I'm a Mikuni fan. They make good reliable carbs that are simple and fairly easy to fit. Their biggest advantage is the over-abundance of different jets, needles, slides etc that let you tune the carb to your bike. Unfortunately, unless someone you know has already worked it out, you will need to experiment somewhat to get things correct. Advice is to buy the carb that will suit, jetted approximately right and at the same time buy needles and jets either side of that so you can tweak it. Sudco are a good place to start...

email (option): cas.vanderwoude@gmail.com

Ian, what fuel ?

If Alcohol - damn near anything will work as long as the fuel flow is good - amal, do dope float jets, etc. and are good.

If petrol;

Dell Orto - spares can be tricky to get - but they are good, we use them on the B50's without bother - Mark Cook of PSS/CCM is best supplier

AMAL - again good, simple to tune - I am having trouble tuning my B25 carb, but that's something else other than carb.
Modern Concentrics have serious life, wear problems, the MK2 is nearly as bad.

Mikuni - I presume you mean the round slide VM series - thousands of options, can easily disappear up own orifice tuning these .....
Brilliant for two strokes, probably not really needed for a four-stroke.

Kehein - incredibly cheaply made, but a very good carb - jets etc are significantly more expensive than others - also far, far far, too many Taiwanese and Chinese copies ..

Various flat-slides/D-slides etc. - variations on above.

Wal Phillips fuel 'injectors' - a joke at the time and even worse now - OK on dope, but even then really need (SU) float chambers to get reliable results.
Almost always incorrectly set-up ............

And yes, I've f*****d around with all of these, with fuels from pump-petrol to toluene, and beyond ....
And still messing - currently using an 30mm OKO carb on B25, with good results!

email (option): ginantonik@tiscali.co.uk

Mark Cook - PSS/CCM - spares for Dell Orto (he sells them for CCM/B44/50's)

Allensperformance - for Mikuni jets, at least they are genuine parts ...
He can be a bit of an unhelpful 'know it all' but persevere, depends who you speak to.

Kehein - probably above, vut make sure they have a 'K' in a diamond stamped on them - then they MAY BE genuine, though it seems the Chinese may have started to copy this.

AMAL - from Burley Fuel

As well as what fuel, are you bothered what it looks like ?
For the fast SV ???

Seems as if they are letting SV's run in the class below (500 SV considered an honorary 350 ..) to bring UK sprints in line with AMA classes ....

email (option): ginantonik@tiscali.co.uk

[' As well as what fuel, are you bothered what it looks like ?
For the fast SV ???']...

I'm not too concerned about the appearance of the carb...For esoteric reasons I would rather fit an AMAL if I can get new MK2s in the appropriate size as it is nominally a British product and I am familiar with tuning and setting them up on non standard engines..

Fuel will be petrol during initial setting up and then, possibly, a change to Methanol when the dustbin fairing is fitted...

That will depend rather on initial results with petrol and whether or not I decide to cut the fairing...

It is for fitment to my M20...Ian

email (option): ian@wright52.plus.com

I actually like Amal's, but have used Mikuni's for racing.

At a guess you would be looking at 26-28mm for a SV - I like the MK2, but I think the 26 and 28 use different body sizes (you need to chk Burley website).

Only 2/3 needles to chose from, most of the mid-range tuning is done on the needle jet.

Low speed have separate jets, there are air-jet options for high speed changes.

Slides are available as alloy, chromed alloy, brass and chromed brass !!!!!

There are a number of float jet options - included for dope, unlike other carbs.
easy to convert to alcohol - float needle and perhaps seat (SV500 - may not be necessary) and main-jet - before buying a dope needle, try a massive needle jet with the standard needle (drill out an old needle jet).

they have a choke system that actually works.

Would you buying new or used?

If used make sure you buy a four stroke carb - that's not been f****d with !
the mk1 concentric and monobloc - needle jets and needles will fit - but are incorrect for the MK2 - same for 2 stroke use.

Pilot and main-jets are same for all range.

Its easy to convert from 2 stroke to 4, but more expense and you need to change needle and needle jet, as well as smaller main jet etc.

