l20b Carburetor

[green9333]

I'm building an L20B motor for my 521 truck. Bigger pistons, Modified cam, A87 peanut head with bigger valves and stiffer springs. The motor came with some Hitachi HJL38W flat top dull side draft carbs. I have received different opinions so I'm looking for help. These are not the same carbs that were on the 260 Z that everyone says were junk. these were for the 1600 and 1800 motors and were suppose to be competition carbs. I guess my question is do I rebuild these or do I look for something better for my L20B. I don't want to be under carbureted. any suggestions would be great.

 

Thanks Dave!

[datzenmike]

They are not competition but were offered as the carburetors only on the Japanese Market 510 Coupe. They are round (early)or flattop SUs but with only 2 mounting bolts and 38mm diameter. All Z car were 4 bolt and 45mm or 46mm diameter openings. I have a set of flattop and these are supposed to be from the latter L16SSS and the L18SSS.

 

As the stock L20B carburetor is 30/34mm the twin 38mm has to be better for flow.

 

Another advantage of the side draft twin SU is the air flow is basically a straight line to the combustion chamber where the stock down draft has to make a 90 degree bend.

 

SUs are constant velocity carburetors in that they maintain a constant speed of air flow through the venturi by raising and lowering a slide. The carburetor does not automatically open fully when the throttle is floored but opens only as needed as the engine revs up so they are impossible to bog. Thus they do not need an accelerator pump. 

 

 

 

[green9333]

datzenmike, you always have detail info. Thanks!

So these 38mm SU's should be good? still trying to find a rebuild kit. What do you think about going fuel injection?

 

trying to attached a photo like you did but cant seem to figure that out as well. 😭

[green9333]

https://imgur.com/37Vx3C0

[datzenmike]

I think you'll like SUs better than a Hitachi downdraft. EFI? if you have individual throttle bodies sure, but expensive and complicated. They do run very well, better cold starting, mileage and idle. 

[Crashtd420]

Jenvey makes some nice throttle bodies and even ones that look just like weber side drafts....

 

https://www.jenvey.co.uk/

 

It does start to get pricey since you need an intake, a management system , and fuel system mods....

 

Another option is using a downdraft sniper efi unit.. they are around 1000 or so... they only thing is you need to fabricate is an adapter plate but it is doable.... I'm not sure on hood clearance with the L20b in a 521 though...

[datzenmike]

There is one more option and that's motorcycle carburetors. Same idea as SUs but one for each cylinder.

 

 

These are R-1 carburetors. If good enough for a 1 liter engine at 14,000 it should (and does) be good enough for a 2 liter engine at 7,000. 

 

Hardest part was making my own intake. Cost was less than $20 but I spent more like $300 learning what not to try.

 

 

Used an aluminum tray for a heat shield, probably not needed.

[Stoffregen Motorsports]

Accelerator pumps make cold starts a lot easier. One trick with SUs is to crack the throttle while cranking. This lifts the well jet and allows fuel to flow up and into the air stream.

 

I personally don't love SUs, and I feel that a properly modified DGV can make more power, but SUs can be modified too.

 

I've never been a big fan of the motorcycle carbs, but I also have never tried them. They look a bit goofy on an L motor, but I bet they are easy to tune and probably have huge hp potential.

[banzai510(hainz)]

Maybe do a 38/38 Weber DGEV they simple carbs cheap. Cam better be above 470 lift otherwise I think your waisint time money on valve train parts.  precision shim out of australia has lash pads that fit stock retainers so you only need valve spring however rocker arms are now expensive for new ones

best is to buy used dual mikunis  w manifold at steal of a price

Edited November 30, 2021 by banzai510(hainz)

[Stoffregen Motorsports]

10 minutes ago, banzai510(hainz) said:

 rocker arms are now expensive for new ones

best is to buy used dual mikunis  w manifold at steal of a price

Factory rocker arms are usually good to go, and can even be resurfaced if need be.

 

Where is this elusive supply of cheap Mikuni carbs? I'd love to know.

[banzai510(hainz)]

I had 2 Japan 44 sets sold $600 with Nissan manifold(one was brand new and other slightly used) and cable and a Italy Solex set( unknow condition I sold for 150

sold  sets 3/4years ago 

 

you can find them when you least looking.

 

got another New looking set which was 250$ on the self no manifold but is set up for A motor center pull set up

Edited November 30, 2021 by banzai510(hainz)

[Tucson620]

6 hours ago, Crashtd420 said:

I'm not sure on hood clearance with the L20b in a 521 though...

Was gonna mention this - tight clearances between air cleaner and hood with a down draft carb. (*tight = paint exchange)

[Crashtd420]

4 minutes ago, Tucson620 said:

Was gonna mention this - tight clearances between air cleaner and hood with a down draft carb. (*tight = paint exchange)

That's what I have heard,  I believe the sniper does fit but I cant remember if the guy had the l20b or L16 that use it... I'm thinking L16...

It was a 510 also.....

If you think you might try it there are specs online , you should be able to get an estimate if there is enough room...

[banzai510(hainz)]

weber sells a low air cleaner. the tall standard one will hit the hood

 

https://www.ebay.com/itm/112869200384?epid=658251317&hash=item1a47875200:g:pY8AAOSw3FFgEFxl

Edited November 30, 2021 by banzai510(hainz)

[datzenmike]

1 hour ago, Tucson620 said:

Was gonna mention this - tight clearances between air cleaner and hood with a down draft carb. (*tight = paint exchange)

 

Had L20B stock carb in my '71 521 and no problems. The L20B is only 3/4" taller than the L16/18

[banzai510(hainz)]

the weber adapter raises this up also that why it hits. esp if one has the taller 1pc carb adapter then add the taller Aircleaner

[green9333]

good stuff guy's, Thanks for the info!

I think I'm going to spend the money and buy a new webber dual side draft 40's setup. expensive but I had to skimp on my already over spent l20 build.

[datzenmike]

Never seen down draft or side draft Webers that the owners didn't have to mess with the jets and 'tuning' where the original stock carburetors work all the time. No one has ever exchanged stock jets and air bleeds. Why was that? Is it because the Weber simply can be tuned or messed with?

 

If this turns out to be the case you might think about a wide band air/fuel gauge rather than 'reading' plugs and the ass dyno.

[Stoffregen Motorsports]

The theory below, as I understand it, of how carburetor air volume (or CFM) affects the jetting on a particular engine is probably not wholly accurate, but I think it gets the point across.

 

Tuning on a small single carburetor seems to be less important, probably because of the strong signal produced by all the air rushing through a small orifice. When you have the same engine pulling through a much larger volume area (multiple carbs), the signal produced is divided and becomes less strong. Since the signal of air is what pulls the fuel out of the jet, you may need to force feed fuel to the engine and jetting becomes more important.

 

So yeah, I wouldn't bother bolting a set of dual carbs to a stock engine without doing some modifying. Larger cam, mild porting and maybe big valves would be the least I would do to take advantage of the dual side draft carbs.