Concentric main jets

Diferent compression ratios, diferent concept. diferent airspeed through the carb.

Previously robert_tuck wrote:

Diferent compression ratios, diferent concept. diferent airspeed through the carb.

These carb settings are best viewed as a guide. Optimal settings can vary from bike to bike and of course very few machines are exactly as they left the factory. Add in the changes in fuel composition over the years and it's not really surprising that whilst the manual may be correct for one bike, it may be quite a way out for another.

Previously Jonathan Soons wrote:

On the attached page from the manual, the main jet sizes seem wrong. When the 650 and 750 use the same monobloc, the jet size goes up for the 750. With the concentric the jet size seems to go down. Is the manual correct?

Hello these jet sizes are the wrong way around , 650 should be a 220 jet size and Atlas should be a 260 jet size , Jet sizes are smaller on Concentric carburetors , yours anna j

Jonathon.

The numbers in your attachment are actually correct for a 930 Concentric if..........

There are two of them fitted to a 750 engine (Atlas or early Commando)

or on a single carb as fitted to a 650 engine (eg Mercury)

Attachment is Norton page from Amal Carb Manual for Classic bikes. Note that generally a 106 needle jet is used with this carb (& most others)

Attachments
norton-carb-settings-doc

Herewith some settings I found. Apologies for the poor quality of the images. Hopefully still readable. It appears the pdf is corrupt so ignore.

Dominic

Attachments
Carb%20settings%20750_Page_1.jpg
Carb%20settings%20750_Page_2.jpg
w

Hi, if it helps, I have just got the carbs sorted on my 1965 Atlas. I am running a pair of concetric 930's 220 main jets, 106 needle jet, mid position clip on the needle, 2 1/2 slide. I use standard 95RON unleaded, flat top pistons, standard cam, re-built mag. Starts first time hot or cold, tick over is good and reliable. I found No.3 slides would work if you set the mixture screws to give good running at 2000 rpm, but the mixture was on the rich side at tick over. The 2 1/2 seem just right. Plugs show good colour when chopped at full bore or less. tick over was unreliable until I swopped to "stay up" floats and aluminium float needles. Fuel level is set to Amal specs. The only non standard part of the engine (apart from maybe the concentrics and the flat top pistons)) is a non spigot head. I got a good one cheap and milled of the spigot on the cylinders.

Hi, the aluminium needles have the viton tip, and are available from Burlen. The older brass ones together with the plastic floats were causing filling problems on my bike. There is an Amal service mention of the filling problem. It is a lot simpler to set the fuel hight with the "stay up floats as they have stainless tags that bend. Forgot to mention I don't have an air filter.

Mercury Carb Settings

Attachments
mercury-650-carb-settings-doc

Hi, in the past I have tried a pair of monoblocs, a single Amal Mk2, all of which worked but had various problems. The concentrics with the set up I have described are now perfect on my engine. I synch them at tick over with vac. guages, and then synch them again at about 3K RPM. the play in the cables allows for any slight differential between the two settings. I use a double cable twist grip and bind the cable outers together so the wind deflects them to the same amount. Otherwise they flap about in different positions and upset the synching as you ride. It all helps to smooth thing out. I hope my settings work well for you. I found the mixture screw had a large effect well past tick over and was helpful in determining the correct slide cut away.