I found the table in Des Hammill “How to tune Jaguar XK engines” and that’s precisely what it means. The SU are so nice and simple to understand, just great (I just hope I never get ‘stuck’ again).
Once the piston is steady (no increase in demand - the piston has already moved up against the damper until it all stabilized), a thinner needle should let more fuel past. So if it’s the same at idle and thinner further down the needle, it will have a richer mixture when driving and especially when accelerating. I’ll report back here when I feel better about the state of tunes. Thank you for the help, I feel much more confident now!
David
…sorry it’s not oriented right, we have the same issue and I still don’t understand why it does that sometimes
H4, how cute!
If I have UM, I’ll probably try UB and if that doesn’t feel right, I’ll buy another set until I’m confident enough to drive it some real distance with the settings “right”. I’ll sneak in an exhaust test when I get around to it.
I wonder how this would react if you put a gulp valve on the manifold or those little spring loaded valves on the throttle plates. To be fair you are talking about a very stock system and the actual observed results that are probably common but unknown. Rarely do we get this type of interesting data and I certainly haven’t seen anything like it in the books. SPEED PRO SERIES has a very good book on SU carbs by Des Hammill. I recommend it. It gives very good info about how these carbs respond. If you have a depression above the float from a restriction you could try opening up the plugged float chamber hole for the non-existent tickler. For that you might buy a used spare from any similar H or HD carb with a 2" float bowl. Remember though, the float level in SU carbs is FAR less critical than in Detroit cars.
Back to my car. It is a 3.8 with HD6 carbs. It is running rich at idle, very lean on the highway. 10.3 to 17.
Here’s my plan. I believe I need to find any vacuum leaks into the manifold or carbs. I have a SU carb rebuild kit coming.
Then I will plot the lambda/AFR at various throttle openings from idle to WOT under load. I have a wideband O2 sensor on each pipe for a better idea of which carb to adjust. I am using Innovate DLG-1
With everything tight and my fuel consistent, I will look to needle profiles to even out the AFR (air/fuel ratio) at various needle settings. The dashpot and vacuum advance will make up for load/acceleration/deceleration needs.
There is a nifty chart maker at http://www.teglerizer.com/cgi-bin/needle100db.cgi
It plots the size of the needle at each 1/16 inch down.
|WO3|1000|950|910|878|845|822|800|780|755|735|712|690|670|650||0|
Another tool is a graph maker http://www.mintylamb.co.uk/suneedle/
It plots the numbers and compares three needles. I am changing from a TU to a W03. The HD8 have different needles.
From my research, and a dual AFR gauge or dyno time, you should be able to make your cat purr and growl as she ought.
In a week or two, I will let you know what result I am getting. Wolfgang Gatza from Dreieich, Germany got his V12 to run excellently by shaping his needles. (Oct 2014 post)
Again, I will get everything running as it should, then play with needles and carb adjustments until it is running right at all ranges. Wish me luck!
Sounds good and I wish you good luck, sorry for hijacking your thread for so long.
If you want to create your own profile, I just read that using a file is better.
Andrew Waugh recommended it on the German forum. If you file a flat on one side of the needle, you can adjust more precisely and most importantly, you can see the difference. And you can compare the needle to the other one if you’re satisfied. Makes sense to me. For metering purposes it will change the aperture of the jet just the same as a perfect round profile.
You’re perfectly equipped to adjust your profiles! What a dream.
David
Since you’re open to adding sensors I would suggest EGT gauges. They should tell you more about what is happening than how much fuel/air it is getting. My 420 running on HD6s was showing up to 1400F at highway speeds when it was lean. I adjusted the mixture to get it down to 1200. In small planes we adjust by the EGT.
I put a SU rebuild kit in both carbs, put in red springs, got a WO3 needle from Joe Curto, put 30W oil in the dampers, and tuned the carbs, using the original air filter. I also put in a fuel pressure regulator set at 2.7 psi.
Since I have two O2 sensors, the readings of the front and rear carbs look fairly similar.
