Single carb for a 650

Dear Peter,

If your 650 has a standard cam a 230 will be OK. If it has an SS cam it will need either a 240 or 250.

Regards,

Les Emery

Peter,

Wise choice, this sounds about right. The slide will be No3.

I have a 30mm Amal Mk 1 Concentric on my 1964 650 engine and the carburration is just perfect. Due to other reasons I have not run my 650 for two years but the carb was never an issue. Excellent throttle response, light action and good fuel economy using BP Ultimate E0. Tick over ultra reliable too. I use Champion N4 plugs in it. Hope this helps.

Hi

For a while now I have been trying to make Mikuni 30mm carb work on my 650 Dommi. I have finally given up and bought a Concentric 930/301 Prem. It has come with:

Main jet 230

Nedle jet 106

pilot RJ17

and a something I can't read3

Does anyone running a650 with a single concentric know if this is likely to work??

Thanks in advance.

WELL these are all wrong for a start a 30mm is too big the jet size wrong , I run my norton Manxman 650 on a single carb now, and its a Amal 1.1/8 inch or 28mm 389/76 mono bloc with 290 main jet 106 needel 25 pilot jet mid notch and I am still runing a bit rich , I use Bosch plugs W7DTC and have been for three years no problems the original main jet was a 320 but this runs way too rich and I have tryed a 300 main jet too now down too a 290 main jet so am going to try something ease like getting a better float leavel or go to a size 4 cutway side, where on a 3.1/2 cutway side now, hope this helps a bit and I have a Amal Manual thats a 1.1/2 thick in there its tell you the correct size from imperal to metric and a 1.1/8th of a inch is 27.9mm to be on the money or 28mm nearest demoinator your anna j

The Mercury 30mm single carb spec is a 280 main, 25 pilot, .106 needle jet and position 3 on the needle. I have a 32mm concentric on my Mercury with a 290 main - works perfectly (sea level).

This is all about Concentrics, Anna, which will have different settings to a Monobloc. I am struggling with richness on my 650 with a Monobloc. I am going to tweak the float height, and if that doesn't work, will try a 30mm Concentric. It shouldn't be too big, because, if you look at your Amal book, later, 1967-on 650SS models were fitted with two 30mm Concentrics, with 280 main jets, 107 needle jet and #3 slides! It would be good to hear from anyone who is running a twin-carb 650 with those settings....

I have a piece of paper from Southampton University which acknowledges that I once was quite good at number crunching. So I did a few basic calculations and arrived at the results below.

1&1/8" = 1.125"

1" = 25.4mm

So 1.125" = 1.125 x 25.4

= 28.575mm

Therefore 1&1/8" = 28.575mm

So a 389 Monobloc carb with 1&1/8" choke size equates to a 28.575mm choke size. Does this news matter?

Also.......the Mercury did not use the same cylinder head as the Manxman or 650SS. It used an adapted version of the Commando 750 head which has slightly better porting. It also came with a 30mm Concentric as standard which suited the head....as it did on the Commando.

Finally........the anomally. For reasons that nobody has explained the 2 into 1 manifold on the Mercury engine has/had a 28mm choke which of course put a step in front of the incoming mix. Why??

When I rebuilt my Mercury I made sure that the carb id matched the inlet tract. Have dremel will deburr!

A step in the inlet tract will disrupt the boundary layer and cause turbulence which can result in better mixing of the fuel/air charge and can result in better running. A case in point is the 350 Gold Star where it was discovered that fitting the larger carb from a 500 boosted the max power considerably (and somewhat counterintuitively).

Previously ian_cordes wrote:

This is all about Concentrics, Anna, which will have different settings to a Monobloc. I am struggling with richness on my 650 with a Monobloc. I am going to tweak the float height, and if that doesn't work, will try a 30mm Concentric. It shouldn't be too big, because, if you look at your Amal book, later, 1967-on 650SS models were fitted with two 30mm Concentrics, with 280 main jets, 107 needle jet and #3 slides! It would be good to hear from anyone who is running a twin-carb 650 with those settings....

hello Ian Well The early manxmans had twin 1.1/8th carbs with interted sleeve fitted and small jets like 250 main jets 20 pilot jet 3 cutway 106 needel jet, one float bowl shared between the two cars , but they soon whent down a size to 367/265 and 376/266 and a 240 main jet 25 pilot jet 3.1/2 cutway 106 needle jet I am not shure what the reson was but there must of been one somewhere , I not touched my Nortons for well over 2months now, there just sitting on there benchs covered up so the dust does not get on them , and that where they stay until I am well enough to get them out, later I may sell one of them and buy a electric motorcycle or build one from scatch , who kowns what i do next, not even me , we just go with the flow for now, yours Anna J

All of that is true, Anna, with regard to Norton quickly reducing the size of the Monoblocs from 1 1/8" to 1 1/16", which makes it all the more puzzling that, when Concentrics were introduced, they went up to 30mm (1 3/16"), with 280 main jets, a 107 needle jet, 25 pilot and #3 slide. It obviously worked, and no doubt upped the power output. I have never heard anyone complain on here of issues with that set-up. Maybe it highlights differences in efficiency between Monoblocs and Concentrics, perhaps?

Odd then that the first 750 Commando's were fitted with a pair of 30mm Concentrics with 220 main jets, .106 needle jet and # 3 slide, delivering less fuel when one would thing the larger engine would be asking for more.....

Just checked Amal's carb book for the Atlas, and early ones had 1 1/8" Monobloc's with 420 main jets, reducing to piffling 350's in 1966. Come 1967 and the Concentric's, they had 220 mains, 25 pilot, .106 needle jet, and #2 slide cutaway, similar to the Commando, apart from the slide. Strange that 650's had bigger jets than the 750's; unless the book is wrong, of course..... No wonder we struggle to set our old bikes up....