I might experiment by taking springs, piston/bells from another set of carbs and see if I see a difference. Good idea.
Yup, did drop tests on the three. All good. The engine runs super strong when the mixture is correct. My full load test drives are fun, but only when I pull the choke to allow the AFR the engine wants.
Just a thought, but if your float bowl needle valves are leaking a little bit or the tops of the float bowls are cracked that would allow a small amount of additional fuel to flow into the carbs when the demand is low. That would obviously make it run rich when it was idling. When fuel consumption is increased the small leakage would be simply added to the amount of fuel that was demanded by the carburetor and would have no effect. Take the suction chambers and pistons off, turn the key on and look down at the jets and see where the fuel level is. If it is flowing out of the jets then that would explain your symptoms.
That is a huge hint: that indicates strongly either a problem with the needles and/or the float levels.
The floats are fine and the needles are in good shape too though. But, yes, I do need to see if pressure may be too high and fuel is pushing levels too high.
Once again, verify float level by removing the pistons and sighting down into the jets. The traditional method of setting float drop can produce inconsistent results if the float pivots are worn.
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Sounds like it is time to try the next richest needle set. UO Idle area of the needles is the same so you should still be able to set the idle properly. I have had totally stock cars that needed richer needles for no apparent reason. End result is they ran good when done. You have checked everything else and pulling the choke makes the mixture richer. If that fixes it, then you need a richer needle set.
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I see this is your first visit to Jag-Lovers. Welcome
Since you are a new guy here let me add just a bit of weight to 2x comments already made:
@Wiggles is a retired Jag mechanic with ~45 year wrenching on Jags.
@Dick_Maury was the Operations Officers (I may have the wrong title - but it’s close enough) for Coventry West - a recently-defunct Jaguar repair and restoration facility out of Atlanta with likely another 35-40 years of experience with Jags.
That is nearly a combined century of Jag-related experience,
The are 2 of the E F Hutton’s of Jag-Lovers – when they talk, people listen
Hang around a while and you’ll ID the 3-4 other E F Hutton’s on board here (sadly for me (owner of a Series III) these Hutton-clones tend to center on the 6-cylinder variants)
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I had a similar problem to you last year. I have a stroker 4.7l motor in one of my cars. It had a set of street cams from Terry’s. Ran badly, the engine was actually kind of dead. Put a degree wheel on them, found they were 6 degrees retarded (they or the slot in the sprocket were badly cut) which explained the bad idle etc. Vacuum signal was very low at idle. Had a local grinder make me a set ground to XM2 spec - again a mild street grind, and installed them at 3 degrees advanced. Vacuum was now 22 inches, idled like a dream, as you’d expect, and the new cams brought the engine to life to say the least. However, there’s always a however, to get the carbs right I had to set the jet at full height, and the big idle screws on the right side of each carb had to be screwed fully closed. Where it was getting it’s a/f to idle on baffled me - the butterflies were fully closing - no light visible around them. Then strangely enough over the course of about 500 miles the performance got worse, idle in particular. They are now tuned to a normal condition screws wise and the car is back to a gorgeous idle.still with 22 inches. No clue what’s going on, though I suspect the increased vacuum has something to do with it. My experience is that these engines don’t ordinarily show vacuum in the 20 inches range - more like 15 -16 inches.
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I’ve been lurking for a long time, but finally feel like I can contribute too. I like being in the company of giants because the sum of all the years of experience usually pans out that even the weird stuff has been experienced and resolved by someone sooner or later.
I’m fortunate to work on E-Types almost every day so I don’t need to own one… yet. I’m looking to dig into the V12 world too. We have a Series 3 in the shop, but someone swapped in a Chevy LT1.
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Yeah?
Tell that to my parole officer.
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Very interesting. If I can’t resolve the carbs, then I need to look at valve train. How easy is it for the XK motors to skip a tooth? Is it possible a loose chain would cause this strangeness? This engine sounds good. I don’t gear any unusual or loud valve train sounds. You’re pointing me in that direction at least.
The other VERY important thing I need to consider is that this is a customer’s car, and I can NOT waste their time or money. I already feel bad it’s taken so many hours of diagnostics. But, testing and ruling out items is part of the job.
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Very difficult to skip a tooth, but there were some fitting plates the bolt the cam to the sprocket that can slip. It might be worthwhile to check the cam timing.It’s simple - probably an hour open and close.
Seems this may be an indication the throttle plates are not sealing or there is a vacuum leak
Have you also checked the brake booster for vacuum leaks?
Tom
Is that Officer K. N. Filter??
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I have a similar problem. Not nearly as bad: When I tune for best idle it idles beautifully and is way too weak off idle (and still slightly weak at higher rpm).
It’s all fixed ⅓ turn richer but then it‘s not as smooth and smells at idle.
I‘ve played around with valve timing on another xk engine and believe that it will either idle catastrophically or idle well and run out of power. With age the chain adjustment increases overlap and makes the idle worse. The cheap cam plates with the fewer teeth should be replaced.
I exactly get the advertised 0-60 time so it’s hardly low on power.
It’s not dampers, damper oil, piston drop, vacuum leak or unsynchronized carbs. And since it idles so pretty it should be fine mechanically. Blob of solder on spare needles in the idle section? Run a file down the side of the needle in the ‚weak‘ ranges? I still have a few UM needles to play with…
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So once again. E85 is stochiometric at 14.1:1. Gas is stochiometric at 14.7:1. If you are running UM needles, they tend to run lean on modern fuels. This may be more noticeable on some engines than others, but it affects all XK’s. You can’t adjust this out, because you are only adjusting one needle station. As load on the engine increases, the problem will become more apparent. The only solution is a richer needle. UE has been a popular choice, I run UX. UO is much too rich for my engine.
Here is a chart which compares four common needles, station by station. Along the x axis you will find the sixteen stations, along the y axis you will find the flow area through the jet:
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I know, thank you. I have UB in the car and when tuned for a good idle it really lacks fuel from 500-1000 rpm or so and above it runs nice. If I tune for good power the idle is crap and CO high. The UM were worse, yes, but not such a huge difference as to explain what I have now.
I probably need something like the UE then which gets richer earlier? Wouldn’t I want pretty much the same diameters as the UB from say 6 onwards?
If someone knows a needle that is fatter just off idle or thinks I am on the wrong path please let me know as that would save me some trouble. In theory the UB should be fine, right…
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I should have the profile for UB. I’ll work out the curve and post a new graph tonight or tomorrow. What engine and how many carbs?
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Has anyone ever thought to drill a snall hole in the throttle disc? In the world of Holey carbs and big cams, they tend to run rich at idle, due to the transfer slot location, so drilling a small hole in the throttle plates let just a tiny bit more air in at idle. Just thinking out loud here.