Wear is a bit of a problem - use a bloody air filter!
internal corrosion is something else ..... personally i've had no problems, but I have bought s/h parts which are a terrible mess, internally!

Let me know what size you are looking at and I can see what I have.
I don't generally sell any of my carb bits, as they have limited value (collectively I must have £100's of jetting) but I can see what I have.

email (option): ginantonik@tiscali.co.uk

Hi Nigel..The carb is for my (very) non standard M20 sprinter...

Suffice to say I'm looking at something in the 34-36mm range....

I'll be buying new...If Amal can't supply a Mk2 then I'll start to try and find one 'in stock' at a dealer somewhere...

If that fails my final option will be a Mikuni or similar...

I like the Mikunis but with such a heavily modified engine to set up life will be simpler if I at least have a carb I am familiar with!....Ian

email (option): ian@wright52.plus.com

Hmm I think you may be optimistic even @ 34mm - the short stroke B50 for Bonneville will only be using a 34 or 36mm for +60bhp @ 8000rpm
I would think a 30mm is pushing it.
The B44 MX used a 32mm and it is thought they generally produced something like 35bhp

Amal will be able to supply, go on there website and you can select a spec. - main-jet/pilot/slide/etc. giving a final price.

Phone them - ask for advice - but do not be surprised how small a carb they may/will recommend. 'Undersize' carbs are much easier to get the jetting right.

What's your inlet tract like - if its up-draft to try and straighten air-flow, then you may struggle finding ANY modern carb that will cope.
You then need something with a separate float chamber.

email (option): ginantonik@tiscali.co.uk

I've had an answer from AMAL..Due to 'tooling issues' the MK2 won't be available for 6 months (minimum)...

This confirms what I had heard 'through the grapevine' that AMAL are in a dispute with the company that makes the castings for the MK2... ..

A GP is a possibility, though a substantially more expensive alternative....

Nigel...Regarding carb size I think we may be looking at these engines in a fundamentally different way...

That doesn't mean I think you are wrong or that I am necessarily right..Time will tell whether my approach will be one that works...

It's worth noting the side valve KR Harley had a 1 1/2" (38mm)carb on a 375cc cylinder with a 97mm stroke revving to 6500rpm...Ian

email (option): ian@wright52.plus.com

Ian - the Linkert carb on the KR that I had a look at (or was it KTT?) has pre-atomizer thing in the middle like a Weber carb - the available area for air flow was nothing like a 1.5" carb!!

Over carb'ing is one of the most common ways of cocking up an engine, very, very few engines are carb/air-flow limited.

Getting a SV to produce any power is a real "triumph over adversity" thing - HD stuck with their SV for political reason - certainly no logical reason.

A similar reason existed in the UK where engines were designed with a small bore x long stroke - purely because of tax regulations enforce at the time (2 3/4 hp Douglas - was a350 producing more than 2.67BHP!!).

A SV has terrible air-flow, particularly at higher rpm, the cylinder pressures (BMEP) are so low, even @ low rpm, they have to rev the bollox off them to produce any horse-power.
You can fit whatever size carb you want, but it will not effect the airflow through the ports/valves, etc.
It will however effect take-off and acceleration because of incorrect mixture.

Current thinking is the inverse of Irvine's "tuning for power" - something which is of as much use as a historic novel.

Small ports, high airflow velocities, resonant inlet/exhaust systems - something Richardo, Helmzolt and Kaadan were all advocating (and using) in the pre-war period.

Getting a SV to produce any real power is a CHALLENGE ... guess thats why people try .....

Oh yes, carb size ..... take advice, where you can, filter out the bull-shit, and give it a try .... then try again, and again ......

email (option): ginantonik@tiscali.co.uk

Jerry Branch did a very good article on making the KR move - some interesting figures quoted by a renown tuner...

Carb - 1 5/16" Linkert
RPM - 700rpm !!!!!! that's 22.5m/s - no wonder they had piston trouble - its pushing modern F1 speeds.
BHP - 48-51 BHP .... very impressive for a 750 - SV, no wonder they were competitive with Goldstars and Manx's.