The readings bounce all over, but are close to the holy grail of 14.7 at idle. They read 13.8 at 70 MPH, and 12-13 in the mid range. When I push the accelerator, the numbers go down, hence richer, just as I want them to be.
One weird thing, though, I closed the idle screws all the way to get my idle down to 700 in gear. I do not know what feeds the engine, as the butterflies are completely closed.
Nevertheless, she runs well now and my AFR is close enough.
Seems like you got it close enough for reasonable function, but you must have an air-leak if you have to close those screws fully. You’ve compensated for the leak fairly well it seems. Places to check are the throttle shafts, misalignment of the butterfly plates on the shafts or gaskets.
I had the opposite problem on an xk120 engine that I souped-up and put a pair of HD8s on. I couldn’t get enough air in to keep it idling even with the bypass screws wide open and had to drill holes in the butterfly plates.
UB needles are in, all filters and some debris in the float chambers flushed clean. About time! (The filter has been cleaned too, but it’s apparently ineffective).
Looks like the rear carburetor was far too lean. I believe it idled well via the balance drilling, but as soon as the throttle opened the rear three got extremely lean.
That also explains why it drove okay during cold starts, with a slightly rough idle, but with light brown plugs. On the last day we nearly didn’t get up the hill as soon as the asc (choke) had shut off, the mixture got too lean to ignite and the carb spat back.
I would be suspecting a stuck rear jet or float, but not sure. I had looked at the jets as I wound them down… maybe not on the second attempt.
Theory: I tuned everything well for UM needles. That meant misfires when accelerating. Then something got stuck and further attempts to enrich it only made it worse because I always tried to rectify the leanness during idle.
With the UB needles I got it to idle very well and it doesn’t stink anymore. I have yet to drive it under some load and do the fine tuning (neighbors…), but a „stall test“ to 1500 rpm went well. Far better than before. So in conclusion, the UB are very good so far and the stock UM, as many here in the forums say, are too lean. I think I will test it now (edit: rain… tired… tomorrow) and see if I get back in style. Thank you @ptelivuo ! And again sorry for the crosstalk.
David
That’s why I had three filters on the Jag: one inline, near the tank, once inline after the sediment bowl, and the three wee brass ones in the float bowls.
I have the in tank filters, the glass bowl filter still with its original element (it lets lots of fuel through, I checked) and the wee brass sieve in the float bowl lid is a bit of a joke. Which is good because the banjo bolts are forever-tight.
May add another cheap inline filter just before the carbs.
All that debris is after I drained the tank and flushed it a little! Maybe also from inside the line; I was worried it might be silted up but thankfully not. I blew through with air.
Hm, I would have. It’s cooled by fuel (you can hold a lighter to a plastic bottle if it’s full), it’s on the intake side. Carbs are always cool and the V12 distributor filter, same style, isn’t harmed by the heat either? (Neither are my shoes when I dry them in the valley)
On the exhaust side I would worry.
I‘ll definitely watch the temperature down there when the rain stops and I‘ve got it hot.
Then again if you say so I‘ll rather get a metal one. Very least they don’t feel as cheap.
I have a plastic one in my engine bay but I have it down at the bottom of the rubber line near the frame where it connects to the line that runs back to the tank. Been on there for years and is fine.
I should think so.
Went for a short test drive so it got hot and it all went well. Died once just after the asc went off (when gunning the engine I assume it was startled) but from then on it behaved well. But it needs some fine tuning still. Maybe just me but it also felt a little slower.
The filter location wasn’t warm at all so that’ll be fine. If anything new happens I‘ll make a new thread.
Me neither, and I think it’s impossible to crush that single special washer. I tried what happens without cracking the float bowl and don’t think it’s an issue. Maybe they changed the design.
XJ runs fine now. Starts immediately cold and hot, runs good when the hisser switches off (always scary), no hesitation, no noises and it’s smooth. Piston lift test gives results like in the books, I think there was some blockage in the rear valve or jet area in some way. I was new to carbs, I think I know enough for the moment… much better than injection.
Edit. It runs sweet! Just drove 3 mins to post a letter. Restarted immediately. Not a single hiccup!
