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Another embarrassing break down

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JP,

Well spotted!

I didn't know what is was - it just appeared one day.
Actually, it is a pea hen. I've started to feed it crushed corn and dog pellets so now we'll never get rid of it. Ha!
In the evening it actually flies up into one of the big poplar trees to roost for the night.

Cheers!
 
You do realize that Peacocks and hens are terribly noisy things. Beautiful yes, but I hope your neighbors are a decent distance away and that you are a sound sleeper.

Regardless, it sounds like you've done a good bit of detective work. I remember from previous discussions about the HS4 on the 1500 that the most common needle is whatever was stock for the MGB. I don't remember off hand what the MGB used but I'm fairly certain it wasn't the #5 needle shown in your picture.
 
Glad to see you've solved your issue, and that your needle is correctly positioned.
You may well need a richer needle, but if you do it will be to richen the mixture higher up in the rev range. At low speeds, all needles are pretty much the same and I interpreted your problems as being low down.
You seem to have a misunderstanding about how SUs work when you suggest that the carb needs a richer needle to supply a 1500cc engine. The carb has to supply an air/fuel mixture, not just fuel. If a carb is too small, it'll tend to run rich, not lean, due to the high airspeed over the jet and increased suction caused by that.
If you are indeed running lean, and it does sound that way from what you write, then gently operating the choke while you have that stumble when cruising should clear the problem. However, with the jet as far down as you have it now, it's hard to see why you're not running very rich at low to medium revs. That's why I think you may well have a problem other than the needle. If you don't hsave an air leak, then if it were me, I'd look to ignition, being, as I am, an Englishman of years who firmly believes in the old adage that 99% of carburetter problems are ignition. Try a few more degreees advance.
 
Doug,

Yes, the peahen has let loose on several occasions - fortunately during daylight hours - quite hilarious, sounds like something out of Chitty Chitty Bang Bang!

As far as the needle, it is the one supplied by Burlen Fuels (SU) in the carb when new and VB also shows the #5 to be "std". VB show a #6 as "rich'.

My plan was to get the #6 and measure it, in 0.1" increments, with a micrometer, as well as the #5. Install the #6 and if it runs rich then modify the #5 "accordingly". I'll do this carefully, using a drill press with sandpaper.

Incidentally, both Moss Motors and VB show just the needle, and not a needle assembly. Does the needle just pull out of that top hat (needle holder)?

Roger,

Thanks for the added insight into the workings of the SU.

Let me see if "12 flats down" is safe for me to pull out the choke. If so, I'll try your suggestion. I'll double check for air leaks also. As far as timing, if I remember correctly, I have it set at 14 BDTC (@ 850 rpm) - this gives 28 BTDC (@ 3500 rpm), so I have a little room for more advance, I suppose.

Cheers!
 
Most spring loaded needles come "with" the top hat on them. When you price them, the spring loaded needles cost a lot more than fixed ones. I assume that's because of the top hat.

I will check my books tonight regarding MGB needles. I am certain they were not #5s.
 
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