Amal Concentric Mk 1 Jet Holder New vs Old

[Keith Beach]

Ive been reading Rob's excellent articles in Leading Link on the Amal 626 Mk 1 Concentric.

On my monthly carb strip down to check for and clean out the gunge caused by ethanol laden fuel I was checking my current carb against his points to check. All seemed well apart from the jet holder which was part no 622/168. This is the holder which Amal changed to post 1968 and which drops the main jet about 1/8 inch lower into the float bowl. Having reads Rob's article and seeing comment on the internet that the original length jet holder (part no 1034/080) is preferable in a two stroke Ive salvaged one out of an old one of my Amals and have got a new one on order.

So why do the main Amal suppliers supply you 626 carbs with these longer four stroke jet holders although you have explained to them they are for your Greeves ? I note that their websites mostly tend to list only the 622/128 holder.

What is the impact of the longer jet holder on two stroke performance ? Why did Amal change from the original part ?

best wishes

Keith

[John Pattinson]

Concentric

Hi Keith, I have tried the two stroke jet holder in my Hawkstone-no difference. This was a far bigger carb and I was looking for a cure for gassing up. The finer points of carburation are not the same thing on a scrambler. Trials carburation must be perfect for you to trust your bike and ride well...John P.

[John Wakefield]

Might be worth having a word with Neville Lewis (ex Surrey Cycles) a well known expert & rebuilder of Amal carbs. His number is 01483 268484

[Keith Beach]

Thanks John and John. I thought that it may well have little impact changing jet holders but was curious about the reasoning behind the change. I tried contacting Neville but that number no longer existed. I have tracked down a new number but have not got hold of him yet. I will let you know what he says.

Many thanks

Keith

[Terry Sewell]

Amal Jet Holders.

Hi all,
Just going to throw my four peneths in. I chased around many years ago to find out some answers to Keiths questions. I got in touch with amal and most of the gentlemans answers went over my head. This is what I can remember. It has to do with the air pressure pushing down onthe surface of the fuel inthe float bowl. ( that is why we have to get the level right.) The fourstroke engine produces more suck, that is why the main jet sits lower and the twostroke is not so good at sucking, that is why it is higher.

Thats given you the bases to find out more. As I said they came back with so much technical data, it lost me. and thats not hard to do.

Have fun researching. Terry Sewell

[Keith Beach]

Thanks Terry that's helpful.
best wishes
Keith

[Brian Catt]

The technical term is "inlet manifold depression" I believe and you need a "suckometer" to measure this when balancing multiple carb fitments. My 2x SU Volvo was easy enough, then got asked to assist on a 6x SU Jaguar racer...nightmare! AS Terry said, 4 stroke have more suck than 2 strokes, so these need exact fuel levels and why you get bad bogging when it's wrong

[Rob]

The carb I eventually got from Amal's that worked, although for a 2-stroke had the longer jet holder and the bike has run on it ever since. I rather think the changes were made all those years ago to make things work better on a 4-stroke and the poor old 2-strokes could take it or leave it!

I'm currently working on a home-reconditioned carb with home-made solid slide, etc. and all the right bits for the Anglian. I bet it will run like poo after all the effort and I will revert to what I've got now!

I will write up the details for LL.

[Keith Beach]

Well I changed to the two stroke jet holder today and tried a test ride with a 3 slide, 106 needle jet and the needle in middle position. The bike ran terribly - way too rich. I ll change back to the four stroke holder me thinks.

[46T Sprocket]

What size of main jet are you currently using in your 626 carb?

David.

[greeves246]

Carby

Fit a dellorto, bryan wade did to his griffon 390 ! P***ed of amal though ! on the day.



i put a phb26 on my bultaco totaly changed it
hell of a lot of torque rob

gary

[John Wakefield]

Amal's were not good in their day, ok may be on a scrambler or road bike where low speed running was not a problem, but apart from Villiers using their own, there was no British alternative, for other manufacturers, the 'new ones' now made in China are frankly crap.

[Keith Beach]

Hi David, I am using a 140 main jet.

I have been reading about this inlet manifold depression and "sucking" Brian on the back of your and Terry's comments. I wonder if I have never been able to get rid of knocking and pinging at steady 20 mph type speeds because there is not enough suck, so running lean until accelerate or shut off ?

Keith

[46T Sprocket]

Hi David, I am using a 140 main jet.

I have been reading about this inlet manifold depression and "sucking" Brian on the back of your and Terry's comments. I wonder if I have never been able to get rid of knocking and pinging at steady 20 mph type speeds because there is not enough suck, so running lean until accelerate or shut off ?

Keith

Hi Keith,

Lack of suck and a lean mixture is normally compensate by fitting a larger main jet.

Your crank seals may also need replacing? It will never draw mixture in efficiently if the seals are not in good condition.

Interestingly I could never get it right on my Pathfinder with the original Bing carb and fitted a smaller bore carb MK11 Amal (24mm from the original 26mm) to increase the venturi effect. Velocity increases as the bore is reduced. Different and sweet engine!

If you are experiencing this at low revs you will probably need to look at the pilot jet size you have fitted and increase the jet size.

Your timing may also be too far advanced. If it is too far advanced the explosion created by igniting the compressed gasses is coming in just a bit to soon and you have a conflict situation arising as the piston gets near the top of its stroke. Knock and pinging is a sympton of this. Try retarding your ignition it a bit at a time and see if that has any effect before messing with the jets.

Your crankcase seals though need to be in good condition to eliminate any issue there before doing adjusting anything else.

Detonation is destructive and needs to be prevented. Info here on the subject http://www.f1moto.com.au/view/2-stro...-detonation/54

Regards,
David.

Sorry forgot to add that a good indicator of a lean condition is the state of the electrode on the plug. Even running lean a plug can be a black sooty colour fooling you into thinking your mixture is rich. However If you see tiny wisks that are a pale white or silver colour on the arm over the centre or on the centre pole itself this is aluminium being eroded off the piston.

[Keith Beach]

Thanks David Crankcase seals are fine only replaced recently, the knocking has been present on a whole range of timing settings and main jet sizes. It's not present on low revs nor when accelerating only at steady throttle openings. This seems a common problem with Anglians and I live with it most of the time but just occasionally fettle when people suggest things I haven't tried.