Idle mixture screws only improve vacuum even when bottomed out.

[ryno1234]

First off, full disclaimer - this isn't about a MerCruiser, I have a Yamaha Stern Drive but my question would be universal and I know MerCruisers had Rochester Quadrajets on them like my Yamaha does... This forum has WAY more eyes on it.

I have a Rochester QJet 4MV. I just bought it from a remanufacturer, so it's "new"(ish).

When adjusting the idle mixture screws, my vacuum gauge showing the manifold pressure only continually shows higher vacuum the more I lean out the mixture. This continues all the way until I bottom out the mixture screws. Oddly enough the engine *sounds* rougher, but the vacuum continues to improve. Backing off the mixture screws, the engine smooths out just a bit, the vacuum drops just a tad.

All the while, my exhaust is black (although less black the leaner I go, but still...) These seem like signs that I'm running too rich.

Are you supposed to be able to bottom out the mixture screws and still have the engine run? I would think it would kill the engine (unless fuel can come from some other place I'm not aware of).

Any tips?

 

[kenny nunez]

Look down at the Venturi boosters, is there fuel bubbling out of the brass jet? If so then the idle stop screw is holding the throttle blades open enough to start the next circuit. Be sure that the ignition timing is around 6-8* at 5-600 rpm. Usually a Qjet likes to be 3-5 turns out from seat with around 15” vacuum. The metering screws should cause the engine to stop when seated. Are you sure that you have the correct gasket under the carburetor? Is this the correct marine carburetor?

 

[ryno1234]

Hi kenny nunez , thanks for the response.

I am not seeing an fuel bubbling out.

I'm not sure what my ignition timing is but I've ran the boat like this for several years and my last carburetor was this exact model number. I just bought a new one to replace the old one and I never had these issues.

I should also note that if I take my RPMs up to 1,000 or so, the black smoke goes away. I can still see some smoke, but its not as thick and its hard to tell if its more gray or black, but looks gray to me.

I used the gasket that came with the carb, so I assume it is the correct one.

I'm not opposed to taking the carb back off and looking at it further if there is something I'd be able to visually tell.

Last thing: I did tighten down the mixture screws decently hard the first time I did it. Probably harder than I should have because I noticed that they would continue to turn with a little pressure. Is it possible I ruined something that would cause this?

 

[kenny nunez]

If you damaged the seat by over tightening the screws then you cannot control the idle mixture anymore
With the carburetor removed flip it over and examine the area where the needle points of the moisture screws protrude and check for metal pushed out.

 

[ryno1234]

kenny nunez

Attached are some related photos after taking the carb off this evening.

It does look like the mixture screws have caused a protrusion in the casting and also looks to have some cracks.

Is this the likely cause?

Also, I noticed my gasket which mates between the carb and intake manifold doesn't cover the full base of the carb. You can see in the photos there is an exposed area. I'm not sure if that is an intentional design element or not.

 

[Baylinerchuck]

Wow, not sure I’ve ever seen idle screws pushed out that far.....yikes.

 

[kenny nunez]

Obviously what started the problem was the wrong gasket that caused a vacuum leak. KD makes a tool that fits the metering screws that is basically a length of speedometer cable with sockets on both ends for the different metering screws and a hexagon handle. I used it just the other day to set the mixture on my jet boat. I probably bought the tool 30 + years ago. It is a must have if you have a Qjet.
The bottom line now is you need another carburetor.

 

[GA_Boater]

Compare the old gasket to the new one, that will answer the question about being the right gasket.

Order another carb. You can try to return it, but I don't think you will be successful.

 

[ryno1234]

Reflecting back on it, I don't think I tightened both screws down so tightly, but both show damage. I think this may have been preexisting. Luckily the company I purchased the carb from was willing to take it back.

Thank you all for your help and suggestions.

 

[Lou C]

If you are going back with another QJet the gasket I have always used is a lot thicker than one of those paper gaskets. And while some Qjets have mixture screws with a slot, mine has these d-shaped ones that require the tool that Kenny is talking about. Mixture screws should be lightly seated that's all. If they are stiff going in then find out why. Sometimes if the boat was was used in salt water you can get corrosion in the threads but you can use penetrating oil to free it up with no damage to the threads. In addition do not overtighten the mounting bolts/nuts for the carb, just get them snug, if you use the right gasket it will seal just fine. If you overtighten them it will warp the air horn and then it won't seal. Even though these carbs have been out of production a long time you can still get most of the parts for them, the key is starting with a good core in the first place. That's the problem with QJets now, there are not a lot of good cores left and some of the rebuilt units are made up of several different carbs but not really right for your application. If you can't get a good Qjet might want to try an Edelbrock instead, at least that way you can go new. That's what I would do...because some of those reman carbs will never work right, and chronic carb problems will sour you on boating pretty fast. So for new your choices are Edelbrock vs Holley and most seem to agree that the Edelbrock may be less finicky than a Holley.