15-07-2020, 07:53 PM
Hi. I run an SU H1 with a HC head. This carb came originally from a Morris 8 series E. As standard the piston weight is used alone, there is no spring. I presume springs were a later refinement to allow a lighter piston for reduced inertia, and also to cater for carbs that weren't vertical. Did a previous owner modify your carb ?
I seem to recall that the needle has a deliberately accentuated taper near the idling position, so that you can set idle mixture with only a small variation in jet height and therefore avoid changing the mixture too much at greater speeds and loads. All the standard tuning advice is slanted towards getting the idle mixture right, as there is an assumption that the needle is the one developed (not without some effort) for the precise needs of the engine and will therefore not need altering. Of course for an A7 such a needle was never officially developed, so choosing the best one is down to experiment.
SU carbs aren't very sensitive to float chamber fuel level, and springs and damper oil will hardly affect idle, so the fact that your jet adjustment needs to be right up one end is a puzzle. Is the choke linkage all OK, are all the washers and glands in the right place ?
I seem to recall that the needle has a deliberately accentuated taper near the idling position, so that you can set idle mixture with only a small variation in jet height and therefore avoid changing the mixture too much at greater speeds and loads. All the standard tuning advice is slanted towards getting the idle mixture right, as there is an assumption that the needle is the one developed (not without some effort) for the precise needs of the engine and will therefore not need altering. Of course for an A7 such a needle was never officially developed, so choosing the best one is down to experiment.
SU carbs aren't very sensitive to float chamber fuel level, and springs and damper oil will hardly affect idle, so the fact that your jet adjustment needs to be right up one end is a puzzle. Is the choke linkage all OK, are all the washers and glands in the right place ?