Need a Quadrajet guy

Fritz Maurer · Jun 29, 2022

Is there a part missing on this dashpot? Carb is not native to this truck but according to the number (17059213) its from a 1979 pickup with a 350. When I got the truck the carb was a mess so I rebuilt it. Got the rest of the carb all set up, but there is not enough dashpot screw to reach the lever. Am also confused about the dashpot using the only ported vacuum nipple on the carb. They had the distributor hooked up to manifold vacuum. Engine is a 366.

Janicholson · Jun 29, 2022

If the primary throttle plate has been taken off of its shaft, it could be installed incorrectly and hanging the shaft part way open. OR the automatic choke could have indexed the throttle into fast idle mode. You need a vacuum diagram from the engine the carb is from to be certain of the settings. Jim

MarkB_MI · Jun 29, 2022

Will the lever touch when the engine is running? As I recall, the purpose of the dashpot is to help kill the motor when you shut it off so it doesn't diesel. If so, the adjustment serves as an idle stop.

Steve@Advance · Jun 29, 2022

The dashpot is vacuum controlled to let the throttle ease closed for whatever reason, possibly to prevent exhaust backfire if it had a smog pump or AIR emissions on the manifolds.

Chances are the dash pot is stuck or the diaphragm bad. I wouldn't worry about it. The idle is set by the screw down below, the fast idle set by the choke cam, so the dash pot not really necessary.

There is also a choke pull off servo, did you check or replace it? That is important.

Float should be replaced too, they would sink.

If the fuel filter has the check valve spring in the end, take the spring out. Useless and can restrict the fuel flow.

I don't know how available tuning parts are for Q jets anymore, used to have boxes of that stuff. That's what made Q jets so good! They were tune able, could keep my OCDC going for days! LOL

It's common to connect the vacuum advance to manifold vacuum. Did you check the vacuum advance? Did you check the centrifugal advance? Both are very common to need attention regularly.

Steve@Advance · Jun 29, 2022

Mark, I think you are talking about the electric idle positioner. It did serve as an anti diesel and as a idle increase when the AC was on.

Fred47M · Jun 29, 2022

I don't have great answer, Chilton manual doesn't get too into that. But I am running a carb off a 79 400 sbc (probably same carb as yours) on my 84 350. I just switched everything over, that and choke pull off electric choke etc. I didn't plug that it in because it didn't move, later version has a wire. So I don't think it's too critical as it runs excellent weather it 100 or -25f. My vacuum advance is run to were the dashpot is on yours.

rustred · Jun 29, 2022

for starters, the 366 engine uses the 4V holley carb. the quad carbs had an electric solenoid there for dieseling. when the key was turned off that closed the throttle plates. that dont look right to me.

Fritz Maurer · Jun 29, 2022

I havent had it started yet, but the dashpot is shown with the plunger extended. All diaphragms have been vacuum tested, all parts in the float bowl are new. The dashpot screw will just touch the lever with the choke open (throttle at low idle) and the plunger extended... but still too far away for it to work like it should. Maybe Ill just delete it, I just thought there was some simple part missing. I couldnt find anything on line about that particular dashpot. Thanks guys

MarkB_MI · Jun 30, 2022

> Mark, I think you are talking about the electric idle positioner. It did serve as an anti diesel and as a idle increase when the AC was on.

Yes, I'm familiar with that device. I was thinking this dashpot is a variation on the solenoid.

UncleTom · Jun 30, 2022

What Jancholson said, if the primary butterfly's were taken off the shaft, they could be in wrong. There is only one way the plates can go back on and that is the original position. Otherwise it will prevent the throttle from closing properly.

Bruce(OR) · Jun 30, 2022

I'll throw my 2 cents in.
Looking at that pic, it appears your throttle is partially open.
Looking at that item, the vacuum chamber is built so that engine vacuum actuates that item pulling the idle up.
Once the engine is shut off, that part loses a vacuum signal and the throttle closes more.
Anti-diesel would seem to be the desired effect.
I disposed of dozens of Quadrajets many years ago.
Your application might not need this item.

Distributor to manifold vacuum source is a cheating trick.
It is a work around for spark knock without having to rebuild the engine.
With ported vacuum, You step on the gas, Vacuum signal increases, timing advances and the engine rattles itself up to speed.
With manifold vacuum, you step on gas, vacuum signal is less, timing advances slower and the engine gets up to speed.

As for carb adjust, the GM Lean drop is the best I have found. Radio antenna is good, or the door frame/handle is fair.
A glass of water is a good indicator and when you can balance the nickel, your doing good.

Need a Quadrajet guy

Fritz Maurer

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Janicholson

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MarkB_MI

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Steve@Advance

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Steve@Advance

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Fred47M

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rustred

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Fritz Maurer

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MarkB_MI

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UncleTom

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Bruce(OR)

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