Needles for SU carb conversion

[Jagboi]

I'm working on a 1972 V12 for a friend that has the 4 SU HIF44 carb conversion and I'm trying to tune it. I have a wideband AFR meter and if I set the carbs set to give a nice idle AFR, then it is very lean on the road at cruise speed.
Conversely, if I set the AFR to give a good AFR on the road (13.5:1), then it is massively rich (10:1 and black smoke) at idle.

Needles that came with the kit are BBG, which is what SU recommends. I had a set of needles from a 4.2 XJ6, which are BDX and that improved things considerably, although it's still a bit rich at idle.

Has anyone used other needles and can comment on their effect? I don't want to start ordering batches of needles to try and see what might work by trial and error.

I went through the table of SU needles and plotted the different curves, and I'm considering a BEP needles, as that has a leaner idle profile, but similar to the BDX in the rest of the load range.

[politeperson]

Just an idea, as I had a similar situation with a tuned 4.2 running SUs.

The carbs had been rebuilt, however the engine would not rev up easliy. It was fine at idle though.

I spent ages making adjustments to the timing and air screws etc. No difference.

The solution took a few minutes when I had figured it out. It was not getting enough fuel.

I took out the needles, laid them on a flat surface then filed a flat on each needle. About three strokes of a hand file on each one.

That increased the orifice just enough to increase the fueling and gave an engine that ran very well.

I bought a set of proper richer needles, but it did not run as well as the filed needles, so back in they went.

Who would have thought?

[christopher storey]

I am not surprised about your results. The AFR is supposed to be set at idle . An AFR of , say ,13.5 :1 will give an idle CO of perhaps 5%. The needles are so constructed that from that point up to a cruise setting , say 2500rpm at about a one-quarter throttle opening, a CO reading of perhaps 1% CO, (or possibly even less than that) will be obtained. What you seem to be doing is setting the cruise mixture at about 5 times what is needed, and thus it is no surprise that the mixture is horribly over rich

[Jagboi]

Perhaps I expressed myself unclearly. With all the other SU's I have on other Jaguars, the correct needle gives correct fueling at both idle and on the road. With the needles that are in the car now I can't achieve that. I can tune it to either have a good on the road behaviour with a very rich idle; or a good idle, but excessively lean while driving.

I should be able to have good driving and good idle, as it is now it's essentially undrivable somewhere in the load range. I know that SU's are to be tuned at idle, but when idle is correct AFR, it's undrivable on the road because it's so lean.

So I'm looking for suggestions on a needle that will give a correct amount of fuel across the entire load range.

[jagwit]

if I set the AFR to give a good AFR on the road (13.5:1), then it is massively rich (10:1 and black smoke) at idle.

I don't know what exactly "on the road" means but 12.5-13.5 is what you should have at full throttle conditions and off-idle.

For steady cruise, level road situations you should have 14.3-15.5.

[Jagboi]

I don't know what exactly "on the road" means but 12.5-13.5 is what you should have at full throttle conditions and off-idle.

For steady cruise, level road situations you should have 14.3-15.5.

By on the road I meant driving at a steady cruise. I agree with your target AFR's, that's what I'm trying to achieve.

With the needles SU supplied if I tune for an idle AFR of 12.5-13, then steady cruise AFR is 18 or more and it pops and bangs and backfires and struggles to make 2000rpm. If I tune to bring the cruise to the target values, then the idle AFR is 10 or less and produces black smoke. A single jet height setting will not give correct idle and cruise AFR's. Right now it's an either/or situation, I have to choose to have either idle or cruise massively wrong.

As I mentioned, changing needles to ones from a 4.2 XK engine made things a lot better, I was hoping that someone has been in this situation before me and a good recommendation on the needle to use that would bring me closer to ideal.

[christopher storey]

Perhaps if you tell us what the needles are, we may be able to give a better idea of what may be wrong. The letter code is etched on the shank about half way down the shank of the needle. Also, are they correctly mounted in the piston? The shoulder should be flush with the base of the piston . It occurs to me that whoever did the conversion may have mistakenly re-used the Stromberg needles !!

[Jagboi]

From post 1, needles are BBG. As supplied by SU in the carb conversion kit. Yes, they are flush with the bottom of the piston.

[Gfhug]

Have you asked SU/Burlen for their comments and suggestions?

Geoff

[abowie]

I assume you've seen http://www.mintylamb.co.uk/suneedle/

If you google susearch3.xls you can download that tool; also very good.

I have 4xHS8's on my XJS but I suspect the jetting will be totally different to the HIF44.

If you can't work out a stock needle, Julian's recommendation of filing the needles to improve your leanness at speed is a good one but can need you to go slowly, just a few strokes each time until you get what you want.

