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Twin Su tuning
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Hi guys..its been a while since a updated my progress on those twin webers..i had loads of trouble with my webers,so i traded them for twin su's with cash differance afcourse..now my bakkie is running loads better with way better fuel consumption and response..now i'm strugling to get them tuned just right..its begins to splatter at about 4500rpm and when i go for second gear it pulls dead and splaters alot and sound like only 2 cylinders are igniting..plz if i can just get some advise from you guys on what damper oil to put in,damper oil level,spring lenght and tension,needle sizing and how to identify needle sizing,and air-fuel adjustment..or just any advice on what may be causing the problem and overal tuning steps to follow..plz help me!!

Posted on: 2013/1/22 21:31
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Re: Twin Su tuning
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Damper oil: 20W or 10W30 is what Datsun recommended.

Posted on: 2013/1/22 21:37
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Re: Twin Su tuning
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This all depends on which engine you are fitting the S.U. carburetors.

Are they genuine Hitachi sidedrafts, or SU sidedrafts? Got a photo?

Quote:
spring lenght and tension,needle sizing and how to identify needle sizing,and air-fuel adjustment

Posted on: 2013/1/22 21:40
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Re: Twin Su tuning
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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.

Posted on: 2013/1/22 22:16
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Re: Twin Su tuning
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Posted on: 2013/1/23 8:05
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Re: Twin Su tuning
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thanx for the replies so far..really apreciate it..was just wondering what tool should i use on measuring the vacume and where can i buy one?

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Posted on: 2013/1/23 13:35
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Re: Twin Su tuning
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Those are Hitachis rather than SUs. They only use one of two springs, so it's much easier than an SU. The lighter spring is used on A-series. L-series engines used the heavier spring.

You do not need a tool to meausure vacuum, you can stick a piece of hose into each carburetor and listen to the sound. Adjust the balance screw until the sound is even.

If that is a stock A14, the stock A12GX carbs should run well on it.

To identify the needle, remove it by taking out the set-screw. The number is stamped on the thick end. M83 was the original GX needle
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Posted on: 2013/1/23 20:47
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Re: Twin Su tuning
No life (a.k.a. DattoMaster)
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Hitachi's are a direct copy of the SU, to the point where the floats and needles are interchangeable, and probably other bits, but that's all I've used in mine.

the tool you need to measure the vacuum and balance the carbs is a carb balancer or synchroniser. There's a few different types. Its going to be much quicker and better to see a visual gauge of the vacuum than trying to judge it by ear. You used to see them in any auto parts shop over here years ago, but carbs are a bit old fashioned these days. No idea about where to get them where you live, but would be very surprised if you can't find them on ebay.

Posted on: 2013/1/24 0:18
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Re: Twin Su tuning
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Posted on: 2013/1/24 0:25
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