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Idle Speed Adjustment

Idle Speed Adjustment

Knappe, Cannara, Dwyer

Dave writes:

Can anyone describe how to adjust the idle so even a blind man could do it. I have been to the archives and they were not specific enough. I think I have located the adjustment screw near where the accelerator cable connects to the engine by some large return springs. Is that Where I adjust the idle. :-)

From Doug Dwyer:

Setting idle speed can be a bit of a bitch on these cars. Don't try to adjust the idle at the throttle body. The idle speed is adjusted at the air distribution block which is a few inches ahead of the throttle body. It's an aluminum casting with some hoses going hither and thither. The adjustment is made by turning the allen-head screw (5/16" ?)which is located in the upper area of the block and a bit tricky to access (doesn't matter if you're blind or not----it's mostly done by feel). I use an Allen wrench which is attached to a socket and turn it with just my fingers. Others have cut, bent, and twisted Allen wrenches to ease access to the recessed screw.

The problem is that the nether regions below this metering block are usually all befouled with internal combustion by-products and all the adjusting in the world makes no difference. often, too, the adjusting screw is all but seized in place. The only choice is to remove the block and clean it. Order a new gasket first. Remove the hoses from the distribution block and remove the block itself. You'll probably be amazed at how gunked up everything is. Remove the spring valve and the adjusting screw. Clean all in solvent and, to whatever extent possible, clean the area inside the intake. Thoroughly clean the adjusting screw and use some anti-seize (or at least some oil) before reinstalling.

There is also a mixture adjustment on the Air Flow Meter (AFM). If you care to venture into this area we can give you directions but first..... Check everywhere for vacuum leaks. A small vac leak will usually give you a slightly high idle but a larger one will give a low, lumpy idle. You'll also want to adjust the throttle plate to the specified .002 clearance (this adjustment IS made at the throttle body)

From Alex Cannara:

You have the right idea and you need to start with what others have said about vacuum leaks, etc. The next thing then is to set the closed position of the throttle butterfly to .002" (on left side) and this is done by removing the intake elbow and releasing the throttle cable's tension, where you indicated, and using the stop adjustment (5/16" bolt and locking nut). The reattached cable should have only the slightest slack in it.

With the leaks and closed setting done, reassemble the intake path and run the engine 'til warm. Check the timing is set right. Turn the air-bypass hex screw (5/16") full right (closed) for slowest idle. Then open until you get 750 rpm (in P or N and no A/C). If you can't (ours must be almost closed), recheck for leaks, even around injector nozzlesand hosing on and behind the Extra Air Valve (between middle injectors),maybe even removing the aluminum block the adjuster is in and cleaningthe spring bypass valve, etc. (note how the parts come out!) Once you get the idle speed in range, use the air-bypass on the AFM (5mm hex,upper left corner) to adjust either for smoothness or CO spec, as youcan (ours is about 1/3-turn open [CCW]).


Hi all,

I did some Jag-work today, replaced all water hoses, changed coolant and did a tune-up.

Everyone who tried to adjust the idle speed on a XJ6 SIII knows that it's rather difficult to get at the adjuster screw because the air flow meter is in the way. If you take the air flow meter out, you can reach the adjuster but the engine doesn't run, so no idle adjustment can be carried out.

The adjustment screw can be turned with a 7/32 allen key.

My idea was to cut a piece of such an allen key, long enough that it protrudes the hole where the adjuster screw sits in (by 7mm approx). After removal of the air flow meter I glued it in place in the adjuster screw with a little epoxy (careful, do not lock the screw!). It will stay there permanently.

Adjustment can easily be done now by means of a 7/32 wrench, while all components are fitted and the engine is running.

Harold Orlando offers an alternative:

I took a 7/32 allen and (after heating with a propane torch) changed the 90 degree angle into a 45 degree angle. It works fine.


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