Jetting for Concentric carb.

cleancut · Post by **cleancut** » Mon Jan 02, 2006 11:45 pm

Please help me with jetting sizes for my 1964 model 31.
The carb had been changed from monobloc to a concentric
with the markings 928 over 10.
Any advice would be appreciated.
Many thanks, Andy.

Chazzyb · Post by **Chazzyb** » Tue Jan 03, 2006 9:31 am

The only thing you can tell for sure is that it's a 928 (28mm choke diameter, 900 series). Originally, the '10' would have determined the factory jet/slide configuration but anything can be changed. I'd suggest you strip it to find the current jet, needle position and slide cutaway size, then follow a tuning guide. I'm assuming it runs at the moment?

No doubt someone will be along shortly who knows about twins and concentrics.

Don Madden · Post by **Don Madden** » Tue Jan 03, 2006 5:10 pm

Look up the tuning specs for a late Triumph single carb model with concentric carb, ie: TR6. It will give a starting point as they have similar characteristics. My Triumph factory manual for late 60's gives: Bore size 30mm, Main jet-230, Needle jet-.106, Needle type-STD, Needle position-2, Throttle valve-3 1/2. The pilot jet, of course, is fixed. These recommendations are for having an air cleaner fitted.
Cheers, Don.

Don Madden · Post by **Don Madden** » Tue Jan 03, 2006 5:14 pm

Addendum, (addendumb?) to previous post: Forgot the mention the 930/10 is specified for a T120 left side. Only minor differences to the single fitment.

ant · Post by **ant** » Tue Jan 03, 2006 8:37 pm

give surrey cycles a call and ask them.

alan13 · Post by **alan13** » Wed Dec 15, 2010 11:49 pm

i have a 1961 g80 500 and have a 30mm concentric i want to fit but i dont know what jets sizes , throttle slide cutaway and jet needle to use can anyone help
alan

alan13 · Post by **alan13** » Wed Dec 15, 2010 11:50 pm

Don Madden · Post by **Don Madden** » Thu Dec 16, 2010 2:27 am

Look up the settings for a BSA 441. They use a 30mm concentric, too. Pilot: 25; needle jet: 107; main:220. Be aware that Amal made changes in the concentric from the first, (1967?), to the later ones. By 1969, they replaced the removable pilot jet by one pressed into the transverse passage, replaced the needle, needle jet & jet holder with revised type. These three parts must not be mixed but used as a set. Cheers, Don.

iandusud · Post by **iandusud** » Thu Dec 16, 2010 12:15 pm

quote:
Please help me with jetting sizes for my 1964 model 31.
The carb had been changed from monobloc to a concentric
with the markings 928 over 10.
Any advice would be appreciated.
Many thanks, Andy.

I recently ordered a Concentric from Burlen Fuel Systems (Amal) for a '56 model 20. They told me that to convert from a Monobloc to Concentric you keep all the settings the same (ie pilot jet, needle jet, cutaway) except for the main jet which you reduce by 20%.

Ian