Hello everybody. I’m the owner of this fantastic Jaguar XJS from 1991. In good state, some corrosion managable. I have one question regarding the power. Any possibility to improve it, as i don’t feel "a kick in the butt"when accelerating, may be normal since this car is heavy weight. Any recommandation? Kind regards. Philippe (north France)
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Perhaps a hint on the engine size would help us.
Welcome!
First off, you need to ensure that all the basics are good, beginning with compression.
Then, make sure all the complex ignition and fuel injection parameters are correct.
Many on here are VERY knowledgeable about these cars and will soon chime in.
Thanks a lot. Indeed I’m looking in my area for specialist having suitable equipment, i’ll keep you posted. However in case teh power is not there, what needs to be done?
one change I did that made a huge difference…(assuming you have the TH400 auto transmission)…
Change out the Vacuum Modulator for the transmission. The stock one shifts into 2nd and 3rd very early (for smoothness and better fuel economy). And you also have to practically floor it to get it to downshift from 3rd to 2nd.
The XJS V12 will never feel powerful at lower speeds in 3rd gear.
I bought B&M 20234 adjustable vacuum modulator ($32), and adjusted it half a turn from all the way right. Now it takes a bit longer to shift into 2nd and 3rd, and I can get it to downshift with only half throttle. Car feels more powerful now, even though nothing done to engine.
Another thing that really helped give more power, but this is way more involved…removed mid-mufflers (the big ones in front of the rear axles)
One little change that helped a tiny bit too: I changed out the vacuum full throttle switch. The stock one enriches the fuel mixture if you floor it. I bought a Stewart-Warner 77343-04 switch ($62) which triggers the full throttle enrichment at a higher value of 4 inHg. This way, I get full enrichment a bit earlier when giving it a lot of throttle, even if I don’t floor it. (I think OEM one needed 0-2 inHG to turn on?)
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Where about in France are you, I live in France also and have a 91 XJR-S (MY90)
Best regards
Simon
Hello Simon, I’m in the North, city Valenciennes. What about you? Kind regards. philippe
One thing worth checking is the kickdown mechanism. It involves a microswitch at the end of the throttle cable housing on the turntable on top of the engine. Have someone sit in the car and push the pedal all the way to the floor while you watch the switch. After the turntable turns to full open, the cable housing should pull in and actuate this switch. It often fails to do so.
If it doesn’t work right, adjustments are in order – but the first thing I recommend is a minor modification of the throttle pedal itself. It’s an L-shaped bar with a bearing at the corner; one end has the pedal, the other end attaches to the throttle cable. This bar can flex a bit when the pedal is stomped, and can even get permanently deformed. If the pedal hits the floor before it even tries to actuate that kickdown switch, it’s probably deformed a bit. Remove it from the car (it’s pretty easy to get out), mount it in a vise or something, and bend it a bit so the tips are a bit closer together.
Here’s the mod: Do something to stiffen that L shape. If you’re a welder, fashion a small bit of metal to weld between the two legs of the L to triangulate it. Make sure not to attach it too close to the pedal end because your toe might contact it. If you’re not a welder, you can help this thing by simply tying some wire between the legs, but you need to get it taut. You can drill a small hole in each leg for attaching the wire; as long as the holes you drill aren’t too close to the pivot at the corner, you’ll be drilling in a low-stress area.
Done right, you’ll be able to feel the difference in the pedal when you mash it. Repeat the earlier test watching the kickdown switch when someone pushes the pedal to the floor. When the driver can get a clear click from that switch every time, take the car for another drive and see if it behaves better.
Note that some consider that kickdown switch mechanism more trouble than it’s worth, and remove the actuation from the cable housing end and simply mount it elsewhere on the throttle linkage so that instead of requiring the distinct kickdown at the bottom of pedal travel, it actuates whenever you’re at WOT. The original intent – not just on the XJ-S but on virtually all A/T’s of the era – is to give the driver the option of WOT without kicking the transmission down a gear if that’s what he desires, or he can press the pedal a bit harder and deliberately engage that kickdown. But others don’t care about the option, would just as soon the car get up and go when floored.
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Or add a kick down switch inside the cabin…
Instant fun!
Salut de la Vaucluse !
Long story short, I use an adjustable electro/vacuum switch for full throttle kickdown. It closes (and operates a relay, which operates the kickdown), at about 2" of vacuum
Years ago I had a problem with the kickdown on my XJS. A bushing on the throttle pedal pivot had somehow gone missing, resulting in lost travel of the pedal arm and not enough ‘pull’ on the cable to actuate the KD switch
Later I used the full load switch to operate the kickdown…but that’s a bit too aggressive for some people. Including me, nowadays
Cheers
DD
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Manually downshifting is always an option. And the shift gate can be very easily modified to eliminate the awkward 2-1 movement
Cheers
DD
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