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S U Carbs?


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So I own a stock L20b that some day I may rebuild and turn up the power a bit. I'm looking to fit it with SU carbs. I don't really know a lot about them as far as which would be the best to use. Would it be wise to try and find a set off of and older Z car? If so, what sort of info do I need to make a good choice? Thanks.

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I'd throw a set of 38mm dual hitachis on; they came on the SSS L16 engine. Even though they weren't offered as an option on USDM Datsuns, plenty of them made it over attached to the hordes of used engines we imported.


The larger 46mm(from a Z) ones may be too much, depending on the purpose of your L20b. I have those on my LZ23, and even then, they seem like they're a little big.

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Depending on how you 'build' the L20B....you might get away with running some 46s.

That said....you would want to purchase a wide band which would enable you to re-shape (profile) a set of fat needles until a/f ratios were conducive at idle, cruise and WOT

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i agree with hainz. go for a weber 32/36 you can pick them up pretty cheap and its a bolt on. the problem with su's are they have all been ran for a loooong time and most of them are worn out. the big problem is the throttle shafts wear, causing vacuum leaks. after 40 ish years of opening and closing the throttle it wears the hole into an oval letting a tiny amount of air past and you will never be able to tune them properly. this can be fixed but its not cheap, or easy enough to do in your garage for most people. other than being bigger the 46mm z car su's open a different direction than the 38mm su's so you will have to rig up a throttle cable. thats pretty easy, but the linkage that connects the 2 carbs i had to cut and weld back together for it to work. now on to the intake. l20's have the intake and exhaust mated together so if you change the intake to fit su manifold you will have to change or modify the exhaust manifold too. a webber bolts right to the stock manifold. all of this can be done but it takes some work. half the price (or less) and you have a brand new webber thats a direct bolt on in an afternoon.

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Should be a given to 'service' any SU prior to install.


Cost me a hundred+ bucks to have the 40 year old 46mm throttle shafts 're-bushed'...fifty bucks a pop plus shipping and taxes.

Any reputable machine shop should be able to handle the task.


Initially I had a new 38/38 on my LZ23 but then decided to try some side drafts....definitely an improvement







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well hello pat and welcome! yes they do. i prefer atf. i went to the local drug store and i got a plastic syringe that i can fit a small piece of tube on. i use that to suck the old oil out of my carbs and then i add 3cc of atf. here is some more reading. now lets see a pic of your ride!




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Okay, let's say I change direction and decide to go with a set of Webers. I've done some reading and it's looks like DGES 38/38's would work fine, but could I also use a set of dual DCOE 40's? or would that be too much for a stock L20b?


Thanks for all the great info.

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Bigger side drafts like the Webers may not necessarily be too much for the L20, the bigger issue is that they won't be as friendly for street use. Basically you're going to be making a lot of power up high in the rpm range (where the carbs can just dump a ton of fuel) but you'll be struggling and bogging down low.


People will say that it's all about the way you tune the carbs, but I think you'll still be better off running a set of SUs on a street motor. As for possible power loss from the SUs: I have a set of 38mm SSS SUs on my 2.1L motor. It's also making close to 10.5:1 compression with at least a .520 lift cam. Big motor, but I really like the SUs on it for street driving. 


You can upgrade the SUs like I have to make them dump more fuel, ZTherapy sells bigger nozzles and higher performance needles on their website that you can simply replace the stock 38mm parts with. That's what I've done with mine. You lose power at about 6.5k rpm, but how often are you really at 6.5k+ on the street?

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Thanks for the info. You may think that I'm flip flopping, really not sure the direction I want to go. I've talked to the guys at Ztherapy in the past. They have been very helpful. Not sure if I want to part with a grand for carbs, so I will continue to ask what most may think are stupid questions. Getting back to using weber side drafts. Would a single 40DCOE be too much carb for a stock L20b? Thanks again.

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I believe that kit does not come with throttle shaft bushings..............................................

Which should be a given to replace since most are 40 years old


This is true, and the thing that I am currently struggling with on my SUs. I feel a little robbed on the top end with my big motor and I think it's probably those throttle shaft bushings leaking. 

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Cost me 100 bucks to have 2 bushings installed in my 46s....and that was the only thing wrong with them.

Actually,.....no.....one of the throttle plates was NFG due to years of excess slop.

I wouldn't buy a kit until the SUs are 'inspected'................ and I never used one myself.


Your top end shouldn't be affected.... but you could be running lean due to an unmetered leak from the worn throttle shaft and or?

After the throttle shafts are addressed, you might have to profile the needles to obtain the best A/F numbers at idle, cruise and WOT


That's why a wide band is somewhat of a must when fine tuning because most of the time SUs are installed on something other than what they were initially designed for.


My WOT is around 12.4....and that is good

Edited by Sealik
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Part of my problem is that I don't think they really leak that much until the carbs get warm, and my engine is almost over radiated so I know it takes them ages to actually get warm. I did borrow a wide band when I originally put the motor in and tuned the carbs for the first time, been using that tune as a baseline ever since. 


I'm planning on installing an in car wideband (there's a company that does a really nice little gauge that just gathers info from your exhaust bung like a normal wideband) that way I can really find out where my issues are while i'm driving. 


Replacing those bushings has already been on my list for a while, just need to collect some money and wait until I can convince myself to put the car down for a while to send them over to ZTherapy. 

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Change your thermostat if the engine is not coming to operating temp quickly...180 for summer

If your throttle shafts are compromised....they are leaking weather the engine is hot or cold.

Can you move the throttle shafts up and down???   Carefully spray the bases etc. of the SUs with some WD40, idle should increase if they are leaking.

Lateral movement is normal.


Typically if they are leaking one will have issues tuning the carbs.......<..... idle is mostly affected.

People then adjust/richen the carbs to compensate which in turn affects other stages of the needle.


325 bucks for new bushings from ZT?

Crazy.......find a machinist if need be.

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