing I forgot looking at this photo is, DON'T block this air hole that you see at the bottom of the flange with your carb manifold you have made, or it wont run OK...Drill an hole in the manifold big enough for it...

You can see there are no markings on the Copy-Carbs...

The same starting choke set up as the marked Carbs, and this one works very smoothly.

So I stripped it down...

OK... I can't spell... Pilot Jet has 38 engraved onto it. Main Jet 98, as I said the Mikuni marked carbs have 95 If I remember correctly, but we will see when that carb arrives...

So what have I found...

The Carburettor seems very well put together to me and of good quality when you consider the price as little as £18 or less...

Undoing the screw top of the slide cover and cable stop I felt it could have been smoother on the threads, and the spring just lacked a bit of quality. The slide was silky smooth though and no play, needle fitment was fine too.

The rubber gasket between the carb body and float bowl felt a bit inferior, and was stuck to the body instead of the bowl, but has come off all-right the second time I stripped it out. Both jets screwed out OK, and the threads seemed fine.

 The float pin came out smoothly and  the float looks good quality.

The needle valve is only what looks like stainless steel but has no Viton tip on it, I seem to remember the Marked ones have this, we will see.

I must say the die casting on the body and float chamber are first class. Only quibble was when I ran my finger around the inlet choke edge inside there was a bur on the edge that I cleaned off, I wonder if the other will have this. There is also a very small casting flaw pimple on the inside of this choke. But I would not have seen it without my glasses.

It will be very interesting to see if there is any difference between the two...

Jet wise, well you will always have to set the carbs up as above, and there is now jet kits available (The Full Set) for silly money...

All I can say is for £18 for the unmarked ones to £35 for the marked ones it has got to be worth a try using them, and you could have more luck than fitting an old reconditioned carburettor, or even some new ones a lot more expensive... If you do the trial in Scotland, if it returns

try a Villiers carburettor on your bike? yes we will later... 

Right we now get onto the "Real-Deal"...

Mikuni Japan VM 22.

This is the carburettor that basically was copied as, and named "Concentric", by Amal.

 I said, at the time Mikuni Corporation were working on a project with Amal Birmingham in the sixties, so you don't think it was easier to use the system and just design a new body, that would be quicker, to get something onto the market for the factories urgently.

Anyway like I say this Mikuni VM22 is the real deal.

And the one if you can afford it to buy... And has I say 

Allens Performance in Nottingham, do Mikuni kits for most British Classic bikes...

This carburettor is from the Nick Draper BSA C15 so has probably worked well enough to have won the odd trial or three.

The choke bore on this Carb is has it says 22mm and this is the bore size right through...You can just see it stamped onto the carb body...

The starting choke is the one copied by so many of the later batch of carburettors, but this was not copied by Amal for the concentric, too costly to manufacture at the time perhaps...

Right, stripped down, this carb is just more chunky and spills out quality.

What looks like the main jet is not, just a seal carrier between the float bowl and the body of the carb. The needle valve is a sprung version to aid fuel surge in the float bowl...

This is the main jet and holder (No 125) "BSA C15 280" in this one, and screws into the side of the float bowl.

This makes it easy to get at to change jets,

The pilot jet has far as I can see, is a bushed one, pressed into the bottom of the cast in tube, on the left of the photo.

The rubber manifold seal is just the same as the Amal carbs.

 And on this unit the holes have been elongated to fit the same width of stud centres.

Summing up, on this carb, it is quality, that is why so many are used on Classic bikes.

If you look along the choke you will see it has two small fuel holes by the slide, just like the Amal Concentric, and the needle valve sleeve tube is very similar.

Just a good bit of kit...

There are Chinese copies of these carbs to, coming onto the market at the moment, they have "Made in Japan" stamped onto them, but I am told they are not made there.

I know someone does modifications to these for the Triumph Cub and he says when they are converted they are superb? So you have a choice..

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14/12/2020...

Mikuni VM22 T/A ...PB...

The carb arrived from RED Circles in Cornwall, how good a service was that...

So I stripped the carb out the same as the copy one, and there are differences...

The body casting is different and a bit more refined. The flange to the manifold is thinner, and easier to get bolts in.

 I noticed the choke outlet hole into the bore is slightly bigger too.

 No rough edges on this either.

 Taking the float bowl off, it was a better fit to the body, and the rubber seal had been oiled and felt thicker. Main jet is, yes 95, as I had remembered. And the pilot is 15.

The fuel needle valve is indeed viton tipped and with the anti surge spring.

Also this carb has serial numbers stamped onto it, which makes you think yes it is made by the Mikuni Corperation, as it can be traced?

So yes there is a difference to the copy ones, just a bit more refinement.

 I am not saying you will not get the same result with the copy ones, if you get a good one...

 Perhaps I should not have written this? The next time I go to Red Circles they will be out of stock, if you now all buy them...

This Mikuni  carb is a bit more refined than the Copy-Carbs...

This unit just tends to take the edge on quality...

Needle fuel valve has you see is Viton tipped, with a surge damper spring too...

Serial numbers on this carburettor too...

Just put together with a bit more care...

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Other brands of Carburettors Later...

More Later.

Updat2023...05...