Max piston speed for a fully developed modern piston + rings is usually considered to be 20m/s - F1 are pushing 25m/s

I.E. 6500 rpm for a 94mm stroke or 5500 rpm for a 112mm stroke.
These rpm restrictions will limit the max HP.

Assuming a BMEP of 10bar (F1 ~16bar, MotoGp ~15bar, Manx Norton ~14bar) - which is a BIG ask on an OHV let alone a SV.
The maximum BHP @ 6500rpm will be 36bhp - MAX
Dropping it nicely into the 30-34mm carb size range .....

A M20 was pushed through Bonneville @ 116mph - on methanol + nitrous with a serious up-draft inlet tract ...

Now a short stroke 500 SV - that would be interesting, and have been suggested for turbo-charging ........

email (option): ginantonik@tiscali.co.uk

Nobody has mentioned using an SU carb. I'm going to try one on my project which has a standard M20 stroke but a big bore to take a Holden 202 piston to give 626cc.

I think SU's work pretty well with superchargers too Ian, just thought I'd plant that little seed!

Nothing wrong with an SU - I've a pair of 1 1/4"' if anyone wants them.
Very handy for the float-chambers as well.

Modern bikes have all gone onto CV carbs, basically because the venture opening varies according to the engine load (not your right hand) allowing better emission control.

SU's are ok on twins and work well, but singles tend to have problem with the inlet pulses - these can be damped out using a thicker oil in the dash-pot - but you will be constantly refilling it, and it will also give slower acceleration.

Mid-range mixture is altered by the needle - probably cannot obtain as many as you could, but you can always file a flat on the side like a Gardner carb (no ones mentioned them yet!).
WOT mixture via a nut on the bottom and drilling out the nut 'orifice'.
Good luck NigelG be interesting to hear how it works out and don't forget to center the main-jet before starting tuning it ...

A major reason for their use in the 60/70's was they were cheap and BIG, there are far more/better options nowadays

The Stromberg was a much better carb in its day, I used a pair on my 1150 rally Imp.
In same vain, I used the 40mm BING CV carb (std RS100 carb) on my 1050 BMW classic outfit - that gave over 100bhp at rear wheel - easily!

email (option): ginantonik@tiscali.co.uk

I've heard the SUs can be problematic on a single for the reason Nigel outlines...

I did see an M21 with one fitted on a classic run once and it seemed to start and run OK...but I didn't test it myself..

I had the motorcycle version on my Ariel Square 4 and it was a doddle to set up in that application..
Triumph Thunderbirds had them as standard fitment as well for a couple of years...

You could also buy SU conversions for the Bonneville and Commando...I don't know if they are still available...

I haven't tried one on a single myself...

Pressurised induction is a 'no no' for what I am trying..After all it largely negates the gas flow problem inherent in the side valve which represents one of the main challenges in tuning them...

'Modern' side valve thinking is that in some applications water cooling and fuel injection along with greater knowledge of combustion etc. could give the side valve another lease of life, allowing manufacturers to benefit from the low tolerance construction of the top half of the engine...No expensive OHC cylinder heads, cam drives etc. in a side valve!

Gas Gas have done exactly that with their side valve engine for trials use...Ian

email (option): ian@wright52.plus.com

SU's can give problems when used with a drop manifold which you'd probably have to use because of the height of the dash pot, on the Harley's they tended to wet your leg with petrol but they were ok if you used a straight manifold and a smaller petrol tank!

email (option): davmax@ntlworld.com

Dave - you've just reminded me, that the last SU I set up on a bike was my cousins HD chopper.
His spit-back problem was simply solved by thick (as in 20-40) engine oil in the dash-pot -it slows down the rate of opening fo the venturi.
Wet-leg syndrome is not only for H-D's, my Brough's had same problem, the difference was a rather elegant alloy casting, separating leg from carb.

RE Amal supply problem - bugger, thought they were getting their act together. They have installed some new machinery as they were doing quite well.


Please don't go down the mk1 Concentric route - too many manufacturers including Wassel Chinese copies.
Real durability problems as well as primitive low running adjustment - if you jump in that direction then make sure it has pilot jets (all 2 stroke variations have - and are a slightly better carb).


