17085292 Idle Speed

[Raoul Ross]

85 Chev C10 Suburban. I cannot get the idle speed down. Idle speed is 900-1k

Idle speed screw is completely backed off tab. Unit was rebuilt at least once, the mixture screws are exposed. Timing is set to spec. Butterflies are closed on secondary side. Vacuum is 20in. at idle. It has a mixture solenoid but no TPS.

[Kenth]

Vacuum leak or still on fast idle cam?

[quadrajam]

Is the choke fully off and the fast idle cam out of the way?(Adjustment screw down low on passenger side)
Maybe a small vacuum leak?
Check that the throttle cable and trans cable are not tight at idle.
Make sure the link between primary and secondary shafts is not tight at idle.
Has the idle bypass air been modified?
Manifold or ported vaccum to advance can?

I have a couple of that same carb and no such problem.

QJ

[Kenth]

BTW, this carb has no mixture solenoid.
It has the ordinary power piston, metering rods and jets.
The solenoid used is for the "dual capacity pump" feature.

[Raoul Ross]

Excellent point about the throttle/trans cables, I will definitely look into that!

I looked at the secondary linkage, plenty of slack there, definitely in spec. There doesn't seem to be any slack in the throttle shafts either. 20in. of vacuum at idle tells me it's not a vacuum leak.

Manifold vacuum to dizzy at idle. When pulled (to check timing) vacuum goes down to 12ish inches though I didn't plug it. I just wanted to check that somebody hadn't bumped the timing up. We usta do that 'Back in the Day' to impress the customer with your tune-up expertise! 

Dual capacity what? 

[quadrajam]

Dual capacity accelerator pump volume. Used on 85 and 86 trucks for
emmissions purposes. When the engine warmed up to 170 deg. the solenoid
would be energized reducing the pump shot amount by about half. It didnt
affect drivability because of the Q-jet's generous acc. pump volume.

QJ 

[quadrajam]

Oh...and the reason I mentioned vacuum advance is if you dont find a vacuum leak,
switch to ported vacuum and that usually will bring idle rpm down.

Check and make sure the centrifugal timing is not creeping up too early as in weak
distributor springs.

Hurry and fix it I'm getting tired of spelling vackum.

QJ

[Raoul Ross]

I hesitate to change to ported vacuum because of it's unmolested-ness. all the vacuum lines are there and everything is operational. I'd like to get it working as it should. I also wonder how that would affect fuel mileage.

I pulled the dash out to change all the bulbs and fixt a broken circuit trace, the customer really liked that. Did that while replacing the headlamp dimmer switch, a super PITA.

It's kind of a rolling time machine. 102k miles, completely original, former Colorado Forest service vehicle (2WD!). A bit of rust here and there but nothing major.

ANyway, I'll be getting the vehicle back this weekend, I'll keep you'all up to date on what I find.

[Raoul Ross]

So I was able to do more diagnosis yesterday and found a few things, not the least of which is the primary throttle shaft *is* wobbling in the base plate.

The other thing I found out is, when somebody rebuilt the carb and knocked out the idle mix screw plugs, they went crazy and turned them out... a LOT. It was so rich at idle the exhaust burned your eyes.

So through leaning it out I was able to get the idle speed down. That would probably also explain his hot soak issue. He told me if he didn't restart it after sitting for 10 minutes he'd hafta wait an hour before he could get it to fire up again.

Of course the real issue is the wobbly throttle shaft. Interesting that it still holds 20in of vacuum.

[tayto]

sounds like an ignition problem. the exhaust making your eyes burn is unburnt fuel, not "rich"

[Raoul Ross]

sounds like an ignition problem. the exhaust making your eyes burn is unburnt fuel, not "rich"

That is next on the To Do list, pull the plugs and look under the cap.