Concentric Modification
- Thread starter bmetcalf
- Start date
The image is too small to read. Anyone got any ideas?
Bruce, I see that is 1969, I have bought a few of these old carb's, And the hole in the needle jet is very small on some of them, I wonder when they went bigger ?. Cheers Bill.
I have a Norton 850 that I run with a single carburetor. The carburetor is an Amal 1038. It has many of the same parts as the 900 series Amal including the float bowl and the float, as well as the needle jet and needle. I have that carburetor working really well on the Norton. It has to flow a lot of fuel with only one small float bowl supplying the 828cc Norton. That carburetor will not flow enough gas for the 850 without the modified float bowl. I tried it and the float bowl would drain in about two seconds and run lean until I modified it. Mine is modified with two extra holes where the slot is shown in this diagram from BSA. Many people find that with modified Nortons they have the same problem even with dual carburetors. John McDougal told me the reason he had Monoblocks on his 750 Norton race bike was because they flow more. Later on he found out about the modification of the 900 series carbs, but he prefered the Monoblocks anyway. I think that a Vincent with 930s or 932s might need to have this modification if the twist grip is going to be opened all the way for any length of time.
I have a BSA R3 that road raced a bit. It has Big D cycle Jack Wilson (Texas go fast Triumph guys) ported head and runs 3 X 930 concentrics. It would pull well over 8000 down the long front straight at Portland topping out at 140 mph and never showed any sign of fuel starvation. But then that's a 930 feeding a 250 cc cylinder, not a 1038 feeding two 414 cc cylinders.
How do Amal's get away with selling stuff like this ?, I bought a pair of 32mm in the 70s, When I was first looking for Speed.
You could fry an Egg on the cases after getting 110, If I put bigger jets in it would dry up.
With there flow meters etc they should not have sold them.
I wonder how many blokes have blown up there engines because of them.
I know I have a pair on one of my bikes, But only because they were cheap from a junk sale, And I don't go very fast now !!.
Ken S Bundy ? from OZ had the right idea, Junk the float etc, Drill a big hole in the needle jet and fit a remote good float, I did it on my Comet a few years ago, Just to keep my Brain working !!, Lots of Fun. Cheers Bill.
You could fry an Egg on the cases after getting 110, If I put bigger jets in it would dry up.
With there flow meters etc they should not have sold them.
I wonder how many blokes have blown up there engines because of them.
I know I have a pair on one of my bikes, But only because they were cheap from a junk sale, And I don't go very fast now !!.
Ken S Bundy ? from OZ had the right idea, Junk the float etc, Drill a big hole in the needle jet and fit a remote good float, I did it on my Comet a few years ago, Just to keep my Brain working !!, Lots of Fun. Cheers Bill.
I agree Bill......Loads of racers here with many a holed piston using concentric carb's.
I think that Norton Commandos are on the edge of suffering from insufficient flow from the Concentrics. I think that they compensate for this with main jets that are large. It is not the ideal situation. The standard size main jet for a Commando is about 260. My friend has a Commando 750 with a PW3 cam and I think he has 360 mains and it runs pretty lean even with the air cleaner. It seems that when he puts bigger jets in it makes hardly any difference. I have told him to modify his float bowls but he hasn't done it yet. Vincents would be in the same boat, but nothing pushes this system to the limit as much as two 414 cylinders sucking from one of these tiny float bowls.
For comparison a stock Triumph 65o Bonneville runs either 180 or 190 main jets, on a Trident the main jet standard size is 150. Niether of these engines pushes the limit of the 900 series or 600 series carburetors. I think it is when the main jet is nearly 250 that the float bowl and the float needle and seat become a problem. If the float needle restriction was not starting to cause a problem you would expect that a Norton 750 main jet would be more in the range of 190 X 750/650 = 219
You have to make sure you have the float needle seat with the big hole as well. I think the biggest one is either .125 or .156"
For comparison a stock Triumph 65o Bonneville runs either 180 or 190 main jets, on a Trident the main jet standard size is 150. Niether of these engines pushes the limit of the 900 series or 600 series carburetors. I think it is when the main jet is nearly 250 that the float bowl and the float needle and seat become a problem. If the float needle restriction was not starting to cause a problem you would expect that a Norton 750 main jet would be more in the range of 190 X 750/650 = 219
You have to make sure you have the float needle seat with the big hole as well. I think the biggest one is either .125 or .156"