[Jagboi]

I have not asked Burlen yet. However, the kit was bought from Burlen a year ago and it appears they only provide one needle type.

[Jagboi]

I assume you've seen http://www.mintylamb.co.uk/suneedle/

I was using this one actually, I liked the chart better. https://www.classicminidiy.com/technical/needles

[abowie]

That's really good!

[christopher storey]

Going back to the needle problem, I see that both the alternatives you tried are marginally weaker at bottom to mid-range than BBG, but very much weaker at the top . I do wonder whether perhaps you have the wrong springs in. The correct one according to the SU complete catalogue is AUC 1167 which has a length of 2.75 inches under an 8oz load and should be painted yellow on the end coil. It may be worth trying the engine with the BBG needles but without springs to see if this makes any difference. The other points, which may be crucial, are

i.Are the float levels set correctly and ?

ii.Is the fuel pump giving excessive pressure ?

I should also emphasise that under no circumstances should you use anything near full throttle with the alternative needles in use, as being markedly weaker under heavy load they may cause piston failure

[Woolfi]

I am driving a EV12 with a 6,0L motor from an XJ81 and have 20 years of experince with carb-tuning and needle shaping. The car has now 4 originial EV12 Stromberg carbs and 4 taylor-made needles.
The correct AFR for idle is 12.5 to 14:1.
The correct AFR for light cruise can be AFR 15 - 17: 1. I am driving the He-Motor sometimes with 18:1 up to 19:1. Absolutely NO problems since 15 years. NEVER , never a piston can failure, if the mixture is very weak at cruising. IF the pedal is pressed to the flow when accelerating or nearby full throttle, the mixture for acceleration and high speed shall be at AFR 13 to AFR 14. If you drive with AFR 13,4:1, 10% of the gas leaves the motor unburned. But the motor needs the too much gas for inner cooling at high loads.
I say it again: I am driving my motor sometimes with plugs white as snow at part throttle. NEVER had any problems with heat problems. I have mounted a lambda-meter in the car and can see just-in-time , how the mixture of one bank is. At part throttle the total amount of heat is MUCH to small to kill a valve or a piston.
Back to your real problem:
1. You shall adjust the 4 needles at idle to a AFR of roundabout 13 to 14.
2. You have to grind some material off the 4 needles, that they will be thinner and therefor the mixture less weak in a CERTAIN area of the needle. I think you shall reduce the diameter of the 4 needles by 2/100mm in an area roundabout 6 – 12 mm above the piston. You start with 1/100mm, go up to 2/100mm and reduce again the 1/100.
3. Then you mount the needles and make a test drive, checking how the AFR is now. If the AFR-meter shows still a too weak mixture, you can continue to grind off material. Only in small steps. Otherwise you will ruin the needles and the mixture will become too fat. If your car is cruising with a weak mixture, the amount of bad smell inside a roadster will by low(er).

Grinding 4 needles, that the Lambda is correct in any load, is very difficult. You need some experience and a Lambda-meter to do this succesfully.
Avoid to grind the area of 3 – 5 mm above the piston. I have reduced one time 4 needles by 3/100 mm, between 3 and 7 mm above the piston. The colour of the plugs did a VERY big jump from very bright gray to daaaark velvet black. In this low area the mixture is very sensitive to a small different needle diameter. I had to throw these 4 needles in the garbage and start to grind 4 new needles to the shape, they had bevor my last grinding. I am writing all the numbers allways in a tabella, that I am able to see and comparte, what I am doing.

You need something to measure such small differences in the diameter of the needle.
Grinding is very easy. I have done it with sanding paper by hand. Lot of work and bad results. The needles will become never round.
Later I put the needles in a small accumulator drilling machine and put the sanding paper between two finger-tops round the needle. If the drilling machine is turning it makes a perfect round needle. I used in steps 400, 800 and 2000er paper for this. The last finish I did with a piece of 2000er paper, which I use since 20 years for polishing.

I told you my estimate how much you have to change the shape of your 4 needles. Maybe you can find and buy 4 needles, which are very similar to this.

If the mixture still is weak at cruising, maybe (?) you have to grind the needle also up to 15 mm above the piston or another 1/100 m betwwen 6 and 12 mm.
Needle shaping is a lot of trial and error, IF you wand a good result. Grinding away more off the needle, that the mixture at part throttle would be below AFR 14,7, will result in good drivability, but higher gas consumption and bad smell.
The goal will be allways a weak mixture at part throttle. Making the mixture fat, would be the easy way of ‚adjusting‘ the mixture of 4 carbs.

A bought needle will NEVER be as exact as 4 well himself ‚done’ needles. You need some intelligence, few ‚tools‘ and two days, to get a very good result.
You have to excuse my poor english.

