My B31 runs rich despite what I try. The carb looks in good condition and it has all the correct jets etc. Does this look like the correct float needle for the monobloc carburettor? It looks to me like it's off a concentric carb with the thinner piece which slides into the plate on top of a concentric carb float. If this is wrong it will likely affect the float height?
Thanks
David
Iirc the monoblock needle does not have the bottom rim, just the pointed bit.
I'd buy a new one (not expensive) from Burlen or Surrey
Suggest you buy a new pilot jet too in case some one has bored that out.
That Viton tipped brass float needle fits both concentric and monobloc, Amal part 622/197. The Viton tip seals better than the original nylon needles. Introduced back in the 1970s. They also come with an aluminium body and Viton tip.
The first photo shows an original nylon needle on the left and an aluminium bodied Viton tipped on on the right.
The second photo shows the float chamber and needle on my monobloc.
Quote from: JulianS on 11 Jul, 2025, 11:42 That Viton tipped brass float needle fits both concentric and monobloc, Amal part 622/197. The Viton tip seals better than the original nylon needles. Introduced back in the 1970s. They also come with an aluminium body and Viton tip.
The first photo shows an original nylon needle on the left and an aluminium bodied Viton tipped on on the right.
The second photo shows the float chamber and needle on my monobloc.
Perfect. Ages since I had a Monobloc and don't remember the extra bit but the lengths look the same.
I remember reading about a flooding monobloc carb and it turned out to be a fiber washer fitted under the needle holder causing the float to be to high .
That is correct, the later production monoblocs have a shallow recess machined for a fibre washer, they also use a thin concentric type O ring on the flange.
The earlier monoblocs dont have a recess and the joint is metal to metal no fibre washer. They use the thick O ring. If a fibre washer is used on these the float is too high and may even rest on the tickler before the needle seats giving you petrol all over the place.
Photo shows the recess in a later carb.
The brass needle is slightly heavier than the two alternatives so the fuel flow is slightly different.
Use the alloy float needle as the brass one will weigh the float down further making the carb run rich.
Thank you everyone some great advice there. I have stripped the carb down again and can't see any wear on the needle and jet. The float needle holder seems OK and is metal on metal although it is brass and I might put an alloy one in. But what I did notice is it should have a 376/3.5 throttle slide but it has a 376/2 throttle slide. So I think an alloy float needle and a new throttle slide will be the first things to replace
David
One thing that often goes astray is the tubular spacer on the float hinge pin (seen in Julian's second photo).
With the number 2 slide it will run rich.