ITBs not returning

[djz]

Hi guys, I've recently been installing a set of OER individual throttle bodies on to my L28, and I've got a problem where I can't get them to return fully, when you tap the throttle it will return back to about 1500+rpm until you pull back on the throttle shaft. Each TB has it's own return spring, and the throttle shaft has another spring as well.

 

I've taken the throttle shaft out and lubed all the rod ends, my manifold is unfortunately made for TBs/carbs that have the throttle linkages on the opposite sides to the OER ones so I've had to add another rod end on the thermostat housing, the shaft seems reasonably loose, it is pretty easily turned by hand with a little bit of resistance without the TBs connected.

 

I have tried adding extra springs which seemed to help but can't seem to find a position that works well, the springs either make the pedal too hard to push initially or the springs are bending in ways they shouldn't or touching on other things.

 

Has anybody fixed a similar problem to this? Do triple carbs have return springs on each carb? Would changing the length of the linkage from the main shaft to each TB make much difference? I just guessed a length when I made them.

 

Thanks!

 

[Tony D]

Put an additional linkage on the cross/transfer bar, and use holes in it to set your spring tension on the transfer bar for 'feel trim'.

 

What you ARE missing is the stock return spring on the cross/transfer bar---it's a large coil spring that usually wraps around a specific spring arm, and is backed against one of the heim joints. It is a STOUT spring which makes the linkages pop up on release of throttle.

 

One other thing you could investigate is removing your two springs and put a compression spring on your throttle cable (like an old VW) which would pull your quadrant back to a starting point set by a cable stop on the cable itself. Also, your throttle quadrant is set so that you will take a HUGE effort to move it off-idle the way you have it set. If you will rotate it anticlockwise as viewed from the firewall maybe 15 degrees it will take FAR less effort to move it off idle even with huge spring pressure.

 

I set up my quadrant to have my pedal floorboarded and the ITB's at WOT. To do this, I had to alter the pedal stop on the throttle pedal inside the passenger's compartment, and lower the throttle pedal down to almost the level of the brake pedal (darn the coincidence there!) At idle, for smoothest pull the quadrant should have the cable tangential to it, you are bisecting the arc the quadrant will travel through. You may want to try rotating it 15-30 degrees anticlockwise before changing anything and see if you are still O.K... Like I said, you may have to lower the pedal stop, but likely it will make your setup operate FAR smoother than it currently does.

 

Good Luck, Cheers!

[djz]

Thanks Tony, I've added the return spring onto the cross bar, should it be preloaded at all?

 

I've shortened the throttle cable so I could adjust the quandrant to a nicer angle, it's working better now and doesn't have the huge effort off idle any more.

 

Also I've removed the extra springs I added and have changed the spring on each throttle body to a heavier one, it's returning better now but still not fully all the time, I get the feeling that one of the throttle shafts or plates must be sticking or something like that, I'm going to have to remove the plenum again and see if I can figure out which one it is.

[Tony D]

I gave mine 1/2 turn preload as I recall.

The biggest thing, as you noticed, was the quadrant angle not being correct.

Should be very soft to the touch, especially with the S30 pivot setup.

 

Make sure the linkages are the same, the arms are at the same angle, and that the throttles are all balanced---no difference in flow front barrel to back. if you have a twisted shaft the one will stick all the time. To compensate you end up having to open them all slightly, and that puts your idle speed all to hell! Best to idle them closed and use a bypass circuit for idle.