Setting carb idle mixture screws with a vacuum gauge?

[LAC_STS]

I read you can use a vacuum port on an auto carb to set the idle mixture screws.

I obviously dont have a vacuum port on my marine Edelbrock but is the port on the intake manifold a vacuum location I could use? Im talking about the one that has a hose going to it from the starboard side valve cover. Has a PCV valve in there.


I tried doing it by ear but I dont think I am doing it right. By the time I can tell if the engine is stumbling the screws are way out or way in.

The best I got it to be was exactly 2 turns out which I was turning them to as a starting point since I had turned them so much.


I have a timing light that is a tach too. 2 turns out is where I get the highest idle. If I go in the idle becomes eratic and goes down. If I go out the idle does not increase. If I go way out it will stumble.

2 turns out is where the RPM stays nice and steady.

 

[Alpheus]

Re: Setting carb idle mixture screws with a vacuum gauge?

Ifs its running steady 2 turns out and is the highest RPM you can achieve by adjusting them. They are set...

Here is how I adjust them.

If you are planning on doing hole shots or pulling people and want to get up on plane as fast as you can. follow this procedure.

1 get engine up to normal operating temp making sure that the choke is open full.

2 Turn you air mixture screws all the way in till they just bottom out and turn them out again 2 turns.

3 adjust your idle to around 600 to 700 rpm, or as low and stable as you can get it.

4 You can do this by ear,vacuum gauge or with a tach.
Turn your air mixture screws out until you start to here or see (tach or vacuum gauge) the engine rev just a little and just when it starts to bog down a little turn in until max RPM is achieved. Do this with both needles one at a time.

If you are looking for better fuel economy and are just spending your days cruising around and not at WOT all the time follow the first 3 steps accept this time turn your needles in till the engine revs a bit and the starts to bog and turn out till you are at maximum RPM.

5 After your needles are set return the idle of your boat to around 700-750 RPM

The reasoning on this is just that the mixture curve is a little small but you can choose which end you want to run at. For hole shots you want to be at the rich end just for that little bit of extra fuel before the carb switches to the primary circuit. And for cruising you can run a little on the lean side as not to waste the extra fuel dumping at idle it also dosen't carbon up the plugs as fast.

 

[LAC_STS]

Re: Setting carb idle mixture screws with a vacuum gauge?

Ifs its running steady 2 turns out and is the highest RPM you can achieve by adjusting them. They are set...

Here is how I adjust them.

If you are planning on doing hole shots or pulling people and want to get up on plane as fast as you can. follow this procedure.

1 get engine up to normal operating temp making sure that the choke is open full.

2 Turn you air mixture screws all the way in till they just bottom out and turn them out again 2 turns.

3 adjust your idle to around 600 to 700 rpm, or as low and stable as you can get it.

4 You can do this by ear,vacuum gauge or with a tach.
Turn your air mixture screws out until you start to here or see (tach or vacuum gauge) the engine rev just a little and just when it starts to bog down a little turn in until max RPM is achieved. Do this with both needles one at a time.

If you are looking for better fuel economy and are just spending your days cruising around and not at WOT all the time follow the first 3 steps accept this time turn your needles in till the engine revs a bit and the starts to bog and turn out till you are at maximum RPM.

5 After your needles are set return the idle of your boat to around 700-750 RPM

The reasoning on this is just that the mixture curve is a little small but you can choose which end you want to run at. For hole shots you want to be at the rich end just for that little bit of extra fuel before the carb switches to the primary circuit. And for cruising you can run a little on the lean side as not to waste the extra fuel dumping at idle it also dosen't carbon up the plugs as fast.

Thanks.

I read that same post you made on an older thread. I tried that but by ear and looking at the tach I can turn them out 6 more turns from 2 turns out and it still doesnt sumble.

Thats why I wanted to use a vacuum gauge but I dont know where to hook it up.

 

[Alumarine]

Re: Setting carb idle mixture screws with a vacuum gauge?

Ifs its running steady 2 turns out and is the highest RPM you can achieve by adjusting them. They are set...

Here is how I adjust them.

If you are planning on doing hole shots or pulling people and want to get up on plane as fast as you can. follow this procedure.

1 get engine up to normal operating temp making sure that the choke is open full.

2 Turn you air mixture screws all the way in till they just bottom out and turn them out again 2 turns.

3 adjust your idle to around 600 to 700 rpm, or as low and stable as you can get it.

4 You can do this by ear,vacuum gauge or with a tach.
Turn your air mixture screws out until you start to here or see (tach or vacuum gauge) the engine rev just a little and just when it starts to bog down a little turn in until max RPM is achieved. Do this with both needles one at a time.

If you are looking for better fuel economy and are just spending your days cruising around and not at WOT all the time follow the first 3 steps accept this time turn your needles in till the engine revs a bit and the starts to bog and turn out till you are at maximum RPM.

5 After your needles are set return the idle of your boat to around 700-750 RPM

The reasoning on this is just that the mixture curve is a little small but you can choose which end you want to run at. For hole shots you want to be at the rich end just for that little bit of extra fuel before the carb switches to the primary circuit. And for cruising you can run a little on the lean side as not to waste the extra fuel dumping at idle it also dosen't carbon up the plugs as fast.

Great advice. Thanks.
Would this apply to an older 3.0 as well?

 

[6meter]

Re: Setting carb idle mixture screws with a vacuum gauge?

It only affects the idle circut anyways. Off idle it does zip.

 

[Alpheus]

Re: Setting carb idle mixture screws with a vacuum gauge?

Would this apply to an older 3.0 as well?

Yep. On any carb with idle/air mixture screws...

 

[Alumarine]

Re: Setting carb idle mixture screws with a vacuum gauge?

Yep. On any carb with idle/air mixture screws...

Thank you.
Although now that I just found out my brother flooded the engine compartment I have bigger problems than idle mixture.