Running issues. Does cold air onto the carbs really make a difference

[johnnydog]

Hi Joe,
When replacing the tops of the carbs after they have been off, it is very important to ensure that the piston moves freely up and down, falling on the bridge with a metallic clunk. If either binds at all, this will contribute greatly to its poor running.
With the air collector box off, lift each piston in turn to see if the moves and falls freely. If not, release the three screws for the top and then move the top slightly at the same time checking the piston moves freely. Once it is in the correct position, gently nip each screw in turn, whilst continually lifting / releasing the piston with your finger to ensure it doesn't start binding, doing each screw bit by bit until reasonably tight. If you tighten each screw fully before going to the next screw, invariably you will have to undo them again to get it just right. The movement of the top between the piston binding or moving / falling freely is only a hairs breadth!

[Joe Wright]

Ah okay thankyou. I will check that. Good to know

[Alec]

Hello Joe,

I have to disagree slightly with Jonny.
To centralise the jet you have to screw the jet adjustment fully up, slacken the jet securing nut, which clamps the jet to the body, press the piston down while tightening the jet clamping nut. Wind the jet adjuster back down twelve flats and check that the piston falls freely.

Alec

[johnnydog]

Hello Joe,

I have to disagree slightly with Jonny.
To centralise the jet you have to screw the jet adjustment fully up, slacken the jet securing nut, which clamps the jet to the body, press the piston down while tightening the jet clamping nut. Wind the jet adjuster back down twelve flats and check that the piston falls freely.

Alec

I am not talking about centralising the jet Alec; I never referred to this procedure. That has to be done as you say. I am just referring to ensuring the top is correctly seated when replacing it, relative to the free movement of the piston. If it isn't quite seated properly without continually checking the free fall of the piston whilst tightening the three screws a little at a time, this can result in the piston binding / sticking, leading to poor running at it won't lift correctly. Similar procedure to Strombergs.....

[Alec]

Hello Johnny,

an off centre jet is the only reasonI know that causes a (clean) piston to stick. I've never followed your procedure and simply fit the chamber and screw it down. Unless it is damaged in some way that's all that is required, in my view

Alec.

[johnnydog]

There is a tab on the side of the piston on the SUs that fits in a slot in the body of the carb, to ensure the piston fitted in the correct rotational position. This tab is secured by a small screw. I have known this tab be knocked slightly to one side due to careless handling, and it then binds on the sides of the slot with a metallic scraping noise which is detected when lifting the piston to check it's free movement when the top is replaced. Not relevant to Joe's questions, but also worth checking if the piston binds when assembled.

[Clifford Pope]

The HS6s on my engine have spring-loaded pins underneath for lifting the pistons. This can be checked at any time, with no dismantling.