Fast Idle on L-Jet

[MidLifeCrisis]

Hi,

After troubleshooting multiple air leaks, I think I finally eliminated any "sputter" when accelerating hard. However, the idle is way too high... about 1,400-1,500 rpms.

The adjustment screw for the throttle plate is barely engaged (less than 1/4 turn). I bottomed out on the adjustment for the idle on the air bypass line. The rubber seal at the bottom looks to be in good condition. I can clamp off the AAV line and air bypass line with no change to the idle.

What could be causing such a high idle if all the normal adjustments are set for the lowest possible idle?

Thanks,

Michael.

[ghnl]

The throttle 'butterfly' should be nearly 100% closed at idle so that all idle control is at the idle speed adjuster. And if the O-ring ain't new, replace it. (I don't recall the size - I just took the old one with me to the plumbing section at Lowes and bought one that looked about the same size.) The O-ring gets hard and takes a set so that turning the idle adjusting nut does not squeeze the O-ring to make the ID change which is how the idle speed is adjusted.

And make sure the TPS (Throttle Position Switch) is adjusted properly. It tells the computer when the throttle is at idle, not at idle or wide open.

[MidLifeCrisis]

Thanks.

I have the TPS adjusted correctly per the manual, and the throttle plate is open by a hair. I'm not sure it could be the washer since pinching off the air bypass hose doesn't effect the idle. I wonder if my AAV is stuck open?

[Tifosi]

Throttle plate 'open by a hair' is likely too much then unless you've got false air somewhere. A hair is just too thick basically, unless you're talking frog hair.

The adjustment should be like 1/4~1/3 a screw thread more than touching the stop arm.
We're talking just enough to unseat the butterfly when at full rest. Nowhere near a 1/4 turn of the screw even, more like 1/64, a degree or two, a 'smidge', or an amount so tiny you aren't quite positive it even turned unless you've got a dial indicator on the outer edge of the butterfly proper (not the stop arm) to see that it did indeed move a few thousandths of an inch.

Still, clamping the AAV and bypass hose shut at the same time should have some effect on the speed as both bleed air into the plenum (the AAV has a wee small 'nick' in it that'll pass air even when fully closed) leading one to think that perhaps false air is indeed the issue.

What happens if you back the throttle stop screw out enough for it to lose contact with the stop arm? In theory it should have no effect at all if the AAV and bypass are functioning correctly and there's no false air, though a change of perhaps a couple 100 rpm is more the case.

[MidLifeCrisis]

Quote:

Originally Posted by Tifosi Throttle plate 'open by a hair' is likely too much then unless you've got false air somewhere. A hair is just too thick basically, unless you're talking frog hair.

[Tifosi]

You can move the TPS while the engine is running, but it's far more efficient (and accurate) to adjust it using a multimeter connected across the proper terminals with the engine off.

As it's more or less just a switchbox with no analog function, there is no 'progression' of any sort, just on idle (no throttle deflection switch), off idle (no switches active), and 55~57+ degrees throttle deflection (high throttle deflection switch active).

[ghnl]

While the engine is idling remove the oil fill cap. If there are no intake air/vacuum leaks the idle speed should deteriorate - slower & irregular. If the idle character/speed does not change noticeably then it is likely there is an air &/or vacuum leak.

Are you measuring the 'frog hair' amount by the throttle stop screw? It might be a good idea to remove the air duct and look/check that the throttle plate is essentially closed with the throttle at idle position.

[MidLifeCrisis]

Quote:

Originally Posted by ghnl While the engine is idling remove the oil fill cap. If there are no intake air/vacuum leaks the idle speed should deteriorate - slower & irregular. If the idle character/speed does not change noticeably then it is likely there is an air &/or vacuum leak.

[ghnl]

Make sure the throttle linkage is not preventing the throttle plate from closing. On our Spider both the horizontal & vertical rods are adjustable for length.

[MidLifeCrisis]

Glad to see that I'm thinking like the pros... I already checked this

Thanks.

[ghnl]

How are you measuring RPM's? If using the Alfa gauge it might be a good idea to verify its accuracy.

[MidLifeCrisis]

I know my tach is generous by about 20%, so I'm using a dwell tachometer to measure the rpm's.