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I have a Rochester Quadrajet 4MV carburetor that has 2 mixture adjustment pins in the front of the carb.

As I understand it, you tune these by continually tightening the mixture screws while watching a vacuum gauge. The vacuum will increase continually until it hits a peak value at which point it will begin to decrease. You find the point right before where it begins to decrease and that is your perfect mixture.

My question is: What if the vacuum never begins to decrease? I've seen it continually increase as I lean out the mixture up until a point where the vacuum will stay constant (but not decrease) all the way until I've bottomed out the mixture screw.

So, 2 questions:

  • Can I just consider that peak value I hit as the correct mixture even though I'd be tuning to the front side of the peak as opposed to the backside of the peak (since my vacuum never decreases, I don't know where the "backside" of this peak is).
  • I would think tightening the mixture screws all the way shut would kill the engine due to cutting off the fuel supply. Am I missing something? Do the mixture screws always allow some fuel to flow even in a closed state?

Update:

I noticed that the exhaust from the boat was awfully dark. I closed both mixture screws and noticed it still is dark in color. From what I understand, this means a rich condition. The carburetor is a new-ish (reman'd) carb just installed, thus my reason for tuning it. I'm lost at this point and have concerns about running it totally "lean" (although it doesn't seem lean). My inclination is to back off the mixture screws a 1 1/2 turns and call it good, but I'm also concerned at the thought of running it too rich, which it seems to be already. Any insight?

Update 2:

I found that the inside of the carburetor was cracked around the area where the mixture screws entered the barrel. This undoubtedly created fuel leaking into the intake that was uncontrollable.

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I believe you are going about it slightly wrong. Here's what I found out about the adjustment for the Q-jet:

Start by completely warming the engine and setting the idle speed where it’s comfortable, and the engine will still idle in gear without experiencing problems. First, check to make sure both idle mixture screws are adjusted equally. Next with the transmission in Park and a vacuum gauge hooked to read manifold vacuum, adjust the idle mixture screws in small increments until you can get the highest vacuum reading with the leanest idle screw setting. Remember, turning the idle mixtures screws in (clockwise) creates a smaller passage and a leaner idle mixture setting. With your foot on the brake pedal, check to make sure the engine idles comfortably in gear. If not, the carb will need further tweaking.

(From: StreetMuscleMag.com)

Realizing your boat does not have an "automatic transmission" where you can load the engine will make a little bit of difference. I believe where you are going astray is by wanting to keep adjusting the idle mixture expecting the vacuum to drop off. I believe you should actually be looking for the spot where it stops gaining vacuum. You state in your question you do see this point. The other thing is, take small incremental adjustments, doing both screws at the same rate. I think you'll get to your best mixture fairly easy as you already have the basics down for what you want to do.

I would think tightening the mixture screws all the way shut would kill the engine due to cutting off the fuel supply. Am I missing something? Do the mixture screws always allow some fuel to flow even in a closed state?

I'm not sure on the Q-jet if it will kill it entirely, though I doubt it would be running optimally. I think if you get to your high vacuum at idle and leave it there, you should be in good shape.

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  • "I'm not sure on the Q-jet if it will kill it entirely" It should unless the throttle plates are open too far and it is feeding fuel through the primary jets. – Moab Aug 24 at 15:25
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Qjets are bad about the secondary metering well plugs leaking fuel which causes a rich condition, this explains why vacuum never drops off. Most Kits include a metering well closed cell sponge to stop this. This is not a permanent fix as they eventually leak through the sponge, and needs replaced occasionally.

This is a more permanent fix to the problem but even these leak but last much much longer. Requires some skill to install.

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