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SU Symptoms on L16


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i picked up a complete pair used su's at the Eagle Rock Swapmeet. seller told me there in good condition and running when removed but been sitting for a while. gave them a quick clean and slapped them on my 510. to my suprise they started up easy and idled pretty good. after the warm up i notice a slight flat spot when hitting the throttle but feels nice right after. i checked for Damper oil but was empty, so i bought some 3 to 1 to fill. now the flat spot is more noticible. didn't have much time to play with them since and been trying to do some reading to get to know them better. also order a Unyson for tunning.


so anybody have some quick pointers or advice as i plan to play with them this weekend. thanks


*P.S. almost forgot, i have a stock back plate with air horns that i haven't installed yet but plan too*

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I use power steering fluid it mine, I don't know what 3 to 1 is.

If you stomp on them, they are not going to react like a downdraft carb, as they have no accelerator pump, so you don't get the immediate response, but I will take SUs over a downdraft every time.

Make sure you use the back plate and heat shield.

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3 to 1 is like a penetrating oil. It just seemed like it responded better dry. It has that flat spot when you give it gas off idle, doesn't even have to be hard even with a slight tap you can hear and feel it. Am going to remove the oil and try different ones and see what happens.


Also I do have the heat shield and the car sounds and feels a lot better with the SU's once past the flat spot.

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It's '3 IN 1' oil and it's not like penetrating oil. It also used to be known as sewing machine oil. It was common in the '50s and '60s as a house hold lubricant for hinges and whatever squeaks.



I don't know how 'thick' it is but the thicker the oil used in the dampers the slower the SU piston will rise. Too sudden a rise and the engine will bog from too much air too soon. (lean) There is no accelerator pumps on the SU so the mixture is made richer by slightly delaying the opening of the piston .


Maybe ditch the 3 in 1 for ATF? Or try different viscosity oils.

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"Maybe ditch the 3 in 1 for ATF? Or try different viscosity oils."


One caution. I remember from an old tech note that you should not use multi viscosity oil, but a single weight.  Can't remember the antiquity of that note, but I have followed it for all too many years.


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Sounds like your lean off idle. Try a thicker oil. Atf (what I use) is about 10w. 3 in 1 is about 5w. Factory su spec is 20w fwi ;) Also have you properly synced the carbs? Everything needs to be the same between the two carbs down to the float level adjustment and the needle position. Make sure that the pistons move freely and fall with a "clunk" at the same rate.

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You might want to confirm that they are no vacuum leaks from the throttle shafts.

Other than replacing the bushings...there are some 'redneck remedies'...






Unisyn.....<......one could use a short piece of garden hose attached to a small funnel.

Place the end of the hose near....in the exact spot relative to each SU

Funnel on ear....listen....works rather well. Doesn't take much idle screw adjustment to change the 'pitch'


Also...I would not install the horns until you receive the Unisyn.....might not fit/seal on the horn 

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Been adjusting SU's for a lot of year's[real one's]. Agree that everything between the 2 carbs has to match. Start with the throttle disc's. Make sure they open exactly the same. Next make sure the jets are screwed down about the same then check that the fuel level in the float chambers is the same. You can do this by removing the dampner bell but be very careful the spring loaded dampner piston doesn't fall out and bend the needle. If it does you are SOL. Very fragile. When you  have them off you should be able to see the fuel in the jet and it should be down about 3/32 or so. Make sure to get the dampner bell's back on exactly the way they came off. I scribe them and mark front and back so I can get them back exactly. 

Pretty simple design. The jet needle is raised by vacuum so that it gives the engine what it wants at the right time. If you want to change the mixture you screw the jet up to lean it out or down for the opposite. If it needs more at some point in the throttle opening you can change needle profiles. Agree that the oil in the dampner piston adjust's the rate of change and heavier oil ought to help out on a off idle flat spot but you may need to richen slightly. First get everything balanced.


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To get the throttle disc's right you want to disable the idle screw's and only adjust the linkage. I screw the idle screw all the way loose then open the throttle disc's enough to slide a small wire under one then I check to see that the other is the same. A unisyn or the listening method would do the same with the engine running but I do this with the engine cold. I'm a little deaf and on some engines the listening method just doesn't work anyway.

You can, of course, check float fuel level by removing the float covers and checking the float height rather than pulling the dampner bell's.

As said, once you get on to how they work you will realize they are about the simplest, easiest to tune carb out there.



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