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

Farmer42

Club Member
There is a needle comparison chart on the Burlen website that gives you jet size against the relevant fuelling and performance for each compatible needle.

There are various tabs within it and you can manipulate or fill in the chart in the basic tab. F stands for fixed needle which is HS6 or Hitachi's, s is for sprung and Z is for zenith/ Stromberg. The number after it is the jet diameter e.g. F100 is fixed needle at 0.100 inch. Most 1 3/4 inch carbs run F100 nozzles. The number at each index point shows the diameter of the needle at that point. The bigger the diameter the leaner it will run at certain revs.

RH needles on these jets will run very lean top end which will be compounded the bigger engine size you have. I would recommend looking at DX or MC needles. If you run HS6 or HS8(2 Inch) you could use F125 jets with compatible needles such as UA that would give you more fuelling at higher revs which would be needed on a bigger engine.

I'm running HS6 carbs with F100 jets and DX needles and it runs great with good torque & performance on a stock E88 head and exhaust setup. I have a L26 engine.
 

240L31

Club Member
I used this chart when I selected my needles. RH needles do work very well, mixture is spot on no matter if full load or cruising. You can check the lambda trace in my dyno chart.

I guess the question right now would be if the carburetor diameter is limiting power and if so, how much.
 

Farmer42

Club Member
I did think the dyno charts were impressive and i could only dream of the BHP and Torque that is shown but I did notice the drop off at higher revs even though the mixture is stable. I think that the RH needles are limiting top end fuelling and you may be better with DX or even possibly RG. Have a look at the needle chart to compare them although I suspect you probably already have!. The only downside is that they are quite lean at low revs and do take some choke to start from cold. The RG needles, although smoother acceleration through the range and better at higher revs, were a bit too problematic at startup for me which is why I stuck with DX.

Given that you have a 3.1L, you may also be better off with HS8, 2" SUs running 0.125 jets a bit like they run on the big Jags.
 

Farmer42

Club Member
The lambda chart does show a stable mixture but I don't know if it is showing whether it is lean or not. I am not an expert in those things. However, I think it would show a stable reading for that needle as the carb has been tuned for that. I guess my point is that if you have a needle with a higher fuel flow throughout the range, you can tune the carb to increase the airflow to compensate for it thus keeping the mixture correct but because you have more air and fuel entering the engine, the power should increase. It takes a bit of fettling to get it set up but I did notice the difference on mine even with the mixtures the same.
 

toopy

Club Member
The only downside is that they are quite lean at low revs and do take some choke to start from cold. The RG needles, although smoother acceleration through the range and better at higher revs, were a bit too problematic at startup for me which is why I stuck with DX.
You could of rubbed/filed them down a little at the idle position and/or higher.
 

240L31

Club Member
The mixture is stable at around 0.9 which is perfectly fine. If the engine could breathe more air, the mixture would run lean at high rpm. I guess I was a bit unclear, my question is not whether power is limited by the size of the carburetor but how much :D
 

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.




IMG_20220618_202558.jpg
 

240L31

Club Member
This is a common misunderstanding about how SUs work. With thicker oil, the piston doesn't move up quick enough, resulting in a very high vacuum. This high vacuum pulls much more fuel out of the nozzle during acceleration.
 

240L31

Club Member
I just can't accept that I'm still slightly rich during acceleration. So I bought a couple of different SU HS6 dampers to understand the carbs better.

It turns out that the piston damper rods are completely different:

1. Clearance (amount of dampening)
2. Free length before orifice closes and oil flow gets slowed down (starting of enrichment)

With these parameters in mind, plus experimenting with different oils (went from atf to really heavy gear box oil) I'm almost certain that I will finally get the AFRs spot on. I'm aiming for 0.88, right now I'm more at 0.8-0.83 during acceleration.

I'm doing this just for fun. Without an AFR Gauge there would be absolutely no reason to change anything at all (runs great).




IMG_20220727_200140.jpg
 

240L31

Club Member
So I went for a test drive with the newly fitted airbox and different piston dampers.

1. The dampers didn't make any difference in acceleration AFRs. I don't really understand why, the piston is so much easier to lift compared to the stock damper

2. The engine really picks up a second time from 5000-6000. Before there was a drop in torque. So this could be either the air box or the stacks (I guess the latter)348167.jpg347006.jpg
 

Rob Gaskin

Treasurer
Staff member
Site Administrator
So I went for a test drive with the newly fitted airbox and different piston dampers.

1. The dampers didn't make any difference in acceleration AFRs. I don't really understand why, the piston is so much easier to lift compared to the stock damper

2. The engine really picks up a second time from 5000-6000. Before there was a drop in torque. So this could be either the air box or the stacks (I guess the latter)View attachment 52294View attachment 52295
Why the stacks? That airbox design didn't just happen it was finalised after much experimentation and dyno testing.
 

Ian Patmore

Well-Known Forum User
I presume the airbox pictured was designed with itbs, but with carbs they work much better with a trumpet of some sort, even Nissan as stock had air trumpets fitted into their OEM airboxes. Probably why that airbox lost a small amount of power compared to no airbox (I think) when run with triple carbs on a dyno, as designed for itbs.
 

jonbills

Membership Secretary
Site Administrator
The airbox is designed to run with trumpets, not instead of.
Mine gained power with triple carbs with the airbox.
 

Ian Patmore

Well-Known Forum User
Glad that's cleared up, I read what Rob G wrote as it was designed to work without.
Any idea what aspect of the design made the gain, as I think there is room for improvement (for triples/SU's anyway)?
 

jonbills

Membership Secretary
Site Administrator
Glad that's cleared up, I read what Rob G wrote as it was designed to work without.
Any idea what aspect of the design made the gain, as I think there is room for improvement (for triples/SU's anyway)?
I remember reading something standing waves, harmonics and reflections off the inside of the airbox vs inner wing. And of course, its cooler air.
 
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