Monoblock Carb ??

[dave55]

Bit confused with numbers on the carb i took off my bike today, hoping someone will have the answers. Have done a quick search on here and looked at thread put on by Ellis  but the number differ to mine so here goes, this is what i have .
Bike 1960 A10 GF swing arm. Engine has been fitted at sometime with 357 Spitfire cam and was running ok when i bought it.

 . Amal 376/6.1 stamped on flange . Throttle valve 376/3.5. Needle C 3rd groove from top .
Main jet 270. Needle jet 106 . Jet block 1.06  . Float 376/ 083

My question is doe these jets etc sound right ones for this bike and cam, the manual shows carb is correct one for standard bike but what does the 6.1 bit stamped on flange mean ??   
Heres hoping the Gurus on here will know or point me in right direction Thanks

[Klaus]

Hello Dave,
The Amal Carb 376/61 is for Velocette 350 Viper with bore 1.1/16, pilot 30,cutaway 3.5, Main 270,Jet 0.106, 3 groove.

GF 1960-61 should have Carb 389/48 with bore 1.1/8  Pilot 30, cutaway 3.5, Main 250 , Jet 106 , 3 groove

cheers Klaus

[dave55]

Thanks Klaus, the plot thickens now , i am wondering if previous owner has fitted an older head at sometime . The carb body and cyl head inlet tract are same diameter. I have seen in a Bsa manual that 376 carb was used also on A10 at some point.
I will look out for a 389 , at least the jet and needle are somewhere in the range on my carb
            Thanks Dave.

[chaterlea25]

Hi Dave,
The numbers on the carb do not matter very much, once the carb choke size matches the head all will be fine
At that size  the 376 and 389 choke sizes overlap
Fitting a 389 will make no difference except less room at the carb top for the cable
You are not clear with your post/ question?
You said the bike ran OK when you bought it, what has changed?

John

[dave55]

Morning John , what changed was i took carb off to clean out as there was a lot of crap in the tank. I also had noticed bike was difficult to start when cold ( ran ok when warm ) and choke cable appeared not to work, i found it was just sat into carb and no choke slide , brass tube or spring inside the carb. previous owner when i bought bike said dont bother with choke just put yer hand over bellmouth when cold. So while i removed carb to clean it i noticed missing parts and also some wear on body and slide which carries needle and decided to get them resleeved . This will be back next week , meanwhile i have ordered correct choke kit for carb from Burlens...( Superfast delivery ) When carb comes back i can fit the choke and an air filter and check it all works
and go from there , i just wondered whether main jet had been increased to 270 because the different camshaft had been installed by previous owner when engine had been stripped , he told me that it had done less than 1000 miles since and as he was 82 found he could not manage riding anymore as bike too heavy for him .
Thanks Dave.

[dave55]

Monobloc carb came back from being re sleeved on slide and body and rechromed nuts and screws where it had worn off or looked rough

[Slymo]

As Chaterlea says the numbers on the carb aren't crucial. If you look at the various Amal pages there were several different carb setups for basically the same machines what is important is to recognize that the carb has three separate sections that intersect briefly.  The idle circuit controlled by the air screw and with a monobloc the removable brass idle jet (accessible from the exterior of the carb under the hex nut) the needle jet that operates from just above idle to about 3/4 throttle and is controlled by the needle, the brass jet in which it slides and the slide and it's cutaway which affects its air and its timing and finally the main jet that operated above three quarter (or thereabouts) throttle and the slide. The secret of getting the carb right is to define by riding where your bike is misbehaving. Also to recognize that changing one section i.e. the needle jet will affect the idle circuit. I for instance had a bike that ran perfectly except on light throttle where is seemed hesitant and fussy. Turned out the needle jet was worn out and replacing it sorted the problem although it required that I richen the idle jet by a full turn in. Listen for pinking = too lean or black smoke too rich. Also as you mention good cold start bad hot = too rich in the idle circuit. The spark plug is the ultimate tell tale. ride the bike in its problem range and stop immediately and check the plugs (without serious burns if possible) this will give you a good idea of which way you need to go mixture wise.