Bog/hesitation on sudden throttle input

I’m getting my XJS back into roadworthy condition, but have found a throttle response problem. It’s not quite like other’s I’ve seen posted, though the solution may be similar. Cold or warm the reaction is the same. Out of gear sudden throttle input causes a half second bog, in gear there is what feels like a 1 second hesitation between throttle down and acceleration. For those familiar with carburetors it acts similarly to a bad accelerator pump. With slower throttle input the reaction is normal. No power loss noticed, just no sudden acceleration possible.
In the course of various repairs I have:
Checked the TPS for bad spots with oscilloscope, no cut out or dips in voltage.
replaced first fuel regulator (leak)
replaced fuel pump (dead) and filter
rebuilt injectors and hoses (leak)
replaced a bunch of leaky vacuum hoses
checked the vacuum hose to ECU for blockage

Any other ideas on where to look next?

I’d be all over the throttle plate adjustment procedure, making sure they open in sync and at the right time. Make sure your throttle bushes haven’t cracked and fallen out. I just fixed an XJS last week that had a big GULP before accelerating and it was 95% due to broken missing throttle shaft bushes.

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What year is your Jaguar and what engine and ignition system does it have?

Paul

Sorry, thought that was in the signature, I’ll fix that.
1986 imported in '87, V12 HE, Lucas CEI

Appart from the throttle bushings and butterflies sync, I would also look at the second FPR (it’s the B-bank one that does indeed regulate the pressure) and your Base Fuel Map setting of the ECU. As you fixed your vacuum leaks, you also changed the vacuum input at the ECU, and therefore the fuelling.

Worth checking your distributor advance mechanism hasn’t seized. Take the cap off and check the rotor arm isn’t fixed solid. There should be a few degrees of movement. I can’t remember if it has vacuum advance too as well as centrifugal but check that too by applying vacuum to the pipe to see the baseplate moves.
The lubrication of the stem felt under the rotor arm is often neglected

Italian Tune-Up.

20 char

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I checked to ensure the distributor was not seized then did an Italian tune up. Not gone, but noticeably improved.
In the mean time the brake light switch stopped working. So I opened it up, cleaned it out, filed everything flat, and put it back together. I have brake lights again.

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Right on !

I use my V12 Jag as a daily driver and it sees quite a bit of short run, city driving. A slight bog on opening the throttle is my que to give 'er the boot…with no bashfulness. The bog always goes away.

Cheers
DD

I agree.

And besides the (well known) problem with the nylon bushings I’ve discovered that wear throughout the entire linkage system can make the sync-ing up a bit frustrating. I remember it be much easier years ago, on my previous V12, which had nowhere near the same amount of use/wear.

Cheers
DD