Concentric

[Hubie]

Hi All,

I have a 928 MK1 Concentric on my a10 Flash and was hoping someone could advise the best jetting.  It has a 106 needle jet, 210 main, haven't checked the pilot jet.  It runs great at low and mid RPM's but runs out of a bit of puff around 60mph, opening the air slide makes it worse.  Any help would be great.

Cheers,

Hubie

[Triton Thrasher]

What do you mean by "opening the air slide?"

You can't go fast with it closed.

[Hubie]

By air slide I mean the choke!

[Brian]

Now lets see if we all understand each other here.

You ride it all the time with the choke (air slide ) closed and if you open the air slide it wont run or makes it worse.

If that is the case you have a serious fuel starvation problem. Your bike should have a Monobloc 389/45 on it with a 250 main jet and a .106 needle jet with the needle in the middle (3) groove.

The concentric doesnt have a pilot jet so no need to worry about that unless its a concentric designed for a two stroke.

I would pull the carby off the bike, dismantle it and give it a thorough clean out, check there is a good supply from the taps. I would probably put a 240 main in to start with. Also check the fingers on the float are properly located in the groove in the needle and the needle isnt sitting on top of them.

You should only need the choke to start the bike on a cold morning, once the bike is starting to warm you should be able to take the choke off (fully lift the air slide) and have the air slide fully open from then on.

I am not a great fan of concentric carbys but they usually work ok, unfortunately yours must have something seriously amiss to require the air slide to be closed all the time.

Dont ride the bike much until you can get this sorted Hubie as it must be running way too lean.

[Hubie]

I think I've confused things!  The choke is not on unless the bike is being started, afterwards I can immediately turn it off and it idles fine and runs fine.  The bike only feels like it's running a bit out of puff once I get towards a full throttle opening.  It had a 130 main jet in it which has been changed to a 210, but I have a mate who has a spare 270 which I will try.  Needle jet is 106 and circlip is on the middle position.

See how she goes....

Hubie

[MG]

From my experience, the main jet in a concentric can be one or two numbers smaller than the one in the monobloc that had originally been fitted. So a 230-240 would be okay for you, but if you can find a 250 somewhere, it will be a good starting point.
You can try the following procedure
Run the bike on almost full throttle, just at the point where you feel it starts to lack power. Hold the throttle in this position and slowly close the air valve (choke). This reduces the amount of air being sucked in and cures the lean mixture, which I think porbably is your problem. You should notice an increase in power/acceleration. This way you know that your main jet is too small.

Brian,
there are concentrics that have a pilot jet, also for 4strokes. These were the Concentrics Mk1/2 that were produced shortly before the Mk2 arrived. You also need a different float bowl gasket for these, because the pilot jet is fitted at the joint of the carb and the bowl.
Guess why I know that....

Oh, btw, Hubie, you know how to read your spark plugs?

[Brian]

I have a concentric on my plunger A10, I put a pilot jet in it myself after not being able to get the pilot mixture right. It took a bit of drilling and tapping but I can now change the pilot jet size to what I want and I now have a bike that carburates correctly at the idle and just above area.

My main complaint with concentrics is that they are made of recycled rejected coke cans, well thats my theory anyway ! Seriously though, they are very poor quality and wear quickly and distort when you try to tighten up the flange nuts. They work ok and are cheap to buy but thats about all.

[MG]

Brian, the original ones are far better, the casting quality of the pattern ones is a bit questionable.

[MG]

forgot to mention: I'm using a 930 conc. with a 250 main jet plus an air filter on the A10. So if you use a 928 with velocity stack a 250 mj will probably be okay, but probably a bit too large with an air filter. I found the best setting to be a 106 needle jet with the needle on the middle groove, just like your carb is set. The pilot jet is a 30.

[trevinoz]

Brian,         
            Of course the Concentric has a pilot jet!
The first ones had the jet screwed into the body above the bowl as you have probably done, but apparently the jet was too far from the works so the carby was redesigned with the cast-in jet which is all but inaccessible.
They are difficult to clean when they inevitably get blocked. I use a piece of wire plucked from my wire wheel and access the jet through the air screw.
Another issue with the Concentric is the change in needle and main jet holder from the first series to the second.
It makes life interesting when they get mixed up.
trev.

[chaterlea25]

Hi All,
FYI Amal concentrics produced in the last year or so (UK) have reverted to the separate (removable)
pilot jet
One issue with the pressed in jet is that size couldn't be altered and as far as I know it was only a 15
I tend to drill through the pressed in jet when its blocked and fit the screw in version.
I stripped a concentric on a Velo last week and it had the 2 jets fitted both blocked of course ???? ????
HTH
John O R

[Brian]

Trev I meant they dont have a changeable jet.

If you take the air screw out and look in the hole with a torch there is a brass sleeve in the fuel delivery side which is the "jet", the catch is if its the wrong size you cant get the idle mixture right as was my case. I drilled the sleeve out larger and then drilled and tapped the body where the bowl meets the body and fitted a changeable jet. You also have to drill the bowl out to accomodate the head of the jet. I used a 5mm thread because I have heaps of pilot jets that size. (left overs from my sons racong days on CR Hondas)The end result after playing around with different pilot jet sizes is a bike that idles like it should with the air screw 1 1/2 turns out allowing some adjustment either way.

I bought this carby new. I also contacted two of the major Amal suppliers in the UK at the time and both told me that the ones with changeable jets are designed for two strokes, not that there should be any difference, and the new ones currently available also list the ones with changeable jets as being for two strokes. Once again I cant see why there should be any difference or indeed if there is any difference other than the pilot jet.

Just for interest I had added a picture of how the pilot mixture works on a concentric for anyone interested, the adjusting screw controls the amount of air coming in.

[MG]

This information is given on http://www.ingfatrygg.se/amal.html (I think that's the site where you got the pic from):

3. The needle jet/main jet asssembly for four-stroke carburettor was altered to give easier fuel lift at cranking speed:

Needle jet got a small hole in the hexagonal section
The main jet was lowered and a longer needle fitted
The jet holder was extended to enable the above

So if you have problem setting up that swap-meet bargain on your four-stroke watch-out, it may be set up for a two-stroke!

no idea if that is correct

[Brian]

I just went back and had a look at the Amal and Surrey sites which I havent done for some time and as John says they dont list seperate carbys for two and four strokes these days, they must use the same body and set it up to suit.

I bought my carby new but it would be easy to get caught with a second hand carby by the looks of it. With the cost of the new ones these days its not worth buying a second hand one.

[ebsbury]

All the differences are listed in here;

http://www.amalcarb.co.uk/TechnicalDetail.aspx?id=11