Misfire at 50mph

[Steeeeeve]

Hi fellas
I recently bought a 65 mini mk1 that had been sitting for about 10 years. 1275cc.
After setting the timing and adding fresh fuel it started right up.
I drove it around for a few days and it runs well at low speed.
The first time I drove it at 50+ mph it started to misfire after about 20 seconds of 50+Mph driving. Then would run well if I backed off. I suspected it was starving for fuel so I replaced the fuel filter.
I took it back out and it acted the same. I get up to 50 and after short while it stutters. Back off a bit and it runs fine.
It seems like the float bowl is running out of fuel and when I let off the gas it fills back up and it's good to go. It's a hif44 by the way.
So I'm looking for suggestions on what to try next?
Strip the carb?
Replace the electronic fuel pump?
It has new plugs.
Thanks in advance!

[greyghost]

replace the pump.
i had the same issue (at much higher speed), the car would start to surge.
the SU pump appeared to be working fine, it just couldn't supply enough fuel at sustained high revs.
replaced the pump with something a little more modern, and the issue has totally gone.

i would prefer to use the stock pump, but unless i can get it to "reliably" follow enough fuel it'll sit in the shed

[smithyrc30]

Check the tank breather is not blocked, check the carb float chamber vent is not blocked, check the fuel pump is actually pumping fuel and the diaphragm is not torn. Make sure you have got oil in the carb dash pot damper. Then strip the carb and clean it. If you get that far make sure the float needle valve is clear and flowing fuel, the piston 'fall time' is correct, the needle jet is the correct one, clear, set at the right height and the float level is correct.

If it has been sitting for that long the fuel line under the car could do with checking as well, probably. Not sure if you have but I would change all the rubber fuel lines as well.

[mini63]

rule of thumb.....always check the electrics first.
simple stuff like condensors can often lead to wild goose chases.
HT lead continuity/resistance are are also easy to draw a line through too.

[carbon]

Could be running weak?

Would be worth pulling choke out slightly, if this makes a difference then check what needle you have in the HIF.