Weber mixture adjustment

[Paul Chapman]

Can some one please advise in which direction to turn the 4 mixture setting screws on weber carb's. to increase / decrease the amount of fuel that enters the engine. i e - does turning the screw in make the mix ' leaner ' and turning it out make it ' richer ' or is it the oppersite to this ?

Regards

Paul

[gjz30075]

does turning the screw in make the mix ' leaner ' and turning it out make it ' richer '

Yes. In for leaner, out for richer. This doesn't affect the entire rev range, you know.

Greg Z

[summerinmaine]

From a tech article:

When you are sure that the carbs are drawing the same volume of air, visit each idle mixture screw, turn the screw counter clockwise (richening) in small increments (quarter of a turn), allowing a good 5 - 10 seconds for the engine to settle after each adjustment. Note whether engine speed increases or decreases, if it increases continue turning in that direction and checking for engine speed, then the moment that engine speed starts to fall, back off a quarter of a turn. If the engine speed goes well over 1000RPM, then trim it down using the idle speed screw, and re-adjust the idle mixture screw. If engine speed decreases then turn the mixture screw clockwise (weakening) in small increments, again if engine speed continues to rise, continue in that direction, then the moment it starts to fall, back off a quarter a turn. The mixture is correct when a quarter of a turn in either direction causes the engine speed to fall. If that barrel is spitting back then the mixture is too weak, so start turning in an anti-clockwise direction to richen. During this procedure, the idle speed may become unacceptably high, so re-adjust it and repeat the procedure for each carb barrel.

ps. Full disclosure: I'm in the midst of rebuilding my carbs, so we'll find out tomorrow if I know what I'm talking about!

[summerinmaine]

Please also keep in mind that a pair of Webers means you have four separate "carburetors" where each pair in a single unit share a common float bowl. The result is that, due to individual cylinder requirements, and carb tolerances and prior history of use/abuse, the settings on each of the four will be individualized. They should be close to each other (due to Weber's excellent design/manufacturing practices), but could easily vary by a discernible amount and still be "right" for the particular cylinder.

[Paul Chapman]

Hello Greg / Jim

Thanks for the info. My S3 started to run a little rough so I adjusted the carb.sync. and idle speed at the weekend and it is now smoother however the all 4 plugs are now coated with soot so I suspect that the engine is now running rich. I intend to alter the mixture settings later this week to improve the colour of the plugs, does this sound like the correct way to proceed ?

Thanks again

Paul

[summerinmaine]

Hello Greg / Jim

Thanks for the info. My S3 started to run a little rough so I adjusted the carb.sync. and idle speed at the weekend and it is now smoother however the all 4 plugs are now coated with soot so I suspect that the engine is now running rich. I intend to alter the mixture settings later this week to improve the colour of the plugs, does this sound like the correct way to proceed ?

Thanks again

Paul

Paul,

First off, I'd trying running it for 15-20 miles, and then check the plugs again. There may have been left-over soot from your tuning exercise.

Mixture settings are certainly the place to start in my mind. And since it's a new issue, there's no reason to suspect a fundamental tuning/jetting problem, I wouldn't think. However, couldn't a sticking or worn needle valve cause the float bowl to overfill a bit and thus richen the mixture? I'd be inclined to check that as well. The needle valve and seat are part of a basic Weber gasket set, suggesting to me that wear is a regular issue. But, only one adjustment at a time, of course.

BTW having now torn down and rebuilt my Webers, it was interesting to see all the changes that had been made by the engine builder to accommodate the new cams and other tuning tweaks. I decided to fit #200 needle/seats instead of #175s as I believe I was suffering a bit of fuel starvation when running at WFO throttle. Now I'll just have to source some new starting jets, as I buggered them in one carb after trying to undo the work some gorilla did in torquing them into place! They still work though, so it should be fine for now.

[Paul Chapman]

[quote]However, couldn't a sticking or worn needle valve cause the float bowl to overfill a bit and thus richen the mixture? I'd be inclined to check that as well. The needle valve and seat are part of a basic Weber gasket set, suggesting to me that wear is a regular issue.[quote]

Hi Jim

Yes I will go for a short run and check the plugs again before making any mixture adjustments, that does make sence, and I have a couple of Weber sevice kits so I can also change out the the needle valves.

[summerinmaine]

Hi Jim

Yes I will go for a short run and check the plugs again before making any mixture adjustments, that does make sence, and I have a couple of Weber sevice kits so I can also change out the the needle valves.

Good luck! Let us know how it turns out.

[Paul Chapman]

Hello Jim / Greg

After a couple of hours this afternoon with a screwdiver and plenty of patience things are much improved and the S3 is back on song. I went for a 5 mile run to get everthing properly warmed up first and then adjusted the mixture by fully closing each screw then opening it until the engine speed stopped increasing then back in a tad, adjusting the idle speed between each one. I had three attempts at this, the first try resulted in the engine hesitating slightly to accelerate smoothly ( I suspect the settings were a little lean ), the second try resulted in the exhaust 'popping' a little on engine over run when changing down ( maybe to rich ? ). After a further tweak things seemed to be much better after a further 5 mile trip although I had to adjust the idle speed again to slow it down by approx. 500 rev's. The plug's are now a brown colour again except No3 ( still blackish ) so I have closed the mixture screw a further 1/8 of a turn.

I hope to take the Elan for a good run out this weekend weather permitting and will check things again.

Regards

[billwill]

I usually use a Colortune spark plug when setting the idle mixture.

In case you do not know what this is, it is a sparkplug with a quartz body, so you can SEE the colour of the flame.

[robertverhey]

I find that setting the mixture screws with the engine at fast idle (around 1500-2000rpm) provides a more useful outcome...at that point the carbies are progressing from idle jet to progression holes, on their way to main jets....so getting the mixture just right at that point can optimise "pick up" when accelerating....there might be a slight trade-off in terms of idling smoothness, but it's usually pretty close either way
Robert

[StressCraxx]

Get all four in sync, makes a big difference.

I find using the colortune according to the instructions tends to make the idle circuit run lean, popping back through the carbs. So I let the mixture go to bunsen blue, then back mixture screw out til it turns yellow. Low speed circuit seems to be more driveable that way. It takes patience. Let the carbs settle with each small adjustment.