Whatever you use PLEASE rubber mount the carb properly on something like the amal mk2 adaptor rubbers. Without vibration insulation you will never get it jetted.
I picked up ~10bhp on a mini engine when we unbolted the weber and held it in place !!!! Despite using thick o rings, Thackeray washers, etc. as I was 'supposed to do'.
The dyno operator spotted the symptoms and suggested the idea - cheapest HP ever!
(shortly before the valve collets pulled through the spring caps .... don't use alloy spring caps!)

email (option): ginantonik@tiscali.co.uk

A Monobloc is a very high flowing (smooth bore) carb, if you can find a good one -wouldn't look out of place on a 'sleeper engine'.
A 1 3/16" has a similar flow rate to VM34 mikuni, according to U.S. tests.

With the re-rating of SV's into one ohv class size smaller (500 SV = 350 ohv), it does lead to some intriguing possibilities.

email (option): ginantonik@tiscali.co.uk

['too many manufacturers including Wassell Chinese copies.']....



I don't know where Wassells carbs are made..

The 'Chinese' Hepolite pistons supplied by Wassells aren't made in China so it's entirely possible the carbs aren't either...

Not that it really matters if they work OK...

I have recently fitted one to my TRW and it was as simple as an 'original' Amal to set up and runs fine..

I've previously fitted them to other bikes with the same result...Ian

email (option): ian@wright52.plus.com

Sorry Ian, but they have bad reputation in the B50 world, they do however have a removal pilot jet, if my memory serves me.

I have a major downer on the mk1 concentric, in all its options - worked very well on my mk6 Bultaco but on four-strokes, we've never had one last more than year, despite fancy slides, pickling to reduce corrosion, etc.
And yes, we run very effective air filtration - I have even added oil to the petrol to try and improve wear and sticking.

My two cousins + a couple of mates run Dellorto's on the CCM and various B44/B50, I run a Mikuni (was a mk2 34mm Amal) on my CCM, a 36mm mk2 on my mk12 Bulto and am playing with a OKO/Kehien flat slide on the B25 (tried everything damn thing else).

All my other two-stroke run Mikuni VM's.

I do tend to mess around with s/h stuff (I'll never learn) and one is certain - s/h mk1 Concentrics are universally FUBAR, Mikunis are usually in very good condition internally, mk2 Amals - somewhere in between.

You go with what you have experience of, if you are happy with the Wassel use them, at least they are cheap.

The interweb is so full of BS, you do need a good BS-filter and there's nothing like your own experience.

email (option): ginantonik@tiscali.co.uk

I'm the 1st to admit that I know little about the M20/21 - however there are certain things that need checking when boring out any of the older cylinders.

The main one, is the amount of metal around the cylinder base, particularly when a cylinder is bolted to the crankcase by its lower flange (as opposed to through bolting via the head) - CI has poor tensile strength and failure between the cylinder casting and the mounting flange is a very definite possibility.

On a linered cylinder, you measure between the outside of any liner (the bored diameter) and the outside world - not from the final internal finished I/d.
This is the 'real' thickness as the liner itself contributes little to the tensile strength.

It was not unknown for manufacturers to thicken base flanges and the metal around them, through the production life of the vehicle as failures happened.

Similarly manufactures would often restrict oversizes - to say 0.040" MAX.
Little knowing that years later someone would offer a +0.080"(or bigger) piston - oops, bang!

email (option): ginantonik@tiscali.co.uk

An unlinered barrel for an M20/21 will bore out to 90-91mm without a problem...I've done a few and have done a lot of miles with them...

BSA used linered barrels for the M20/21 until about 1951 when they changed to an unlinered barrel...

It is generally accepted that they used a poor quality casting with a liner and then upped the casting quality for the later unlinered barrels...

Logic really indicates that is the likely scenario...

However, I would not definitively state you can't run a bored out early barrel..Conversely I certainly wouldn't want to guarantee that would be a sensible course of action...

The fact is the only way to find out is to do it and determine the outcome over a period of time...

If you want to run 90-91mm piston there is precious little material left for a liner...If you want to run a piston that is smaller than that but bigger than the original 82mm piston there is enough material to fit the piston and a liner...

