4 bbl rochester carb mixture screws

[yvrmarc]

I am sure that this is a simple question, but does anyone know where the mixture screws are for the following carb? (see pic below or https://www.mysticmarinediscounts.com/wp-content/uploads/2014/08/SIE 76151.jpg ). My Mercruiser 7.4 is running very rich and I need to adjust the mixture. There is one screw located on the bottom of the carb on the port side does nothing when I adjust it at idle. I have looked all over the carb and cannot find any other mixture screws.

Any help is greatly appreciated.

 

[achris]

 

[Silvertip]

If it is running rich at idle then that's what you adjust. If it is running rich off-idle to WOT then there is another internal issue.

 

[NHGuy]

You'd be smart to get one of those carburetor adjustment tools at an auto parts store. They usually have a springy extension on the end of a screwdriver handle. They usually have a few tips for the common carb adjustment points.
Start out with the mixture screws VERY SOFTLY seated, then open them 3 turns. Start and warm the engine. Adjust each side mixture screw down til it stumbles, then out til it runs poorly, then split the difference. Go back and forth making these adjustments and resetting the idle to 725 or so til everything works at 725 rpm.

 

[achris]

Leave it on the rich side of the best setting. What that means is you'll find a 'dead zone' of screw adjustment that makes very little difference. We're only talking about 1/8 turn, but leave it on the rich end of that dead zone....

 

[yvrmarc]

Thanks to all for your responses. I will head to the boat tomorrow and see if I can find the mixture screw on the starboard side as pointed out achris. To date I have not been able to find it. This carb is not like the other Rochester carbs that I have seen with the two mixture screws so evident on the front of the carb. If I am able to find the starboard mixture screw I will purchase the adjustment tool and follow the instruction above. I will report back with my findings.

Thanks again

 

[NHGuy]

The starboard one is hard to see on my motor too. It's below the fuel inlet and since the carb is black I needed a good flashlight when I first looked for it.
BTW I agree with Chris on the slight richness on your mix adjustment.

 

[yvrmarc]

Hi Again

I am hoping that I can impose on you guys one more time. I found the starboard mixture screw! Thank you!. I got the carb adjustor tool and was able to seat the mixture screws which in both cases were over 5 rotations. I brought it to 3 rotations and tweaked it a bit more while under load in river. That seemed to have solved my mixture problem. However while we were running it in the river we opened it up to about 3500 rpm but it choked out and almost stalled. Upon investigation we notice that the secondary air flaps were not opening. We hooked a vacuum gauge up and had really good vacuum but the vacuum-diaphragm (pod) on the starboard side of the carb does not seem to be opening the flaps.

Is there an adjustment that I can do to the vacuum-diaphragm so that I can get these flaps to open? This is a brand-new carb so I do not expect there to be a problem with the vacuum-diaphragm.

Thanks in advance

 

[achris]

The secondaries open from a lack of vacuum, not in the presence of vacuum. When you open the throttle hard the vacuum becomes less, not more.

Check the model of the carb and make sure it's the correct one for that engine... You didn't give a year, so I can't tell you what is should be (engine serial would be better)...

Chris...

 

[NHGuy]

There is another possibility too. On the left side in your picture, starboard in the boat, is a round vacuum breaker. Some call it a pot. It is to help open the choke a bit when the motor is running and cold, but it has another function, it's supposed to keep the secondary air doors from opening during high vacuum (low rpm) conditions too. If it's sticky it could be part of your issue. With the engine off you should be able to open the air doors with your finger, they are spring loaded and should close when released. Try that, and try them with the motor running at the dock idling too. They should not open manually at idle.

They only open (correctly) at higher RPM when the suction vacuum of incoming air overcomes the resistance of the spring. At that point there's little measurable vacuum in the intake so the pot is open and allows the doors to work. That's why as Chris said there's no vacuum at that point.
I wrote out a post a while back about how to set the tension on the secondary flaps. It was very long winded and detailed. I'll go look for it.

