350 NH carburation

[Alistair F]

My 1958 350 NH is fitted with a standard 376 monobloc with a 200 main jet, 106 needle jet, 25 pilot jet and 3 1/2 cutaway slide, C needle.
It is running very rich at low throttle settings, sooting up the plug and misfiring if ridden in slow moving traffic.This is despite the throttle needle being set in its lowest, weakest position. The carb appears to be in generally good condition although the slide is slightly loose in the bore at the bottom of its travel - tighter further up. Would this be enough to cause this very rich running at low throttle openings?
If so carb replacement might be the only answer - probably a new Concentric Mk 1, any advice there?
Many thanks,
Alistair

[ralfpenkert]

Probably a worn needle jet.
However a Concentric will be a good choice.
Ralf

[simon.holyfield]

My 1958 350 NH is fitted with a standard 376 monobloc with a 200 main jet, 106 needle jet, 25 pilot jet and 3 1/2 cutaway slide, C needle.
It is running very rich at low throttle settings, sooting up the plug and misfiring if ridden in slow moving traffic.This is despite the throttle needle being set in its lowest, weakest position. The carb appears to be in generally good condition although the slide is slightly loose in the bore at the bottom of its travel - tighter further up. Would this be enough to cause this very rich running at low throttle openings?
If so carb replacement might be the only answer - probably a new Concentric Mk 1, any advice there?
Many thanks,
Alistair

I'd change the needle and the needle jet unless you are very sure that it's only done a few thousand miles. I had a very similar problem on my W/NG a couple of years ago and carefully measured the needle. It was very worn and it doesn't have to wear much to make a big difference.

[Alistair F]

Thanks Ralf and Simon,
I will order a new needle jet and needle and give this a try - would prefer to keep the carb original if possible!
Alistair

[ian williams]

ive had similar with my NH with alloy head using a 376/8 carb , been running N5 or HBLN spark plugs .
initially i thought it was running lean because i had the odd backfire when coasting so i kept richening it up but that didnt quite ever solve the hesitation that occurred just off idle .

pilot jet was changed to 30 but i lost idle mix tuning range so went back to original 25 .

i had been playing with main jets and am on something like a 210 now but it worked ok in the range of 200-220 with little difference noted .

i had fitted the needle in all positions and even with it lowered fully it still wouldnt run right , so decided to change it from the 106 .

i fitted a 105 needle jet and now it runs much better and somewhere handy , so looks like it was the needle jet .

engine will run flat out and i have had it up to 70mph once although normally i dont go over 55-60mph on it , have now fitted a 20 tooth engine sprocket which it seems to like .

try a 105 needle jet and put the needle mid position to start with .

cheers , ian

[simon.holyfield]

Thanks Ralf and Simon,
I will order a new needle jet and needle and give this a try - would prefer to keep the carb original if possible!
Alistair

There's a few details here showing the wear in the needle:

https://ariel-square-four.blogspot.com/ ... d.html?m=0

I think the most interesting thing to note is that in the weeks of tinkering it took me to get the W/NG to run right, I had basically put the carburettor back to standard specification!

[JohnnyBeckett]

hi i would just fit a new concentric and forget trying to fix worn carb, i think it was the one of the best things i did to my 350 nh 1954 it starts easy and runs very well

[nevhunter]

The concentric is a much better carburetter than a monobloc... Nev

[Brian_Walker]

Try a 20 pilot, I had the same problem with my 500 (389 monobloc) I had to wind the pilot jet out too far (make leaner) to get it right then went to 20 pilot jet also dropped the needle down further. No point in messing with main jet till the above two are sorted .

[Roger Gwynn]

Nev - agreed undoubtable better but not quite as original. A Honda engine is better but only one person I know has replaced his Ariel engine with one. I use a MKII Amal on my sporty 500 , which is even better but even less original.