Potentially interesting throttle issue

Following @gregma down the perfect idle and throttle engagement hole, I started noticing a slight hesitation when going from brake to acceleration. It’s ever so slight, but it’s starting to annoy me now😁. It’s most pronounced at low speeds, rolling stops, etc. I haven’t noticed it at 50+ speeds, but will test during my drive home today.

My hunch is the TPS is slightly off, but I haven’t started any diagnostics yet.

Happy and appreciative of the thoughts and discussion of this that might make fixing easier - thanks!

Ha, funny you should mention me, I just solved that issue on my car last weekend! Mine was so subtle over the last half year, that I got used to it. But I finally decided to tackle it.

Simply -my throttle rods were adjusted a bit too short! I was able to turn the throttle pedestal a tiny bit, which made the TPS voltage go up from 0.34 to 0.5 or something, but the actual throttle plates were still closed.

But in my case, it was impossible to let the rods out one more turn without having the throttle plates open a tiny bit at idle. I effectively needed a half turn, which is impossible. (Probably why I previously settled on having the throttle rods a bit shorter than longer). So instead I adjusted the actual throttle plate stop screws 2 flats (1/3rd turn) more closed so they matched up better, and if the idle RPM went down too low, I could open the AAV screw to compensate.

I actually left it as is, my RPM used to be 850RPM in P, but now it’s just above 800RPM. But my engine is idling better at lower rpm lately, which is nice.

I still get a slight hesitation when the engine is cold, but that’s just a cold engine, not worth looking into IMO.

Hope your issue is something as simple as mine? Good luck.

Why? Why can’t you adjust the throttle rods to any length needed?

because the part that clips onto the throttle plate mechanism is what screws back and forth onto the threads of the rod. So if I screw it only half a turn, the clip is facing the wrong direction.

Don’t turn the socket, turn the rod.


I’ve been having difficulty getting idle to stay around 700 especially when first running. The aav doesn’t seem to do much and. I too am thinking about adjusting the two rods just a bit. After the car warms up idle is better. However adjusting the rods truly is not the answer is it? Because wouldn’t it make too much idle after warm-up. Seems that a vacuum control like the aav is truly more correct. I would think there would be other substitutes for the obnoxious aav unit. Also I have noticed on early am starting that before the alternator kicks in, that if I put the electric window down my idle stumbles a very little bit while the window is going down…? Electric loss?

What a dope i am, you are right, if both sides are loosened, it should turn.

Sort of like a turnbuckle

The butterflies must stop on their adjustment screws and have the precise gap between them and the throttle intake. It must be very precise and it’s very sensitive. If not the engine will not like it and the idle will suffer.
The rods must have a very small play so you are sure that it’s not them that determine the plate’s position.
It took me hours to adjust mine, precise to 0.10mm and I have a very decent idle.
Also must be absolutely sure that the TPS gives the correct value.

The hesitation it could be many a things, worn or maladjusted TPS, bad spark plugs or leads, too lean base fuel map…

another common issue are the bushings on the throttle axles/rods. The rubber easily disintegrates, causing a not smooth turn of the throttle axle/rod. Many here, me included, have switched to brass and it works great.

Well, definitely TPS needs adjustment, reading 0.28…

Well… what a difference that makes. First time she’s fired up instantly from cold in a loooong time. Still a touch of hunting at times, but i’ll sort that out. Now for the drive test…

I played around with my adjustments today, now that I know (3 years later! :frowning: ) how to properly adjust the throttle arms with the turnbuckle.

So I went back to square one, as the Jaguar Manual says, and made sure the throttle plates were at .002" clearance, the throttle stops were set, and the intermediate linkage bellcrank was on it’s closed stop. I then adjusted the throttle rods on so they fit perfectly, with bellcranks still sitting on stops.

Idled great, but I got a terrible pause on slight acceleration when cold, and a subtle pause on slight acceleration when warm.

The problem is, and I don’t know if this is normal or not, there is a tiny bit of slop with the intermediate linkage. That rod that connects top to bottom is where the slop is. So as the throttle pedestal starts turning and pushes the throttle rods, it takes about 1-2mm movement to open the actual throttle bodies, and you get that ‘pause’ . TPS is going up in voltage so ECU is providing more fuel, but throttles are still closed.

So my solution was: I gently turn each throttle rod by hand to take up the slop until there was resistance. I double checked with the 0.002" feeler gauge on the throttle plate stop screws (Not throttle plates) to make sure there wasn’t a gap, that I didn’t turn too far (it’s easy to do). Also, it’s important to do it on a hot engine. The throttle rods expand! I at first did this cold, and then by the time the engine warmed up, the rods were pushing a bit too much, I had gaps at throttle plate stop screws. Was wondering why idle went up.

I still get a great idle, and now no pause when giving a slight acceleration. There is a wee tiny bit when engine is totally cold, due to the throttle rods contracting. But it’s subtle, and goes away by 160F.

Only isssue, the intermediate bellcranks are no longer on their stops, they have a 1-2mm gap. The throttle stops provide the stop all by themselves. I don’t understand what the bellcrank stops are for?

On a side note, my idle has improved! When checking my throttle plates, sure enough, they were both sitting a micron too low. The 0.002" feeler slid a bit free on top, but wouldn’t budge on bottom. I loosened both screws and adjusted, and now it’s 0.002" top/bottom. The throttle plate is perfectly centered. Don’t know why, but this seems to definitely help smooth out the idle. Air flow pattern?