SU carbs WILL work perfectly on my modified L28 and give DCOEs a run for their money!!!

Skib

Forum User
As the title suggests the gauntlet is down!!!

Don’t get me wrong, I salivate like the next man at the mere sight of the DCOE carbs but this is something I want to crack!

This will be a long post / thread as it will be the musings of a madman refusing to give into conventional wisdom that “them SUs just won’t flow”.

There is a wealth of information out there about how to modify your L series engine, but very little on AFR, needles and carb set up for modified engines using SU carbs.


I made an account just for this thread.

It has been many years since I ran some SU's.
I'm actually pulling a built L28et to build an N/A L28 with SUs, to the confusion of many...
But the car doesn't need all the HP in the world and I want to use the engine as a test mule for parts R&D.

When I first came up building Datsun's and L motors I was fortunate to be local to Z Therapy, and in the same Datsun clubs.
Steve and Pam are awesome Datsun people with an incredible wealth of know how.

So hopefully be able to contribute to the collection of data sooner than later.
 

atomman

Club Member
So, no design aspects to this airbox to aid performance then, apart from cooler air?
I did quite a bit of CFD work on mine to aid flow and stuff , there's a bloke in Oz called Aaron that's making a new airbox for his ITB's that I've been talking with his started CFD work on his to
 

istoo

Well-Known Forum User
Hi bit of a hijack.
I have a fresh samuri engine build earlier this year (Stroked L24 is a 2.75 now, fruity cam, etc. Car was running on 2in HS8s from a jag with BBC needles in it. Was running lean on the RR, and uncovered it was running lean on the rolling round and concluded the fuel pump wasn’t delivering so I have a new pump and regulator to fit. Reckon the engine should be good for 200+ it made 185 with fuelling going from 3 bar at 4000 to 0 bar at 7000 on the RR.

I did take SM and KV needles to the RR session but was told they were the wrong type (floating or fixed)I can’t remember what is in there right now.

Looking for some advice on;
What needles would be a good start for this engine build?
And if they are floating/fixed can you fit fixed on a floating and vice versa?

It’s a minefield. I was hoping to get loads of use from the car this year but loads of things going on Ive barely had time to look at it.

Wisdom appreciated
 

Farmer42

Club Member
If you are still using HS8s you will find a needle chart on the Burlen website that will help with the right needles. For the 2inch carbs you will need the .0125 jet charts not the .0100 as you should have the larger jets to account for the larger air intake. If you had the smaller jets, that may account for why it was running lean.

You can change from floating to fixed needles but using an adapter collar which you can get from Burlen. I'm not sure if you can convert the other way though if the pistons are cast as fixed needles.
 

240L31

Club Member
I still feel the urge to get the very best out of the SUs. Did a small road tuning session tonight :

1. KW1 needles which are supposed to be similar to RH except a slightly leaner mid range

2. Red (stronger) piston springs

Results weren't too good though :

1. Cruise AFRs became really nice around 14.5 but top end it leaned out (no idea why, the needle profile should be no different compared to my current needles at top end)

2. Stronger piston springs: very little difference. I'd say the engine lost a bit of its responsiveness and runs a bit richer overall

All in all I'm starting to think I expect EFI performance from carbs (which isn't possible obviously). But still it's fun to tinker around a bit.
So another update: it seems like OA6 needles are really nice for a stroker engine. They are a bit leaner at idle (so you need to adjust the jet down a bit) and at cruise conditions. I prefer them over the RH needles.
 

240L31

Club Member
For those that are interested: Below my typical workflow how I select the proper SU needle. You need the SU needle search tool by Konrad Crist first. You select a reference needle (your starting point, typically the needle you're driving right now). You can then select needles to be compared.

Below my reference needle is the SM. As you can see, RH is richer everywhere, whereas OA6 is leaner at idle (same as SM actually) and a bit leaner at cruise conditions.

Don't be fooled by tiny diameter differences. You're looking for flow characteristics (open orifice) and typically you don't want a change of more than 10% open area per station compared to your reference.

44566231ug.jpg
 

AliK

Vehicle Dating Officer
Staff member
Club Member
So, play on this stopped a while back as life took over. I finally have the carbs / AFR in a really good place. So it would be fitting to mess it all up again and start tinkering (cue @Rob Gaskin to tell me to just drive the damn thing!! And he be right).

I have these AFRs:

Idle: 14-14.3

Overrun: 15-17 (nice pops and bangs at 16+)

Cruising varies between 14-15 depending on which part of the rev range you’re in, but importantly around the rich torque holes it’s above 13.7 (used to head into low 12s before.

WOT from 11.7 - 12.8 until about 5.8k rpm then heads into the low to mid 13s above that.

The two torque holes are less like holes and more like flat spots.

