Air mixture adjustment

[Kenneth Gear]

The air mixture adjustment screw on my car doesn't seem to have any impact on the car's idle or engine performance.  I can turn it all the way on or all the way out and there is no noticeable difference in engine performance or idle.

Can anyone offer a suggestion about how much impact this adjustment should have and more importantly how to remedy its lack of effectiveness?



Ken G
1971 280 SL
Silver/red

[Mark in KS]

If the idle adjustment screw has no effect on the engine, it means you have a leak in the vacuum lines elsewhere in your system.  The screw controls only air and leakage from other sources are providing intake air that you're trying to control with the screw.

I suggest that you tighten all hose connections that are related to the intake manifold.  Air leaks from the brake booster line are always suspect.

[erickmarciano]

make shure when you turn the screw that is does engage .like ia click when you turn.

1971 280sl
1962 VW
1954 Allstate vespa
1958 gs150 vespa
1962 gs160 vespa

[Ben]

quote:make shure when you turn the screw that is does engage .like ia click when you turn

.......I think you may be confusing the Air Screw with the Mixture Screw. The Mixture Screw on the back of the pump needs to be pressed in whilst turning in order to engage it, and a click is felt as you turn it.

This should only be done with the engine switched OFF !!

The Air Screw is on the inlet manifold near the hosepipe running from the air filter housing. There is no click felt when turning this.

It sound like an air leak alright. Tighten everything as suggested and if still no luck you can sprinkle soapy water over each joint to see if it get sucked in through a poor fitting !

Regards,
Ben in Ireland.
'64 230SL 4sp.

[A Dalton]

The first suspect if air bleed screw does not control idle/intake air is the slide valve is sticking open in the thermo valve. Take the air filter off the sensor and see if there is air being sucked in at this port. It should only bring in air when cold. When warm, there should be none. [above 65C, coolant temp]
 I always disconnect and plug the booster pipe when doing these types of diagnosis, just to eliminate that possible..

[erickmarciano]

sorry   confused....

1971 280sl
1962 VW
1954 Allstate vespa
1958 gs150 vespa
1962 gs160 vespa

[hands_aus]

How does the car run during warm up and normal hot running?

Does the throttle valve close completely?


Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[ja17]

Hello Ken,
I think a lot of us are confused. I assume you are talking about the large slotted air screw at the intake manifold.

My first guess is that the intake stop screw at the venturi is improperly adjusted. If it is holding the butterfly open at idle , you are leaking so much air into the engine, the air screw adjustment will make no difference.

The small slotted screw with the 10mm lock nut at the venturi (intake flap) is often mistakingly used for idle adjustment. This is factory set. The venturi flap (valve) must be allowed to close completely then the small slotted screw is used to just keep the flap from binding. It is then locked down with the 10mm nut. Adjust the linkage rods back to specs.

Check out the photo below.


Download Attachment: M130 Intake Venturi with text.JPG
53.6 KB

 Keep us up to date.






Joe Alexander
Blacklick, Ohio

[Kenneth Gear]

Thanks for the suggestions, just to be clear I am indeed talking about the large screw at the intake manifold.

Ken G
1971 280 SL
Silver/red

[Ben]

Then I reckon you should follow Joes advice, followed by A Daltons !

I've had similar issues and this is the route I went with success!

Regards,
Ben in Ireland.
'64 230SL 4sp.