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| Re: Twin Su tuning |
Subject: Re: Twin Su tuning
by L18_B110 on 2013/1/22 22:16:55
damper oil doesn't matter really. Some guys use light machine oil, some use ATF, some use engine oil. I tried all of them and none of it makes any noticeable difference.
the stock springs wil be fine. I used standard springs even with a decent cam and other engine mods in both A series and L series engines. Don't mess with the spring tension.
You need to go back to the basics and make sure everything is setup right because in my experience with SUs, they're generally not. A Datsun L16 SSS workshop manual is your best friend here. It will tell you step by step how to check everything. I've probably forgotten a few things because its been way over a decade since I've messed around with SUs, but here goes:
-firstly everything inside the carb should be clean. the pistons and vacuum chambers often get gummed up from oil overflow. you will need to clean them regularly. I did it at oil change intervals.
-the needle's shoulder should be flush with the base of the piston.
-the choke needs to be centralised
-starting point for the choke mixture setting is 2.5 turns down
-its very important that the carbs are balanced. you can buy carb balancers and its really the only way to do it other than on a bench with feeler gauges.
-idle mixture can be set at a fast idle (about 1500rpm) by raising the piston a few mm. There will either be a little button to press under the body of the carb to lift the piston for that purpose, or a hole there to inset a bit of wire for the same reason. Failing that, you can do it with a screwdriver down the throat of the carb. Raising the piston a few mm should slightly increase the revs if the mixture is right. If revs drop its too lean, if the engine races its too rich.
-once that's set if it is too lean in the mid or top end, you need to start looking at alternative needles. Genuine SU needles fit. Or modify your own needles if you know what you are doing.
-obviously the floats and needle and seats need to be in good nick too and the float level set correctly.
-it's also important to keep the fuel bowls cool. heat shields and thermal wrap are pretty much mandatory to stop vaporisation.
don't rule out your ignition side for this problem you're having either.
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