Jubilee concentric

the mono lock that came with my box of bits jubilee was off a C15 and knackered so I bought a concentric from surrey cycles, they estimated what the getting should be.

At the moment I tickle the carb until it's flooded close the choke and it starts first kick almost every time and ticks over. Trouble is it won't run without the choke even when warm, unless I can get the revs high enough so it is running on the main jet but as it's running in I'm tying to be careful at the moment!

Next job is to lift the clip one notch and see what happens, but I didn't know if anyone else is running a concentric and knows what jetting they have?

dan

the mono lock that came with my box of bits jubilee was off a C15 and knackered so I bought a concentric from surrey cycles, they estimated what the getting should be.

At the moment I tickle the carb until it's flooded close the choke and it starts first kick almost every time and ticks over. Trouble is it won't run without the choke even when warm, unless I can get the revs high enough so it is running on the main jet but as it's running in I'm tying to be careful at the moment!

Next job is to lift the clip one notch and see what happens, but I didn't know if anyone else is running a concentric and knows what jetting they have?

dan

Hi Dan,

From Amal UK web site:

http://amalcarb.co.uk/carbspec/carburettor/spares/id/5409/

and

http://amalcarb.co.uk/carbspec/carburettor/spares/id/5410/

as they are the OEM, maybe call them for advise on correct Concentric jetting?

Surrey Cycles are (albeit a good one) are a distributor.

Good luck!

Rgds Steve

Hi Dan,

You already know more than me about Concentrics but it sounds like a blockage in the idle jet circuit. Maybe a bit of swarf from manufacturing.

I expect you have a 22mm carb which is slightly bigger than the original monoblock but I don't think this would cause any problem by itself. It should actually be an improvement.

The only Concentric I have ever worked on had a fixed pilot jet which you simply blow through with an airline to clear.

Patrick

Dan

If the problem is with the pilot circuit its worth also checking the two tiny holes where the fuel goes into the intake as well as the jet.

See Pilot Circuit in this link http://amalcarb.co.uk/rebuilding-mark-1-concentric-carburetter

According to this post jet sizes the same as for the Monobloc should be ok. http://www.nortonownersclub.org/noc-chat/technical3-light-twins-forum/584999173?b_start=0#320973014

I'm unsure about correct needle taper or throttle slide cut-away.

Worth looking at this link with pdf download http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html

Andy

Dan

A quiet Sunday afternoon here (awaiting isolastic parts) so I did a bit of digging for Jubilee Monobloc jet sizes etc. Apologies if you already have this info.

This list

indicates specs as follows:

Bore 23/32"

Pilot 25

Cutaway 35 (i.e. 3 1/2)

Main jet 130

Needle jet 0.1065

Needle position 3 (i.e. number from the top)

These numbers match the Jubilee Spares List I have for 1961/62 except for the Needle Jet which is listed as .105

Hope this helps

Andy

Dan

Good to hear that things are improving.

The bush type pilot jet is the norm for standard Mk1 Concentrics as fitted to Commandos and most 4 strokes.

From:

http://amalcarb.co.uk/rebuilding-mark-1-concentric-carburetter

Pilot Circuit

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

The photos in the link show the two different types.

If the problem continues don't worry about probing the jets and the two intake holes with copper wire. Also squirting carb cleaner into the pilot air screw hole may help and will confirm whether those two intake holes are clear or not. This is the exact problem I had on concentrics which had been reconditioned. The process had left some swarf blocking one of the intake holes.

Regards

Andy

The std 25 pilot jet is a 17 thou hole, with todays rubbish petrol the best way to clean it is with a 16 thou drill mounted in a WD40 plastic tube and pun by fingers, also known as a #78 drill they can be bought cheaply on ebay and are a permanent addition to my toolkit and used every spring on all my concentrics.

Dan

A couple of ideas to try.

Could the symptoms indicate weak running being compensated for by using the choke?

