Having finally got the new engine in our car and had it running it appears to be running a tad rich and was wondering what the best jet needle is for a 1 1/4" SU HS2 of moggy minor and mini era the current one seems to be marked OH or OF its a bit difficult to see as the marking is a bit scuffed. Engine is big valves, trials cam slipper pistons currently using a 37 H/C head. Possibly off to the rolling road in the new year when its run in to see what it will do as I am keen to see having bored and ported the block in the bridgeport and undertaken much of the work in our little machine shop.
I can't comment specifically on the HS2 set up, but for the OM series 1" carb (no spring in the dashpot) AN or GG are the percieved wisdom. You could try them, I guess but equally, make comparisons using Minty Lamb..
A No.6 needle with a Blue spring - often works well and I believe is used a lot in the racing fraternity. Others have used AN needles, if you do a search there is a graph on one of the posts showing the comparison.
Yesterday, 10:50 AM (This post was last modified: Yesterday, 11:24 AM by Ruairidh Dunford.)
When you go to rolling road, ask them to advise you.
I have discovered over the years that generalised advice on SU set up for individual engines is very hit and miss, it may get you near but a rolling road will get it correct.
FWIW I run an AN in my very mildly tuned Ulster rep with an HS2.
It runs OK & would certainly do as a starting point.
I've never made a serious attempt to optimise it (there was always something else to do!) but by observation it's a little richer pottering at low rpm than on a high speed run.
(Do you know of an A7-friendly rolling road facility in the Cambridge area? I might be tempted some day...)
FWIW the last time I had to get a SU set up on a rolling road, with a SU specialist, he started with a weak needle and tuned it in by filing a flat along one side progressively working from idle. He got an ideal result and I never had any mixture problems thereafter. This was done after he had set up the piston to rise to near maximum at max depression. This can be done either by adjusting piston weight or spring strength if carb has one.
Burlen Fuel seem to list two types of AN jet Needles.
.090 and .100 for the same AN type.
Can anyone advise what the difference is?
And what is this dimension or reference number referring to... needle thickness maybe?
My SU reference manual makes no statement about this that I can find.
regards
Gary
Gary,
Having just had a look at my SU Reference Catalogue I think the measurements you are querying refer to the Jet sizes. There is in fact a third size - 0.125"
Today, 04:12 AM (This post was last modified: Today, 04:13 AM by GaryCullen.)
Thanks Neil,
So a needle with 0.090 is related to the main jet size of .090?
If so then I didn’t know this relation ship between needle and jet size existed!
and if I have an mis match then may explain the hesitation in accelerating unless the choke is out a bit to make it richer.
I will be checking…
Gary
My next question is: for an A7 running an HS2 what main jet size is used?
Gary
The attached book has useful information including the needle profiles. As a starting point the needle must be chosen from a range designed for the size of jet used e.g. 0.090 inch. Needle dimensions are critical to fractions of a thou especially around the idle end, so unless you are certain that the jet and needle have not worn against each other (they are not supposed to touch) I recommend you start with new ones.
For what it's worth, I run an SU H1 from a Morris 8 Series E, which is a 1 1/8 inch carb with a weighted piston. The chosen needle was GG, and road testing with a wideband oxygen sensor in the downpipe has shown that it is pretty well spot on for Air/Fuel ratio throughout the load and RPM range. The sensor and display are not cheap, but I use them on other cars. A threaded boss has to be welded on to the downpipe and can be fitted with a blanking plug when the sensor is not required. Thoroughly recommended if you are serious about tuning, and much superior to any other method I have tried. The readout is pretty well instantaneous whilst driving.
