1954 NH 350 Carburettor

[Pete.Silson]

is it possible to fit a carb with a 1 and 1/16" bore or is it just too big?

I doubt you will get much improvement in performance with a bigger bore and it's a lot less hassle to get a 1" 276 and use standard jets rather than having to "tune" it for the bigger bore.

[Simon.Gardiner]

I have to say, we pay a lot of attention to things like carb choke sizes but I do wonder if it makes a lot of difference as long as you're ball-park correct and not approaching 'neck wringing' in search of performance.
My FH should have a 1 1/16" carb, To isolate some of it's recent problems I've changed the 26mm concentric (slightly small, approaching 1") to a 28mm concentric (slightly too big, approaching 1 1/8") and I can't tell a difference in performance - although because the thing has been almost perpetually running-in for the last few years it probably hasn't ever had to run above half throttle.
I was expecting the bigger carb to make it noticeably less responsive at the smaller throttle openings though but seemingly not, so far.

SG

[simon.holyfield]

I have to say, we pay a lot of attention to things like carb choke sizes but I do wonder if it makes a lot of difference as long as you're ball-park correct and not approaching 'neck wringing' in search of performance.
My FH should have a 1 1/16" carb, To isolate some of it's recent problems I've changed the 26mm concentric (slightly small, approaching 1") to a 28mm concentric (slightly too big, approaching 1 1/8") and I can't tell a difference in performance - although because the thing has been almost perpetually running-in for the last few years it probably hasn't ever had to run above half throttle.
I was expecting the bigger carb to make it noticeably less responsive at the smaller throttle openings though but seemingly not, so far.

SG

Not wishing to hijack this thread, but how do you have the 28 mm carb set up Simon?

[tony.barnett]

TBH if its `performance' you are seeking then starting with a swingarm 350 Ariel isn't the best place.

Taxi for Barnett!!....

But getting it running right makes sense - get a 1" 276 - set it up as per the books (remember that the float bowl comes in several different angles - you need the right one for a 350 Ariel).

Enjoy bashing it around the lanes

[Simon.Gardiner]

Simes - 928 is set up the same as same as the 926! (ie settings as per the OE monobloc but with main jet 200 instead of 240).
I'm not claiming any of this is optimal but it it seems more or less OK at the smaller throttle openings, which are all it's had so far.
(The later A10 Gold Flash actually went up to a 1 1/8" carb for the same engine tune.)

SG

[ian williams]

hi Robert ;
i have an NH which is primarily based on a 54-55 model , the engine is a 53 bottom end with a later top end using alloy head and it uses a 376/8 monobloc carb .
it runs very well so long as the pilot gallery isnt blocked .
i had to play with the carb a bit to get it running reasonably well , the needle jet i changed to a 105 as the 106 made it hesitate when opening throttle from idle to about 1/3 open , with the 105 needle jet it runs much better .
im running around 5/8" full advance setting via the fag paper in the points and bit of dowel down the plug hole method .
the engine will rev if you want it to and i could hit 70mph without problem , i did change my engine sprocket to a 20tooth for better cruising speed but with the 19tooth i had before it would certainly get the beans up if you opened it up when on the move on a nice flat road .
not the fastest bike by any means but very nice to ride around the local country lanes .

only been out on my NH twice this year as have had cancer and chemo treatment but now things look better and if it stays that way i am looking forward to next year and the bikes again .

hope you get the bike sorted and running how you want it .

[robj_b]

Hi Ian

Thanks for your response it does fill me with some confidence that a 376/8 will give me a little more top end speed as with the current carburettor it starts to struggle past 55mph on the flat.

It is definately a fun bike for pootling down country lanes I just occassionally would like a little more speed to get to these.

Thanks all for your valuable advice it's all appreciated.

Robert

[nevhunter]

Check the Muffler is not adding excess back pressure That will absolutely KILL Power. Nev

[simon.holyfield]

Simes - 928 is set up the same as same as the 926! (ie settings as per the OE monobloc but with main jet 200 instead of 240).
I'm not claiming any of this is optimal but it it seems more or less OK at the smaller throttle openings, which are all it's had so far.
(The later A10 Gold Flash actually went up to a 1 1/8" carb for the same engine tune.)

SG

Do you know if it has a separate pilot jet? I understand most of these 900s have a built in pilot drilling instead of a jet.

[Simon.Gardiner]

Simes
Both this 928 and the 926 are 'premier' concentrics that have the separate pilot jet (screwed in opposite the pilot air screw).
The 'vanilla' concentric just has a pilot drilling (which is just about impossible to clean out properly, unless you drill and thread an access hole opposite the air screw). However, there is also a tapped hole for the pilot system in the top of the float chamber - when the concentrics first came out there was a jet available to screw into that (I've got several) but Amal stopped specifying the fitting of that jet after a while (but they carried on putting a thread in the hole as if something should be there!)

SG