14-08-2022, 06:51 PM
Hi Steve
I use an SU H1 on a reproduction "Nippy style" manifold, rest of the engine is standard. My initial needle choice was GG, and as it seems to run very well I have never experimented further. From your description it sounds like something is seriously amiss.
These early carbs rely on piston weight alone (no spring), and have an oil damper. The piston/dashpot assembly is built with precise clearances, and the original damper oil recommendation was SAE20. I can see how over-thick oil could delay the rise of the piston (causing temporary richness) but not how it could affect its steady state position. Pushing the piston up with the fingers should require a reasonable effort.
I recommend you clean things out and try thin oil as a first step. If this brings no improvement, it's possible that someone has fitted a piston from another model
On the road, the airflow needed for full piston lift equates to something over 20 BHP so this will only happen on a fairly tuned engine. Partial lift doesn't seem to matter, but does mean that the needle profile at the lower stages is the more important.
And as usual when discussing fuelling, check the ignition is OK !
See attached pdf for more original data.
I use an SU H1 on a reproduction "Nippy style" manifold, rest of the engine is standard. My initial needle choice was GG, and as it seems to run very well I have never experimented further. From your description it sounds like something is seriously amiss.
These early carbs rely on piston weight alone (no spring), and have an oil damper. The piston/dashpot assembly is built with precise clearances, and the original damper oil recommendation was SAE20. I can see how over-thick oil could delay the rise of the piston (causing temporary richness) but not how it could affect its steady state position. Pushing the piston up with the fingers should require a reasonable effort.
I recommend you clean things out and try thin oil as a first step. If this brings no improvement, it's possible that someone has fitted a piston from another model
On the road, the airflow needed for full piston lift equates to something over 20 BHP so this will only happen on a fairly tuned engine. Partial lift doesn't seem to matter, but does mean that the needle profile at the lower stages is the more important.
And as usual when discussing fuelling, check the ignition is OK !
See attached pdf for more original data.