[Goatracer1]

!965 Tripower 389 built to basically stocks specs. Car runs great but it is too rich at idle. Idle mixture screws are screwed all the way in. Runs smooth but I can tell it is still rich. I'm replacing the short block this winter and don't want to damage new one. I've checked for fuel leaking into the motor from all 3 carbs and have seen nothing. I've lowered all 3 floats an 1/8 inch to see if it helped. It didn't make any difference. I have had these carbs for 30 years or more and they have been on at least 3 other cars with no problem. Any help would be appreciated.

[wbnapier]

Maybe get an O2 sensor so you know for sure where you are?

Mine are screwed in all the way and I am at 13 AFR.

[Kenth]

Worn, incorrect size or type of inlet valves.
Too high fuel pressure.
Too much opened primary throttle blades due to too lean idle circuit causing nozzle drip.

[Goatracer1]

Same needle and seat that were always used. Possible but same fuel pump used with original AFB. Blades not open excessively. Like I said this Tripower has been used on several other cars with no problem.

[Jonsie]

Don't believe the idle screws should be all the way in, sounds like the needles have the incorrect length and/or taper. Pontiac used several different idle screws for different 2 barrel carbs

[Dick Boneske]

Normal setting for any Tri-Power is two turns out for the idle screws. You've checked most of the obvious causes. I would take out the center carb vernturi cluster and check the idle tubes, Idle bypass, and idle vents. The idle tubes for your engine should be about .033", Idle bypass about .046", and idle vents about .040".

This is far-fetched, but the power valve could be opening at idle if there is insufficient vacuum, or the rod could be mechanically stuck. The main jets and power valve have little effect on the idle mixture, but a wide-open power valve may cause an issue.

Remember, Pontiac used throttle plates for automatic cars with holes that allowed smaller plate opening, to prevent nozzle drip.

I realize you've used this serup on other engines with good results, but something has changed.

[Goatracer1]

I'll open the center carb again and check for a stuck power valve rod. Is there a way to check that the power valve rod is actually being pulled up at idle? There is definitely enough vacuum being produced by the engine whether or not it is getting to the valve I'll have to check.

[carbking]

The power valve actuating valve is in the "down" (wide open) position when the engine is off, which causes the power valve to be wide open. Once the engine starts, vacuum lifts the actuating valve piston (if it isn't stuck).

Quite possible that evaporating fuel could have left a residue on the power valve piston, and since the power valve would have been wide open, possible that the power valve itself is stuck. So:

Check the power valve for proper movement
Check the actuating valve for proper movement
Check the function of the actuating valve. If you do not have a vacuum pump, you can accomplish this check by blowing compressed air at right angles to the vacuum source hole.

You might also wish to test the floats. Brass floats may easily be tested by submerging in hot water. A bad float will blow a stream of bubbles like an intertube with a nail. If the floats are foam, don't even bother trying to test, just replace them.

Jon,

[wbnapier]

I played with mine today. I have an O2 sensor, and to get my AFR at idle down to 13, both screws have to be all the way in (lightly seated). So, something is amiss. At ½ turn out, I am at 10.75 AFR.

I changed the needles as I had an extra set, no difference.

Throttle blades are not drilled.

At warm idle, I am at 10.75 - 11 AFR. 15-16" of vacuum, idle at 850 rpm. No off-idle hesitation.

These carbs were restored about 1-½ years ago and the idle circuit was modified for my engine. 455/469

The exhaust is not visibly grey or black and the car runs decent.

Something in the idle circuit is amiss, but hesitant to dig into since the car runs well.

[tom s]

As you have no AFR guage,what makes you think it’s rich?Exhaust smell?Tom

[Goatracer1]

There is a rich lope at idle if the screws are backed out at all. Not a lean surge. The plugs are all soot. The engine doe not want to idle if the choke is closed at all when cold. 60 years experience working on cars both as a hobby and as a living.

[wbnapier]

Any updates Goatracer1? I'm going to tackle mine this afternoon

[bob d]

What do you use to adjust the idle screws on the center carb? That would be very helpful. Thanks.

[wbnapier]

Originally Posted by bob d

What do you use to adjust the idle screws on the center carb? That would be very helpful. Thanks.

I use a ¼" standard length wrench, I come in from the passenger side. Not so easy to get the driver side screw.

[Goatracer1]

I have the engine apart and won't get it together for awhile. I wanted to locate the problem so as not to damage a new engine. As for adjusting the idle screws, for years I have pushed a short piece of vacuum hose on each screw long enough to turn with my fingers. If the looks bother you they can easily be removed for shows.

[bob d]

Thanks. My idle needles have knurled ends on them.
.

[Kenth]

Original Tri-Power idle needles has 1/4" hex heads w/slotted end.

Aftermarket replacement idle needles may never reach the holes to fully close off idle mixture.
Too long springs on idle screws will also prevent the needles from bottoming.

FWIW

[wbnapier]

OK, I had success.

My issue was the idle mixture screw springs. They were too long and the screws couldn't fully seat, or close to seated enough.

Also, see photo, there is a lot of variance out there. Of the 4 springs in the photo, there are 3 sizes. I had to trim the smallest spring one coil.

I recommend removing the springs, seating the screws to check the gap, then trim the spring if needed.