No Vacuum at the Venturi Control Unit

[bayleif]

I have been trying to get timing, idle and throttle linkage correct on my 1969 280SL manual transmission and I have discovered a problem that leads me to believe that I may not have all of the correct parts for my car.
First: When setting up the throttle linkage, I started at the venturi control unit per the procedure. With the linkage disconnected, I adjusted the throttle valve stop to the point where "the throttle valve should grip slightly without binding". My problem is that it never "grips slightly". I can back the adjustment screw completely out and the value does not "grip".
Second: There is no vacuum measured at the vacuum bore for the timing advance.

I removed the rubber boot between the air filter and the venturi control unit. Using a mirror, it looks to me like there is a very slight gap all the way around between the throttle valve and the venturi body. Is is possible, that I have the incorrect throttle valve? I know the venturi control unit was redesigned at one point. Could it be that the size of the valve is slightly different? This wouldn't be the first incorrect part I've discovered. It looks like the valve is connected to the shaft with two screws. Can I get just the plate part of the valve? I haven't seen any parts breakdowns that get more detailed then just the venturi control unit. If so, does anyone know the correct part # and nomenclature (so I don't have to use highly technical terms like little metal flapper thingy).


Chuck Bartlett
1969 Signal Red 4 Speed

[ja17]

Hello Chuck,
Be sure to check on the other side of the venturi. Some USA cars have a vacuume cell which acts on the linkage and holds it open when the engine is off ao rpms are low. Disconnect the linkage and/or remove the vacuume cell to check the venturi valve. Check to see if the flap will close enough to bind in the intake venturi, then slightly adjust it so it will not bind. These venturi units were all this way.


Joe Alexander
Blacklick, Ohio

[bayleif]

Thanks Joe,
By a vacuum cell, I take it you are talking about a unit that mounts to the side of the ventrui control unit at or near the place where the linkage is attached. I've seen pictures of this in one of the books I have, but my car doesn't have one. I can disconnect the linkage at the venturi unit and verify that as I back off the stop screw the ajustment rests all the way against the stop and still there is no resistance to opening the flap.

Chuck Bartlett
1969 Signal Red 4 Speed

[ja17]

Try forcing it closed just a bit, with the slotted screw backed off, and see if it sticks a little then.
The 10mm nut is just to lock the slotted screw in place.

Joe Alexander
Blacklick, Ohio

[bayleif]

Hi Joe,
Thanks for your responses. I tried attaching a picture (this is my third attempt) I hope this works. It shows the position of the throttle flap shaft against the adjustment screw stop. I don't think that I can get it any closer then this. Even at this position, there is no resistence to opening the throttle. Looking into the throat of the venturi, it appears that there is clearance around the entire flap.
From experience I have concluded that the previous owners mechanic was not overly concerned in making sure that all of the parts he/she used belonged to a 280SL

Download Attachment: throttle.jpg
13.71 KB

Chuck Bartlett
1969 Signal Red 4 Speed

[George Davis]

Chuck, don't know if this will be helpful or not, but after re-reading your first post, I wonder if maybe there are two problems: too small throttle plate, and a blocked vacuum port.  You said you get no vacuum reading, but you should get a vacuum reading even with the throttle mis-adjusted or undersized, unless the vacuum port is blocked or in the wrong location (as it would be for engines with vacuum-advance distributors).  I suggest making sure the vacuum port is clear, and that the vacuum port hole is behind the throttle plate (i.e., on the manifold side of the plate).

I tried looking up a throttle plate part #, but that section is inconveniently missing in my parts manual.  I can only suggest contacting a dealership to see if they can find something, or used parts dealers to see if they can supply the entire venturi control unit for a reasonable price; they probably won't want to sell just the plate.

Good luck!

George Davis
'69 280 SL Euro manual

[W14]

Hi Chuck,

On my '65, the "throttle valve lever" is fastened to the "throttle valve pivot" by a pinch-clamp.  You can loosen this clamp, and re-adjust the position of the lever in relation to the throttle valve.
I am sure this is normally not meant to be adjusted, but in your case you have nothing to lose.  This will at least tell you whether you have the correct size throttle valve.  Who knows, if the previous owner wasn't that particular, perhaps he may have fiddled with it.
You may want to mark it first, so you can return it to the present position.
My throttle valve has a number "8" stamped in the front face (about 6mm tall).

Will W14
1965 230SL 4spd
Victoria,BC

[ja17]

Hello Chuck,
The whole venturi unit is easily removable. Maybe you should remove it and inspect it carefully. Try holding it up to the light and see if the flap is really not closing.


Joe Alexander
Blacklick, Ohio

[bayleif]

I just thought I'd update everyone with what I discovered. With snow on the ground and no place to go, I finally removed the venturi and checked it out. Will, you were right. Adjusting the pinch clamp on the valve pivot shaft did the trick. There was a line scratched on the shaft; evidence that a previous mechanic tried to get it right. He was just a little bit off.
I cleaned everything with carburator cleaner. The passage way to the vacuum port for the retard was plugged. I then adjusted the valve stop so that the valve just binds, re-installed the unit, and then adjusted the linkage shaft that attaches to the venturi. This I think should maintain the throttle linkage adjustments.
Now, if the roads around here ever dry up, I'll take it for a spin

Chuck Bartlett
1969 Signal Red 4 Speed