 

[NHGuy]

To set the idle mixture. With correct idle speed and timing warm the motor til the choke stays open. It's ok to help it open as long as it stays open when you let go. Turn your mix screws in one at a time til the motor stumbles, then back out around half or 2/3 turn. My motor works best that way. A smaller amount of idle fuel won't use as much gas, won't blacken plugs and won't dirty your transom as fast. If it won't holeshot well, add a little more fuel to the idle mix.
If your secondary air spring is not tensioned right the boat won't go fast, it can give you the dreaded quadrabog. The spring that closes the secondary flaps should be set 3/4 turn beyond slack.
To adjust it on the motor you need teeny tools and patience. The tensioner bolt is turned with a torx bit. I think it's a 1.5. You can buy a set of security bits at Harbor Freight for $9 or something. That tensioner bolt head faces straight down on the starboard side of the carb at the rear. The tension is adjusted by turning a slotted cylinder in the starboard side of the carb. It looks like a straight blade screw that's sunk in the side of the air horn. Clockwise tightens it, CCW loosens. You loosen the tensioner bolt while setting the spring tension on the slotted pivot with a small straight blade driver.
Here's where it gets sticky, You MUST put the small screwdriver in the adjustment slot BEFORE loosening the tensioner bolt because if the adjuster goes slack the spring will fall away from it and will cause you to need to remove the carb and flip it over to re catch the spring on the adjuster. Don't ask how I know!

 

[yvrmarc]

This is great information - thank you. I went down to the boat today and started the engine after tuning the mixture screws last weekend. I thought I had it sorted out last weekend until I started the boat today and a huge plum of black blew out the outdrive exhaust and it was running really rough. So I started all over, this time with a vacuum gauge ... I warmed up the engine until the choke was wide open, turned the mixture screws until they were seated and at that time the engine stumbled a bit and PSI was at 10. I turned them back a bit and it seems to run smoothest at 13 PSI, although turning the screws out multiple turns did very little to the engine sound or vacuum. Also, the only place that I could find to attached my vacuum gauge was to pull the rubber hose off the vacuum-diaphragm (pod) on the starboard side of the carb and attach it to that ... I figured that the vacuum-diaphragm had finished its job as the choke was now wide open. Is there a better place to connect my vacuum gauge?

I should tell you that the engine is a 1989 Mercruiser 7.4 / Bravo 1 which I pulled from the boat and had professionally rebuilt. They replaced the carb with a brand new rebuilt version of the one pictured above. A couple more questions if you do not mind:

. I currently have exhaust coming out of the valve cover hoses causing a bad exhaust smell from the engine compartment. Is this normal for a new rebuild with only about 20 hours on it (very little of which was under load).
. I no longer have the blue exhaust after adjusting the mixture screws to their present position, but I do have a considerable amount of grey exhaust ... much more than before. Is this the norm for this engine type?
. Lastly, does anyone know the ideal vacuum PSI for these carbs?

Firstly I really want to get the smell of exhaust minimized as it overcomes the whole boat when cruising. Thereafter I will sort out the secondary's as I think your are correct that they are tensioned too tight (great instruction NHGuy ... I know exactly what you are referring to about tensioner bolt)

Thanks again to everyone for this great feedback

 

[NHGuy]

Yah that's not where to get your vacuum signal. It's vacuum, but it's subject to some carburetor input. Find a plug below the carburetor or on the intake manifold. If you have an iron manifold the plugs can be tough to remove. And once removed you will need to make up a plug with a vacuum barb of some sort. But vacuum testing is best. Highest vacuum at correct idle and timing is your goal.
Here's a link I have used, http://www.gregsengine.com/using-a-vacuum-gauge.html there are plenty more though. Search vacuum testing an engine.

 

[NHGuy]

If the engine ran with both mixture screws seated something ain't right. If you start with 3 turns off the seats and turn one way in, the engine should miss and hiccup pretty badly. Was the timing verified and idle down low too?
There should not be much smoke in the exhaust once the engine is warm and choke is open. But a small bit of dark grey, almost black exhaust is normal.