Idle Mixture Screws does nothing..

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Bill Lawless
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Idle Mixture Screws does nothing..

#1 Post by Bill Lawless »

Hi All,
I was out for a drive yesterday and my engine has a fluttering sound when idling: The right Weber Carbs seam to act normal when I turn in the Idle Mixture screw, RPM's drop off and I back it up till it run faster.. When I turn in the Left Idle Mixture Screws, on Cylinder 3 & 4 they do nothing, even going all the way in??

Weber 40IDF Carbs,
Valves adjusted,
I have a Pertronix ignition. I remember reading that the distributor is the culprit when the Idle Mix screw doesn't react, Is this true with Pertronix?

Thanks Much,
-Bill
 Thanks,
Bill
65' 356 SC Cab Irish Green :D
82' SC Targa
81 VW Westfalia with 2015 Forester Engine
BMW 64'-R50/2, 75'-BMW R90/6, 74' BMW R90/6, Yamaha BW350

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Martin Benade
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Re: Idle Mixture Screws does nothing..

#2 Post by Martin Benade »

Off idle it runs perfectly? Maybe try turning the left idle speed screw in a half turn to see if that gets the mixture screws responding. If not maybe you have a pair of plugged idle jets, although I’m not sure how likely it is for them both to be the same.
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Bill Lawless
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Re: Idle Mixture Screws does nothing..

#3 Post by Bill Lawless »

Thanks Martin, It drives good, but at a low RPM cruise I still hear a faint fluttering, or not 100% smooth..
I have adjusted the idle speed screw.. When I increase the RPM's it's less noticeable but there.

I have checked the float level, but not in a while, maybe I'll pop the cover and check it again..

-Bill
 Thanks,
Bill
65' 356 SC Cab Irish Green :D
82' SC Targa
81 VW Westfalia with 2015 Forester Engine
BMW 64'-R50/2, 75'-BMW R90/6, 74' BMW R90/6, Yamaha BW350

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Bruce Smith
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Re: Idle Mixture Screws does nothing..

#4 Post by Bruce Smith »

Blow out the idle jets, make sure they're open.
Bruce Smith

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Wes Bender
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Re: Idle Mixture Screws does nothing..

#5 Post by Wes Bender »

+1 on Bruce's suggestion. If one side idle screw adjusts and the other side doesn't, it shouldn't be an ignition issue.
Some days it's just not worth chewing through the restraints.....

Olivier Auvray
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Re: Idle Mixture Screws does nothing..

#6 Post by Olivier Auvray »

Butterflies are higher than the first idle progression hole. By this way, fuel is coming out by the first progression hole and the idle mixture screw has near no effect.

On the drawing below, "A" shows the butterfly at the right position.
"B" shows a totally wrong position at idle.
Progressio-_Ports-_Diagram.jpg

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Martin Benade
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Re: Idle Mixture Screws does nothing..

#7 Post by Martin Benade »

Or the same symptoms with the plates in the right place and no fuel to adjust
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Re: Idle Mixture Screws does nothing..

#8 Post by Olivier Auvray »

Martin Benade wrote: Sun May 28, 2023 2:49 pm Or the same symptoms with the plates in the right place and no fuel to adjust
2 plugged jets..... it's not impossible but probability rules push me to think that butterflies are not at the right position: both are at the same level because they are on the same shaft.

We are gambling on the same table and Bill is the croupier: we just need to wait and he'll tell us what playing card will pop up!

Dave Erickson
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Re: Idle Mixture Screws does nothing..

#9 Post by Dave Erickson »

You did not mention anything about using a unisyn (or equivalent). If one side is pulling harder than the other, then the idle screws on the other side will have little effect.

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Re: Idle Mixture Screws does nothing..

#10 Post by Olivier Auvray »

Dave Erickson wrote: Sun May 28, 2023 7:45 pm You did not mention anything about using a unisyn (or equivalent). If one side is pulling harder than the other, then the idle screws on the other side will have little effect.
Weber are coming with air bypass screws: left carburetor can be with "open" butterflies & closed bypass screws / right carburetor can be with "closed" butterflies & open bypass screws.
In this case, you can obtain the same result on the unisyn.
Last edited by Olivier Auvray on Mon May 29, 2023 2:47 am, edited 1 time in total.

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Martin Benade
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Re: Idle Mixture Screws does nothing..

#11 Post by Martin Benade »

I suppose that only happens with a twisted throttle shaft, which is unlikely since I think it idled well recently
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Bill Lawless
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Re: Idle Mixture Screws does nothing..

#12 Post by Bill Lawless »

Just getting back on here....... Thanks everybody for all the info, I got a few things to look at....
But we are in the middle of moving and my unisyn gauge has already made the move so it will be a while before I try that, it was good last year :D
 Thanks,
Bill
65' 356 SC Cab Irish Green :D
82' SC Targa
81 VW Westfalia with 2015 Forester Engine
BMW 64'-R50/2, 75'-BMW R90/6, 74' BMW R90/6, Yamaha BW350

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Bill Lawless
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Re: Idle Mixture Screws does nothing..

#13 Post by Bill Lawless »

FYI, I got it, all 4 Idle Mix Screws do their thing properly now!!! Sounds Good.,
Now to figure out the Idle on my 911 SC, “That’s a horse of a different Color “ to quote the Great And Powerful Oz “

Thanks Everybody..
-Bill
 Thanks,
Bill
65' 356 SC Cab Irish Green :D
82' SC Targa
81 VW Westfalia with 2015 Forester Engine
BMW 64'-R50/2, 75'-BMW R90/6, 74' BMW R90/6, Yamaha BW350

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Wes Bender
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Re: Idle Mixture Screws does nothing..

#14 Post by Wes Bender »

Don't leave us in suspense. What fixed it?
Some days it's just not worth chewing through the restraints.....

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Bill Lawless
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Re: Idle Mixture Screws does nothing..

#15 Post by Bill Lawless »

Well, I did a few things:
Checked the Float Lever and that was right on.
I cleaned the Idle Jet's but don't think that was it, I also blew out the ports from the jets to the Mix Screw and I think that was plugged.. I sprayed WD40 in the jet hole and didn't see anything on the Idle Mix port till I hit it with the compressor..
 Thanks,
Bill
65' 356 SC Cab Irish Green :D
82' SC Targa
81 VW Westfalia with 2015 Forester Engine
BMW 64'-R50/2, 75'-BMW R90/6, 74' BMW R90/6, Yamaha BW350

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