Mixture adjustment query.

[sogood]

As mentioned in another post, I just recently splashed my refurbed Wellcraft Sportsman. The initial trip from launch slip to home mooring was not without incident, suffering some running problems on route. But I wasn't put out too much, happy that I'd get it sorted as time and tides allowed. So, in between work commitments, I got down to the boat and got my issues sorted, mostly caused by a sticking float needle, starving the engine of fuel.

I did the last of my tweaking today and intend to take a trip down the bay tomorrow and put her through her paces. But one little thing that I'm having an issue with, is adjusting the idle mixture. I had read several times, that this should be done with the engine at normal operating temperature, with the engine under load, in the water. I had no success when adjusting it running on muffs, which was to be expected.


But with conditions as stated above, warmed up to temp, engine running, etc. I get no change at all when adjusting the mixture needles. Does "under load" mean just running the engine at idle, or running in gear, tied up? I started with the needles closed down/ in, fully, then opened them up 2 1/2 turns, as given in various "how to's". Any amount of turning, in or out makes no difference to revs. Any thoughts much appreciated. The engine in question is a Mercruiser 5.7 with a Rochester Quadrajet carb. Thanks.

 

[BruceK]

There are two idle mixture and one idle speed screw.
If you are not getting any noticeable rev changes when adjusting the idle mixture you probably haven't closed the mechanical linkage enough. Set the idle speed screw in the water not the idle mixture AFAIK

 

[spannerman]

Tie the boat securely to the quay/dock and disconnect the throttle cable at the carb so there is nothing affecting the throttle plate. Start it and put it into forward gear and adjust the idle screw to give 650-700 rpm, then adjust the idle screws, they are air metering screws on that model carb so in is richer and out is weaker, opposite to mixture screws which meter fuel. You may need to readjust the idle during the process as the rpm will change. When you have it right it should idle around 650-700 rpm in gear, any higher and you will get a hard shift when you select a gear. If you need to kill the engine quickly for any reason just press the black plastic quadrant with the microswitch on the gearshift mechanism on the inside of the starboard riser. Finally reconnect the throttle cable adjusting to give a slight bias toward closed throttle.

 

[sogood]

Many thanks for that. I suspected that it had to be on gear, but wanted to confirm same. My idle is a bit uneven at the moment and I suspect a slight air leak, so new gaskets etc. are on the way. Will update in a week's time when I get back to it. Work keeps getting in the way!

 

[Seastoke]

Many thanks for that. I suspected that it had to be on gear, but wanted to confirm same. My idle is a bit uneven at the moment and I suspect a slight air leak, so new gaskets etc. are on the way. Will update in a week's time when I get back to it. Work keeps getting in the way!

Have u got a back up engine yet sounds like you need one

 

[sogood]

And how would you deduce that from such limited information?

 

[Seastoke]

From you having to be towed to your mooring last time out ,it just makes sense

 

[sogood]

Last time out was the first time out. A sticking float needle was the root cause of my problem. That's why I referred to it as a "sea trial". But I do take your point and I haven't ruled out the possibility of a back up motor.

 

[Seastoke]

Last time out was the first time out. A sticking float needle was the root cause of my problem. That's why I referred to it as a "sea trial". But I do take your point and I haven't ruled out the possibility of a back up motor.

Just a suggestion and a wise move if you go out to sea