Have you checked for air leaks at the carb and manifold joints? This would weaken the mixture - more air entering intake.

Next I'd move on to check float level.

This reference may also help -

https://www.princeton.edu/ssp/65-cub-data/library/amalbritbike.pdf

although Bushman's guide (see earlier post) is also a good reference.

Which groove is the needle clip on?

I'll keep an eye out for updates.

Andy

Dan,

Are you certain that the timing is OK. Retarded timing can give the same symptoms.

Well you have a good knowledge of Japanese mechanics and have checked all the likely culprits. It's possible there is a manufacturing fault in the new carb so I would have suggested trying another carb to see what difference that made. Is the old C15 monoblock completely dead ? It's the same size as the Navigator one -fractionally bigger than standard Jubilee - but the main jet is probably a bit big. Old Amals rarely die completely -they just sort of gradually get more decrepit. But it might prove something !.

How about valve timing - could this be a tooth out ? Have you a handy way of doing a compression check ?

The Boyer timing is set by the electrical components in the box, they all have a tolerance band on their properties so until you strobe it the timing could be out by up to 10 degrees. Try fitting a new boyer box alone and then stobe it, the timing will be different from the old box, the static timing is only to get the bike started so even if the engine does not have strobing marks you need to add them and strobe it. I sent an exhaust pipe on a bike blue for 18" just starting up a bike with a changed boyer box before I could strobe it.

Previously Dan Field wrote:

the monoblock that came with my box of bits jubilee was off a C15 and knackered so I bought a concentric from surrey cycles, they estimated what the getting should be.

At the moment I tickle the carb until it's flooded close the choke and it starts first kick almost every time and ticks over. Trouble is it won't run without the choke even when warm, unless I can get the revs high enough so it is running on the main jet but as it's running in I'm tying to be careful at the moment!

Next job is to lift the clip one notch and see what happens, but I didn't know if anyone else is running a concentric and knows what jetting they have?

dan

If you have not seen it have a look at Bushmans Amal site. I have had a lot of trouble with my Norton's concentric carbs and there have been quite a lot of people having problems. My problem is left hand carb pilot jet keeps blocking (Its on a Commando) I have done the Bushmans mod to get access to the pilot bush, and each time it is blocked. For the fourth time I have the carbs stripped cleaned and soaking in cleaner waiting to be ultrasonic cleaned. A lot of people have had problems with new carbs, badly made with swafe contamination and the blank over the pilot mixing chamber not sealing. I should strip the carb and check it all, including float height and operation. Your problem sounds like it could be pilot circuit.

although there is a big hesitation if I open the throttle wide

Could be wrong main jet or needle notch needs tuning. Choke on improving it makes lean more likely than rich. (but I have just had same symptoms on a B25 and that was a loose HT lead in the coil)

1, Run at full throttle and wait for the hesitation to clear, slightly close the throttle, if the engine power improves increase the main jet.

Hello All,

Project marking is coming to an end so hopefully some time to do other than the day to day maintenance the only transport Brit bike owners is faced with. To the ther Norton!--I have gone allk experimental and bought an AMAL 1600 series carb--a 25mm mrk 1.5. The people there are a mine of information on what jets to try etc--think of all the data the firm built up as one of two carb suppliers to the bike industry (Zenith being the other--and they own them as well now).

I agree most carb things turn out to be electics but--of the ones that remanin carb things--something running rich due to being worn out is usually at fault. I haqve put a 1600 series 25mm Mk 1.5 on my WDB40- (it was found in a skip--the carb not the bike) and was fitted to a CZ. I had lots of trouble but--the AMAL adagage--newer the model of carb the smaller the jets was true. Any waer in the carb body and they will onbject to an air filter. All the carb stuff I have had in the last 5 years has been running rich matters bar one instance where the Rickman would not go over 80--I made that richer and am scared of it again--yippee.

I do bleieve they recommended a 130 main jet for me--for the Navigator--they reckoned that monbloc to concentric involes a 30% drop in jet size