Adjusting Idle Mixture Screws (Updated)

[181Mariah]

1993 4.3LX with a Weber 4 barrel, Looked in manual and can not find where it tells how to adjust screws. Does anyone out there know how to adjust these screws.
Thanks Paul

 

[fuzzybob]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

Paul, service manual 18 section 5C pages 12 and 13. You can download the manual in the "Adults only" sticky at the top of the forum....get the one for the outdrive too. It's a free download. Good luck

 

[dr_bowtie]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

to "correctly" adjust them you turn them in all the way and then back them out 1.5 turns...then you use a vacuum gauge hooked to full time vacuum and adjust the screws individually with the engine running to achieve the highest vacuum...simple as pie...

 

[181Mariah]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

You guys are good, I will need to get a vacuum gage from harbor frieght......
When adjusting the screws, can this be done in the drive way with the engine at idle or what RPM's?
Thanks

 

[dr_bowtie]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

you adjust the idle mixture screws at idle...

be sure to use your muffs or water barrel......

 

[a70eliminator]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

With engine idleing screw one screw in till the engine begins to run rough then back it out just till it smooths out no more, then do the other one the same, now go back and forth a couple times till you have both screws out just past the point of engine running rough, you can then give them another 1/8 turn out just to keep a little on the rich side, I have tried a vacuum guage but honestly I do better just listening closely to the engine, if the guage shows a drop in vacuum you'll also detect a difference in the engine sound, you can even hear the carb whistle pitch change if you listen close enough.

 

[181Mariah]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

Thanks Eliminator,
Just did the carb tune and taking out at 7pm tonight. I will let you know how it goes. Also did a full tune up and new water separtor. Everthing was shot.

 

[bruceb58]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

You do realize that this only affects your idle right? Has nothing to do with how it runs off idle.

 

[181Mariah]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

I see, was thinking he was talking about the mixture screws. Back to the drawing board.

 

[bruceb58]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

Mixture is set with the jets in the carb. These are non adjustable but can be physically changed to different jets. What makes you think you need to adjust the mixture?

 

[a70eliminator]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

I see, was thinking he was talking about the mixture screws. Back to the drawing board.

Idle mixture screws correct, other than your fast idle stop adjustment what else is there, the high speed jets are fixed, you can change them but thats a whole different ball game unless you have gross mismatch just leave them alone, webber does a pretty good job at engineering the CFM match up.

 

[181Mariah]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

When we first got the boat two weeks ago it would take off (4300 RPM) and then stall to 3000 RPM after a minute. After I pulled the cap I noticed that it was very bad. I then pulled the plugs and 4 of them were carcked and set at .010 and one of them broke off when I pulled of the plug wire.
When on the lake I was thinking that it was the carb because the boat sat up for awhile. I tried adjusting the two screws thinking this would help. The boat has 237 hours on it, it must have sat up for a few years for a 93 boat. That would only be 14 hours a year.
I was going to give it a test run tonight but a storm kicked up. I will try again in the morning.

 

[dr_bowtie]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

did you check the fuel filters too...?

reving to full RPM and then backing off usually is a fuel delivery issue....

 

[181Mariah]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

Took the boat out today and it would not go past 3500 RPMs. New fuel filter, anti-siphone valve, complete tune up, new fuel pump. Last week the boat ran 4300RPMs without the tune up but would cut out after 45 seconds under load to 3500 RPMs. I am thinking the carb might need rebuilding. Any other idea's what it could be. Maybe plug wires or coil?

 

[bruceb58]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

Did you look at your timing to see if it was advancing correctly? I believe you have a pointless ignition right?

I would suspect carb as well.

What is the age of the gas in the boat?

 

[jtybt]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

Sounds like fuel flow problem. Either floats set too low(less likely), or some reason fuel isn't getting to the carb. Could be loose fittings in fuel line or other air leak, sucking air instead of fuel at high demand or restriction somewhere. Anti-siphon valve is a point of restriction as is pick-up tube.

 

[181Mariah]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

I was thinking about the timming as well, I am going to check in the morning. Yes it is pointless ignition.
I removed anti-siphon and put in a stright thru fitting on the water today and it did not make difference.
I am lost as what it could be.....

 

[bruceb58]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

How old is the gas?

 

[181Mariah]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

Gas is good, I ran 4 tanks thru it in the last two weeks with 93 octane. I also removed the pick up tube and checked the screen. The fuel pump was replaced from previous owner before I purchased the boat. I would test it but the fuel line is hard tubing and I do not want to cut it.
I just test the coil and it shows 11 ohms resistance.

 

[bruceb58]

Re: Adjusting Idle Mixture Screws

Re: Adjusting Idle Mixture Screws

11 ohms is way to high. How did you test it? You took all the wires off first right?

You can take the hard line off and replace it with a rubber line temporarily if you want to do a pressure test. the only problem would be finding the fittings. Probably worth rebuilding the carb first though.