1972 250C Emissions Vacuum Routing?
 

I have reached the point with my rehab of my 250C where I would like to get the advance-retard vacuum setup looking somewhat original even if I discreetly disconnect it.

I have no diagram on this - it does not appear to be on the CD, and the parts book is of limited help. The setup that was on the car when I bought it is cobbled together.

I have the two electrically controlled valves on the right fender and the distributor with advance and retard fittings.

Could someone with a 114 250 or 250C with reasonably original vacuum plumbing describe the routing of the lines and the colors?

I have but one vacuum source, on the front carb. So I think it goes:

Carb to"Y"
"Y" to advance switch - front switch?
"Y" to retard switch - rear switch?
Advance switch to front fitting on distributor
Retard switch to rear fitting on distributor

Any help greatly appreciated. If it's easier, call me at my 800 number and walk me through it, and I'll write it up for the Forum.

Thanks

Chuck

800 484 7933 x0713

Chuck,

I have a '72 250 and I'm reasonably sure the hoses are hooked up correctly, but it's a little different than your setup.

I have two control valves on the right fender - the rear valve has two hoses connected, both from underneath the valve and the front valve has three hoses connected - two from underneath and one on the top. The rear control valve/rear hose (hose closest to the cab) connects to the rear carb and the rear control valve/front hose connects to the distributor retard (connection facing the front of the car). The front control valve/top hose (Brownish in color) connects to the front carb, the front control valve/rear hose (red) connects to the distributor vacuum advance and the front control valve/front hose connects to the dashpot on my rear carb.

I have tried disconnecting these and advancing the timing etc., per suggestions on this board, but my car runs better with them hooked up. Hope this helps and is not too confusing.

Ryan

Thanks, Ryan!
 

Excellent description.

You seem to have the older (or at least different setup). My front vacuum switch doesn't have the fitting on top, just the two on the bottom.

So I am going to assume that I don't have the circuit from the front switch to the dashpot.

Does your car have the idle solenoid switches? Maybe they bagged the dashpot circuit, which would seem to support fuel shutoff, when they added solenoids?

Questions:

Are both the lines coming off the bottom of the front switch red?

What color are the lines coming off the rear switch?

Thanks again,

Chuck

I have researched this from several sources and it seems to me that there were a number of different emissions set-ups on these cars from 1970 - 1972. Mine does have the idle fuel shut-off solenoids on both carbs. The only colored hoses are the red one from the distibutor advance to the front vacuum control valve and the brown one that leads from the top of the front vacuum control valve to the front carb. All of the other hoses are your basic opaque "plastic" looking stuff. I believe the dashpot is supposed to increase idle when the engine is under load.

Ryan

That's what I thought ...
 

... but then why would they route it through the switch that is supposedly controlled by engine speed?

The Hayne manual says:

"Start the engine and increase its speed to 2500 RPM. Release the accelerator and observe the vacuum diaphragm capsule on the carburetor. The adjustment screw of the capsule (the dashpot?) should be in contact with the actuating lever at speeds above 1800 RPM. At lower speeds, the screw should be clear of the lever."

Chuck:

You have a double diaphram advance and a throttle postitioner. I will take a look at my Chiltons manual tonight (if I can find it) and see what the connections are. Two switchover valves, one for the advance and one for the retard, and I hope your's work -- the fance three-way one is NA.

Usually these are set up to retard timing at idle and on deceleration or when cold, then give full advance when the speed signal is above 2500 rpm or so. This will also activate the throttle positioner that keeps the throttle slightly open on decel to reduce unburned hydrocarbons.

Peter

Peter

No - I have two 2-way valves on the the fender. No 3-way.

Chuck:

I also had the 3-way and 2-way valves on my 71. However, the dashpot on the rear carb was routed to the vacuum pipe on the rear carb. The vacuum source for the 2/3 way valves was routed from the vacuum pipe on the front carb.

As psfred said, the dashpot was used to keep the throttle plates slightly open on decelleration.

My system was "supposed" to work (it never did) by utilizing the 2/3 way valves and relays and switches to advance the timing whenever either the A/C was turned on or the 180C switch on the manifold closed. This then increased the idle speed. Also, the vacuum advance was, I believe, inhibited until the 65C (I think it is 65C) switch closed as was total current to the REAR choke.

I had a diagram of my setup I made once and I had a vacuum diagram I copied from an old engine manual but since I scrapped the whole setup and installed Webers I don't have them anymore.

The vacuum source pipes on my Zeniths were below the throttle plates so that really produced more manifold vacuum rather than "s-port" which would be above the throttle plates. This meant that there was a high vacuum at idle/low speeds and none (or close to none) at high speeds when none of the vacuum stuff was needed anyhow. I think this might be part of the problem when people disconnect the stuff and run a vacuum line from either carb to the advance side of the distributor. You are pulling full vacuum advance at idle and it drops off as you increase speed. Opposite of "s-port" vacuum.

Not sure this helped you. I have a box of old valves and relays if you ever want to add to your "collection"!!!

Dan

So I went to the dealer yesterday and bought the vacuum line various Y fittings and new 3.5mm ID rubber hose for the connections.

Also bought the check valves to connect up the seat back and AC systems, but they are on special order.

I hooked up the emissions stuff as I described it in my first post and the car started and ran OK. Timing was about 3 BTDC with no vacuum and went to about 8 BTDC with the system connected.

This seems backward to me, and I am wondering if I have the advance and retard lines switched. I am coming off the front switch to the fitting on the distributor 'pot that is on what I would call the "front" of the distributor, even though it is facing the rear of the engine. The back switch is connected to the "rear" connection on the 'pot.

Opinions on this?

I did not have a chance to do much testing of the switchover functions as the car is on jackstands in the garage and I was pretty well "gassed" after a couple of minutes.

Also learned 'way more than I wanted to abour replacement vacuum tubing, but that's a subject for a separate post.

Dan,

I am going to double check my hose routing when I get home tonight, but I think my rear carb port is directly attached to the rear carb dashpot (vacuum throttle control damper). I may have misstated in my first reply to Chuck earlier.

I would be interested in those relays and valves if you still have them - it tounds like you had the same set-up as I do on my 72 250. The date of manufacture for my car was 7/71, so I am assuming that I have 71 emissions stuff on mine. I'm in OKC, probably wouldn't be difficult to get together. Thanks.

Ryan

Chuck,

I do have the Haynes manual where the 72 emissions system is described, if you want it, I can copy and mail or FAX it to you.

Ryan

I have the Haynes manual, thanks for asking.

Ryan:

No problem. PM me your mailing address and I will send a "box of goodies" your way.

Dan

Thanks Dan, I got the box today when I got back in town. Parts are exactly like mine - do you know where I can find emissions info on this model? Thanks.

Ryan

Ryan:

I have never seen any written info. A guess would be some "obscure" MB Tech manual or tech sheet that was produced much like the Carb and Ignition tech sheets. Maybe one if the indie MB shops might have one that you could copy. It might also be covered in the MB engine manual but I have never seen one of those, not sure there is a CD available on the engine either.

Sorry, not much help here. Since I decided early on I was going to run the Webers I was not interested in the emissions/vacuum/advance/retard/valve/relay stuff. I didn't try to find any info.

Dan

Ryan:

You might have already tried here but you might give Michael Rush at Silver Star Imports in Edmond a call. I don't know him but I work with his sister Angelia in Tulsa. Angelia says he works with a lot of older Benz's.

Webpage: http://www.silverstarimportsinc.com

Dan