Troubled by results of vacuum readings, effect of air/fuel mixture adjustment...

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May 8, 2022
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The engine is a GM 1984 5.7L V-8 with a Rochester quadra-jet carburetor. The vehicle is actually a 21' cabin cruiser but I believe the information I'm looking for would be the same regardless of the vehicle type. I replaced the original salt water damaged carburetor with a rebuild. They were both Rochester 4-barrels, same bolt pattern, same linkage... so that so far was a quick task. The only difference was the rebuild had a small nipple front where a hose attached. I believe for a PVC valve hook up. The old carburetor didn't have this. So I put a tube on it and plugged it. I backed both air/fuel mixture screws out 2 1/2 turns and it started right up, sounded great let it idle for 5 minutes or so and shut it down. To fine tune air/fuel mixture, I was told to get a vacuum gauge and watch the you tube videos. So I used the nipple in the front of the carburetor to get manifold vacuum instead of ported since it is constant.
I started the motor and the gauge instantly went to 30, buried it. I started closing the air/fuel mixture screws one at a time until both were all the way in. There was no change in the way the engine ran or the vacuum reading. (In the videos the vacuum was between 9 and 10in Hg and it increased or decreased while adjusting the air/fuel mixture). When I unplugged the gauge from the valve, the engine accelerated and that's where I shut it down. I'm obviously new to this. The first vacuum gauge I've ever had and still watching You tube videos but what I really don't get is how adjusting the air/fuel mixture screws had no effect at all. Not even when they were both turned all the way in. I had the idle adjustment set a little high but it didn't move either until the gauge was disconnected. I'm at a loss as to what to look at next and I would sure appreciate any suggestions or ideas you could throw this way.Thanks again
 
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Being in a boat you likely have a "marine cam" which is different-- it makes more and more power as the revs climb vs a car cam that aims for a flatter torque curve.

Your idle vacuum characteristics will be different because of this. But your boat manual should give some pointers.

That said I'm puzzled if your gauge is up to spec, I wouldn't know an engine to pull a "complete" vacuum.

Unplugging the gauge gave a vacuum leak which is why it accelerated. It's important to figure out if that's a "ported" nipple, that is to say, uncovered when the throttle opens from idle. It's possible you shouldn't have had any vacuum there, but the throttle blades are too far open at idle. You can see the hole for the nipple inside if you look down the throttle bore at its blades.

Since you turned your idle screws all the way in, it must be idling on the main circuit, which jives with the throttle blades being too far open at idle.
 
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30" (29.92") of vacuum is the max a vacuum can be pulled at sea level with 27" being usually considered the max. I am guessing the gauge is off.
One of the large ports on the front is to the canister used on cars/trucks or a bowl vent and should not be plugged, the one on the throttle plate is for the pcv and should have manifold vacuum. This is all from memory many years ago, look on the web for info on the ports.
 
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Send a pic of the gauge. I have never seen that amount of vacuum. On a Q-Jet, 2.5 turns from seat is a good starting point; you should see 16" to 20".
Set your timing first and set idle to about 750 RPM. Warm engine and make sure choke blade is vertical. So many people mess up Q-Jet choke operation.

With these things set, you should be close. Fine tune from there.
A ported vacuum port will not change idle speed if it is plugged or not. A full vacuum port will definitely change idle speed.

By the way, the vast majority of QuadraJet rebuilders will butcher them. If you want it done right, contact Lars Grimsrud.
lars.jpg
 

cnewton

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Thank you all for your input. First the gauge I just bought was in fact defective (it's starting point was 12" Hg, not 0) so too much worrying about readings that didn't matter. So I set the idle to 850, the air/fuel mixture screws are both set at about 2 turns out and the choke has got the throttle blades wide open when the engine warms. Haven't had it out yet under any load but more than happy with how it's idling. BTW, took the gauge back, exchanged it, plugged it into the same port I used earlier and got a steady reading of 22" Hg. Plugs are new and the timing is on so this is where I stop before I adjust something that keeps me from chasing blue fin tuna this weekend. Thanks again.
 
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