got my engine going today and trying to get the carbs somewhere near right,
my wideband says on tickover its hovering around 12.5 -13, cruising its around 15.5-16 and ramping it it goes to 17.5ish
i dont know much about this but 17s ramping it seems very lean, i read it should go into the 12s.
also do those other numbers look ok
tuning my su carbs
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Hi
on some (I think most) needles you can fix the needle into the carrier in the bottom of the piston in one of three positions, normally fitted into the middle one, on a good condition needle it gives you an overall richening/leaning of the mixture by about 1.5 AFR, it is so you don't have the jet too far in or out of it's hole in the bottom of the venturi.
With the numbers you are getting I would adjust the main jet mixture so you are getting a mixture at idle around 15:1. raise the idle on the idle screws to about 2000 revs, it will probably not want to do this without a bit of help by richening the mixture on the main jet, if you have to do this just enough to get it to run and note where the needle is relative to the jet Now take some fine emery (1000 grit) and slim down the needle in this area and see what the mixture does at this fast idle, when you have a reading about 13.5/14:1 (with the main jet adjusted back to where it started) then take it for a run and see what the AFRs say for other driving conditions, come back and slim the needle down further away from where it mounts into the piston, don't give it wide open throttle until you are at least around 14:1, for WOT you don't have to be subtle with needle just take a needle file and adjust the needle by cutting a flat on the side of the last 1/4 to 3/8 of an inch. When you get it running with the mixture across the range of about 14:1 use the main jet adjustment and needle position go put the whole mixture at about 13.5 to 14:1 and file the very end so you get 12.5 to 13.5:1 at WOT at 4500 revs ish, your measurements need to be done with the engine steady state, or as steady as you can, defiantly with the throttle in one position so look for long hills and open motorway when there is no traffic.
Now take a micrometer and measure you new fancy custom needles, find an SU needle catalog and find the best match, you will have for calculate the total area of the needle for the last bit you filed flats on and work out the equivalent diameter for that bit.
This all assumes you have the right spring and correct piston lift, check it before you start and get that sorted first.
finally by the sound of things your dashpot in the top of the carb is worn, this is common, you need to put thicker oil in it until it goes a little rich when you hoof it.
Best regards
Mike
on some (I think most) needles you can fix the needle into the carrier in the bottom of the piston in one of three positions, normally fitted into the middle one, on a good condition needle it gives you an overall richening/leaning of the mixture by about 1.5 AFR, it is so you don't have the jet too far in or out of it's hole in the bottom of the venturi.
With the numbers you are getting I would adjust the main jet mixture so you are getting a mixture at idle around 15:1. raise the idle on the idle screws to about 2000 revs, it will probably not want to do this without a bit of help by richening the mixture on the main jet, if you have to do this just enough to get it to run and note where the needle is relative to the jet Now take some fine emery (1000 grit) and slim down the needle in this area and see what the mixture does at this fast idle, when you have a reading about 13.5/14:1 (with the main jet adjusted back to where it started) then take it for a run and see what the AFRs say for other driving conditions, come back and slim the needle down further away from where it mounts into the piston, don't give it wide open throttle until you are at least around 14:1, for WOT you don't have to be subtle with needle just take a needle file and adjust the needle by cutting a flat on the side of the last 1/4 to 3/8 of an inch. When you get it running with the mixture across the range of about 14:1 use the main jet adjustment and needle position go put the whole mixture at about 13.5 to 14:1 and file the very end so you get 12.5 to 13.5:1 at WOT at 4500 revs ish, your measurements need to be done with the engine steady state, or as steady as you can, defiantly with the throttle in one position so look for long hills and open motorway when there is no traffic.
Now take a micrometer and measure you new fancy custom needles, find an SU needle catalog and find the best match, you will have for calculate the total area of the needle for the last bit you filed flats on and work out the equivalent diameter for that bit.
This all assumes you have the right spring and correct piston lift, check it before you start and get that sorted first.
finally by the sound of things your dashpot in the top of the carb is worn, this is common, you need to put thicker oil in it until it goes a little rich when you hoof it.
Best regards
Mike
poppet valves rule!