Regards Wolfgang Gatza

[Jagboi]

i.Are the float levels set correctly and ?

ii.Is the fuel pump giving excessive pressure ?

I should also emphasise that under no circumstances should you use anything near full throttle with the alternative needles in use, as being markedly weaker under heavy load they may cause piston failure

Sorry for the delay, I have been away from the car for a while. Springs are correct with the yellow end. As I mentioned, this was the complete conversion bought new from Burlen, so everything should be correct as supplied!

Float levels set correctly. I have been through the carbs already as I quickly had a failure of the choke valve O rings, the material obviously not comparable with modern fuel (see photo). It cracked and crumbled and vastly over fueled the engine, as the fuel was freely flowing past the choke. That is corrected now with viton O rings.

Fuel pump is giving 2.5 psi, which is pretty much on spec for the SU pump I believe.

I have sent an email to Burlen asking about their recommendation for needles, so far no reply. I'll keep everyone updated when/if I get a response.

[Jagboi]

Wolfgang,

I have no trouble understanding you, and I certainly understand the needles need a different shape. I'm not confident in my abilities to get exactly the right amount of material removed, in the right place, 4 times for each needle. I'd feel much more comfortable buying needles that are close, even if they are not perfect. Close is good enough in this case.

I do have an AFR meter, so that does make the tuning easier, at least I can see the results of what the engine is doing.

[Woolfi]

"I'm not confident in my abilities to get exactly the right amount of material removed, in the right place, 4 times for each needle. I'd feel much more comfortable buying needles that are close, even if they are not perfect. Close is good enough in this case."

You are right. SU or Burlen offers different needles. I think there is a little DOS-programm for the computer, to compare the different needles of SU carbs. With this information and little calculating, you should be able, to chose a 'good' solution.

The only problem is to know, at which position the piston in the carb is, if you drive in the 'area' , where your mixture seems to be too weak. I also have never checked, how high the piston is, when I am driving with 50, 60 , 70 or 80 mph. This you must know, that you can identify the area of the needle, which is not correct.
For the first step I would change the shape of the existing for needles a little bit less thick. Start with a reduction of 2/100mm in this area, mount the needles, make a test drive and check the AFR-gauge. If you reduce 1/10mm this is toooooo much.
I have reduced the diameter of the needles between 4 und 8 mm high above the piston (low part throttle) with 3/100mm, because of poor running and a too weak mixtur. The colour of the spark electrode 'jumped' from bright grey to darker than deeeep black. The electrode looked like black velvet. I hade to throw the needles in the garbage, bought new shape top the previous dimension and rediced 1/100mm. This was enought for 'good' running. The mixture was still weak with AFR roundabout 17,5, but not too weak with less than 19:1. The 6,0L HE-motor is running well also with a weaker mixture, a pre-He would have problems.
If your needle-shaping would not be succesful, you can try with new needles. I would prefere 'taylor made'.
Most important is to shape very few material with care. Test in the car and shape again with care.
Excuse my poor english. I allways prefered books about motors and not books about english lessions in school.
Regards Wolfgang Gatza

[lowact]

One problem with lean mixtures is that the unburnt oxygen tends to combine with nitrogen to form nitrogen oxides, causing smog, acid rain and ozone depletion. Another problem is that there is nothing to be gained by doing this.
As example consider a 6 litre engine with AFR 15.5:1 ticking over at 2400 rpm, throttle opening 10% (say). At this operating condition a 5.3 L engine with AFR 13.7:1 would have the same power, performance and efficiency.
If the 6 L engine was tuned to have AFR 13.7:1, the same power, performance and efficiency would be achieved at a lower throttle opening; approx 8.8%
In all cases speed and fuel consumption would be the same.

[Jagboi]

To round this out, I never heard back from Burlen, but I spent much time analyzing needle profiles and decided on the BDW needle. The BDW is basically perfect, except that it was still rich at idle. Off idle beyond about 1300 rpm the mixture was exactly where I'd want it.

To compensate, I basically did the opposite of what Wolfgang suggested above. Instead of removing metal, I added it. I added about 0.002" of solder to the top of the needle only in the idle range, and only on one side. As it's a biased needle, I put the metal I added in the part where the needle doesn't touch the jet. It took a few tries with the soldering iron and a file to get the amount just right, but I was patient and got there in the end

The car runs much better now, the idle is clean and the AFR are slightly rich at cruise and it enriches under heavy throttle. The car behaves as it should now. It also takes much longer for the CO detector to go off in the shop now, whereas previously when the car was started it would go off after about 30 seconds of running. For comparison, a modern car can idle for 20 min inside the shop without the detector going off.