Once the diff is back on the car and the salt is washed off the roads I fully plan to polish the final 3 stations of the needles as the AFR heads into mid / high 13s with sustained WOT and would benefit from a tiny tinker to produce better power at the top. While conventional wisdom says the last two stations aren’t used, those who’ve done it for years tell me they still make a difference - which makes sense if you think about the mechanics of it.

Another tinker I always planned on with my spare set of SUs is the Vizzard modifications listed below. Not sure if anyone in the club has done them but the article below makes great reading!
IMG_4647.jpegIMG_4648.jpegIMG_4649.jpeg
 

AliK

Vehicle Dating Officer
Staff member
Club Member
Being a tad bored, I pulled out one of my spare carbs to satisfy my curiosity.

The Hitachi carb pistons at full lift don't block the air-flow all that much (unlike some actual SU carbs). Although they will be modified /curved just because and at least not a lot needs to come off!

IMG_4780.jpeg

but just look at the size of that butterfly shaft and the stickie outie bolts holding it.

IMG_4781.jpeg

Easy wins right there. Countersank Allen key bolts with quite a lot of the shaft removed would be a great start. Many sharp edges also can be improved (minus the bridge of course where you do want turbulent air flow).

I'm just putting this up for interest, but will at some point make a Vizard mod thread specifically on what I do / not do.

I was sold these carbs as absolutely in perfect working order but soon discovered the throttle shafts are seriously worn. They came off a race car - so I should have known better that perfect working order means constantly at WOT!! :D more fool me! Oh well, we live and learn. There is easily a 1mm gap between the shaft and the bearing / bush that it sits inside of! See the top of the shaft!
IMG_4785.jpeg


If you’re wondering whether you have a false air source / vac leak from your shafts, just take the carbs off and compare to the photos below.

The light / clean marks are where the air is coming in when at idle or part throttle.

Open side of the shaft.
IMG_4788.jpeg

Linkage side
IMG_4790.jpeg

And for validation, if you look at the bottom of the carb body / butterfly, you will see where the idle air comes in past the butterfly, producing a similar effect! This one is meant to look like that!

IMG_4789.jpeg
 

MCBladeRun

Club Member
Vizard mods are about helping the airflow through the carb? That's interesting about the screws, you could probably help lower the turbulence via that I guess?

I might change the screws on mine but, anything else would probably require more mods elsewhere I'd imagine? Possibly the needle profile for finer tuning?
 

AliK

Vehicle Dating Officer
Staff member
Club Member
Vizard mods are about helping the airflow through the carb? That's interesting about the screws, you could probably help lower the turbulence via that I guess?

I might change the screws on mine but, anything else would probably require more mods elsewhere I'd imagine? Possibly the needle profile for finer tuning?
Yes, better flow, less restrictive and reduced turbulence where it reduces flow.

More air = more fuel required - but whether the constant depression function of the SU carb automatically takes care of it, or if you need to richen up is to be seen!.
 

AliK

Vehicle Dating Officer
Staff member
Club Member
So gang, today I received an excellent response from the chap at Birmingham carbs. As mentioned by someone else (I can’t recall who) - he’s getting set up to make proper bushes and throttle shafts for our carbs! His prices are very reasonable IMHO and I suspect he will be a great UK alternative to ZTherapy for us.

this is looking very promising!
 
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Rushingphil

Club Member
So gang, today I received an excellent response from the chap at Birmingham carbs. As mentioned by someone else (I can’t recall who) - he’s getting set up to make proper bushes and throttle shafts for our carbs! His prices are very reasonable IMHO and I suspect he will be a great UK alternative to ZTherapy for us.

this is looking very promising!

Was Fairlineguy (Neil) :)
 

s2k_adz

Club Member
Interesting read! I've got a set of 2" SUs in the garage unfitted. A friend of mine had mentioned theyre easily as good as the Webber 40s but in the end I found a good set of Webbers so theyre on it. Always wondered what the 2" ones would do...

I also think its time I put a wideband in my car to help with tuning.
 
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Rob Gaskin

Treasurer
Staff member
Site Administrator
My 2" SUs worked well. I suspect the big gain is at high revs with a hot cam, gas-flowed head and exhaust. The inlet manifold needs to be matched to the carbs of course.
 

Paul Atkinson

Club Member
So gang, today I received an excellent response from the chap at Birmingham carbs. As mentioned by someone else (I can’t recall who) - he’s getting set up to make proper bushes and throttle shafts for our carbs! His prices are very reasonable IMHO and I suspect he will be a great UK alternative to ZTherapy for us.

this is looking very promising!
I have back from Birmingham Carburetors my Hitachi carbs with new brass shafts and bronze bushes. All looks really nice. With rebuilt engine will be going back into car in 3-4 weeks time. Come and talk to me at Donnington to ask how they are working.
 

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