Breather pipe

I can't get my Scamp running as nicely as I'd like. It is as if the mixture is weak when driving but correct when static. It's an 850 engine with HS4 carb rebuilt with new needle and jet. The carb didn't have a port for the breather pipe so I've machined one into the small flat where one would normally go. Could the addition of the breather pipe be the cause of my problems?

Breather possibly. Or you might have the wrong needle/spring (you don't say which are fitted) or it might be the wrong carb entirely. Most 850 Minis had a 1¼" HS2 - only automatics had the HS4, with an AN needle and Red spring. The size should match the inlet manifold - are you using the original one?
There might be - if it hasn't been discarded at some point - a metal tab under one of the float chamber bolts, with a number which gives the original specification. Although if you've changed the needle, that may not tell us much.

Thanks - Problem is car came with 850 engine outside the car, a duff engine in the car and a large box of bits. So may well not be the correct carb for that engine. The needle is an H6 as recommended by Mini Spares. I didn't think about the spring as the original looked fine and it's uncoloured.

I had a similar prob.
Although when it was static the advance/ retard part of the Dizzy seemed to work and the timing light moved on the indicators.
When i had it on a rolling road they diagnosed the dizzy advance/retard not working properly. They changed the internals and fixed the problem.

It cost me about £120 many years ago but it was well worth it.
If you do go down the rolling road way make sure they have their own workshop to fix the problems.

Thanks - but it has a new distributor so I doubt that could be the problem.