I bought a 1986 XJS-C V12 HE (with Lucas parts) a few weeks ago, and when I bought it, it had a bit of a lumpy idle (but not bad), and it had all the expected power. However, I very intelligently thought "what better way to drive a 40-year, temperamental British car than to do some light drifting in the rain???"

RPMs weren't super high, I wasn't being 'excessive'. Car was fine, and then I pulled over for a minute, pulled back out, and the power was way down. The engine has become far grumpier and feels very lumpy now, like a big cammed muscle car. I put my hand over the two exhaust tips, the left felt smooth and normal, the right had a different, less frequent pulse to it.

Today, while deciding how to go about diagnosing this, I decided to put my hand partly covering the intakes, and I discovered that the right intake has far less suction than the left, at idle and on throttle. In fact, at idle, the right intake has zero suction force...
I don't think the air filter is the issue because this is quite a sudden issue, rather than gradual, and I even loosened the air box so that a small amount of air could momentarily bypass the filter, and the idle/revs had the same issue.

At no point were there any mechanical or metallic sounds. The previous owner did remove the cruise control unit, therefore the AC unit is not braced sufficiently, causing it to vibrate a lot, not sure if that's related. I did have a nasty belt squeak before this issue, but now it's gone...

I'm going to check all the plugs tonight, but I'm just wondering if this is maybe common enough, or the solution obvious enough that someone may know what I should be looking for?

Thanks!

 

Check the adjustment of the throttle rods. Gunk can build up in the throttle bodies too. So disconnect the rods, they just pop off, and have a look. Clean the throttle bodies, then set their gap, then adjust the rods to match. I think Grant has a writeup about this.