But surely the needle is withdrawn as you open the throttle due to a pressure difference either side of the diaphragm. Aren't they just return springs for when you roll off the throttle? I can't see how they'd affect fueling.

Le_Blur wrote:But surely the needle is withdrawn as you open the throttle due to a pressure difference either side of the diaphragm.

Yes, springs of different tension (giving resistance to the diaphragm) allow the needle to withdraw different amounts for the same airflow going through the carb, letting either less (leaner) or more (richer) fuel in depending on spring tension.

which means lighter springs would effect the whole rev range then. so would need a full rejetting to compinsate.

Oh yes, of course. That makes complete sense. Thanks for joining the dots up for me, Cammo.

lizard821000 wrote:which means lighter springs would effect the whole rev range then. so would need a full rejetting to compinsate.

Springs only affect the midrange fuelling since that's the rev range that the needle controls.

When the needle is fully withdrawn, the main jet is the restriction (control) to fuel flow.

makes sense!

i get the main jet bit, do the slides move during pilot ideling on these VC carbs?? or is there not enough vacume produced?

just curious mate

Good question. The slides are fully down (extended) when idling, the thickest part of the needle diameter blocks fuel from entering the carb venturi, instead the pilot circuit supplies fuel. Hence it's critical that needles are in good condition (not scratched or worn) as they could otherwise let fuel through that shouldn't be entering the carb causing rich running conditions.

It's a common myth that the pilot circuit only controls fuel at low revs/load - it isn't controlled at all and constantly flows fuel at all revs/load. This is why changing pilot jet sizes affects fuelling in the whole rev range. This is a useful adjustment on nc30 hrc type setups where larger pilot jets can fill in lean parts of the midrange - don't change them on standard setups!

Here's a useful pic of basic operation, there's plenty more good CV carb info out there. Slow jet is an alternative name to pilot jet.

When they say 'vacuum port' isn't it more just ambient air pressure versus the high pressure in the carb venturi side that provides the pressure gradient, or is it in fact negative pressure?

Le_Blur wrote:When they say 'vacuum port' isn't it more just ambient air pressure versus the high pressure in the carb venturi side that provides the pressure gradient, or is it in fact negative pressure?

The vacuum piston's motion is controlled by venturi vacuum routed to the top of the rubber diaphragm though a tiny orifice.

It lifts because the area of this diaphragm is larger than the area of the piston itself.

It maintains a constant vacuum above the needle jet.

So i'm thinking from this thread that perhaps my midrange woes could be from an improper sized pilot jet?
Perhaps I should get a slightly larger one and leaner one to nut out my mid range? Does anyone know what the stock size is?