Tatu from Finland chose a Harley piston I believe and used the liner from a Ford Dexta tractor....Ian

email (option): ian@wright52.plus.com

I ran two 32mm MK1 Concentrics on a Vincent twin for many years. Had absolutely no trouble whatever from them nor any others I've had.

I think the phrase - for many years - indicates that in the past they gave no problems.
Modern fuels are playing havoc with modern versions of these carbs.

In the past the materials used (some Mazak alloy) wasn't particularly wear resistant and lets be honest, accurate fuelling to give best power/consumption and emissions wasn't an issue - and probably still isn't an issue with our old iron soldiers (monbloc's and 276's - much better materials).

Starting the damn things, is something else - the B50/44/25 are all inflicted with a pathetic k/start ratio which barely turn them over - you need something which carburette accurately and reliably, or you spend more time trying to start them, than ride them!
I'm out on them every other weekend and until we changed away from the mk1 concentric, we were unhappy bunnies.

But you go with what you know, if your concentrics are working for you, stick with them!
Was just saying that there are better carb out there

Note the carb size - 32mm on a relatively short stroke ohv producing something like a true 25HP per cylinder.
Am looking at some dyno runs on a B50MX - again a 32mm (concentric mk1) producing 34BHP @ 6500rpm.

email (option): ginantonik@tiscali.co.uk

I ran a Mikuni VM on a 500cc AJS single, ended up running well but the price of jets, slides, etc would not do it again, especial on a special unless you have free access to these parts. I have a 600cc matchless typhoon [single]and run a made in Spain 34mm mark 2 amal and I am very happy with it. Otherwise I would use a mark 1 which I have used on a few 500cc AJS singles [28mm-30mm].

On the dyno I got 37.5HP at the rear wheel from a B33 with a 600cc engine running 90 x 94 bore and stroke, Goldie Cams and a 34mm Mk2 Amal...It revved OK to 7000 and produced peak power at around 6500....

It also ran good times on a standing 1/4 mile and had strong midrange..

Great fun on the road in a rigid frame...

I run an Amal manufactured 32mm MK1 Concentric on my 720cc B33 and it starts first time just about every time....

I really haven't experienced the problems described....Ian

email (option): ian@wright52.plus.com

As I said, if you have no problems with them, then use them.

I have (literally) a bucket full of mk1 concentric 4stroke bodies which i was getting ultrasound cleaned, but as no one I know wants them, I have never bothered.

Mikuni's - can be expensive to tune on brit bikes, simply because there's little published jetting data, however they are the main carb in road-racing, where round slides are specified.
A starting point for a 500 is;

VM32/34
6DH4-6 needle
P2-6 Needle jet
2.5 slide
200-280 main jet
25-35 pilot jet

Two strokes and 4 strokes use the same body.

Needle and needle jet rule is same as Amals - if you are at one extreme or other of the needle, then go up/down on the NJ until needle is central again - try and get rid of richness/leanness on pilot jets and or main jet, before starting to change needle profile (there are thousands!).
Splitting back when picking up - slide or pilot-jet (PJ's are cheaper!)

email (option): ginantonik@tiscali.co.uk

Top-hat inserts fitting on the original wrist-pin, with an o/d to fit in the piston-pin holes.

If no provision for circlips - use PTFE or brass pads, rubbing against cylinder.

Alternatively, if dimensions allow, press out original s/end bush and ream/machine the s/e of con-rod to all press fit of original holden wrist-pin.

Tonight's task is to make t/hat bushes to fit a(76mm) XL250 piston into my B25 - I race in the 500 class, so sod-em i'll make it as big as I can without crank-case machining ...

I would have looked at m/c pistons first, as they are lighter -
XT600 flat-top 95mm o/d, xt500 - slight dome 87mm o/d - or a Yamaha 550 grizzly piston - 92mm flat top, or something.
Tend to have smaller wrist pin diameters so only need a special small-end bush.

email (option): ginantonik@tiscali.co.uk

Well I spoke to the engine builder tonight and he said that there is plenty of meat on the M20 conrod so he simply made up some bushes to suit the Holden gudgeon pin. Apparently this can't be done with B31 conrods though, as there isn't enough metal.