Fixing up the W108

[mosu]

I've decided to keep my W108 stock and build/fix on what came from the factory. Lots of coin flipping, but mostly a hard look at my finances led me to this decision.

So....here we go!

As it stands today, it makes some noise, leaks some oil, smells a little funny and generally looks at me with those puppy eyes and says...."fix me!!". I'll document my progress as I go along on this thread. Comments, help and encouragement are always welcome.

First step was to pull the plus and run a compression test again. Let's see where we're at. I haven't run it for two weeks.

Plugs are whitish with hints of brown. Looks like it's running lean.



Next come the compression readings. VERY encouraging given that the motor is cold and has 89,000 miles. And it sat from 1986 - 2010.

Cylinder 1 - 165 psi

http://www.peachparts.com/shopforum/attachments/vintage-mercedes-forum/126746d1420602900-fixing-up-w108-cylinder-1-compression-cold.jpg

Cylinder 2 - 165 psi

http://www.peachparts.com/shopforum/attachments/vintage-mercedes-forum/126747d1420603020-fixing-up-w108-cylinder-2-compression-cold.jpg

Cylinder 3 - 150 psi



Cylinder 4 - 150 psi

[mosu]

Cylinder 5 - 150 psi



...and cylinder 6 - 170 psi



Consistent readings with ~10% variance between lowest and highest. Rings are probably in good condition. The factory manual doesn't list the absolute values. I'm guessing that 150-170 psi is inline with a compression ration of ~8:1 and little to no valve overlap. Does anyone know what good numbers for the carbed M130 should be?

Next big job will be to clean up the engine block. It's just filthy. If there are any active oil leaks, they're impossible to track down.

[mosu]

But, before that, I wanted to test the fuel pump. The car appears to run the carbs dry when run at speeds above 60-70 kph, or up a big hill. If I crank it and idle for a minute to fill up the bowls, it's good to go again. Funny to see the perplexed drivers behind me for the first few times, but it's getting annoying.

Sorry, no pics for this one.

Vacuum gauge on the pump inlet shows a suction of 270 mmHg. Factory spec is 230-320 mmHg. Checks out.

Outlet shows a consistent pressure of 4 psi at cranking speed. The manual calls for 1.76 - 2.65. Hmm, this might be an issue, but I'm not sure what the Zeniths want to see.

At running speed, the fuel return valve is stuck open. Does not close regardless of RPM. At this point I'm not sure if either of these condition contribute to the problem. The plan is to have the Zeniths professionally rebuilt and see how it behaves. The real issue is likely a blockage in the line from the tank. All the hoses in the engine bay are new. I'm not sure if the pump inlet and carb return are connected to the correct hard line. One more thing to double check.

[JeffreyNMemphis]

Does your fuel return valve have a lever that pushes a pin into the valve with throttle on? Is there an adjustment for the lever? The fuel return valve has a diaphragm inside. Are you confident that it is working correctly?

[mosu]

It does and the pin is seized in the open position. Fuel return valve is stuck full open.

Does this have significant effect above the closing rpm?

[cth350]

it means the bowls won't fill fast enough.

Carefully disassemble the valve and soak it a bit in something that removes gunk. (kerosene; simple green, etc).

The diaphragm is likely kaput and needs to be replaced. It's not cheap, so don't help it along by flexing it by hand.

Don't break the bronze housing by over tightening it on reassembly. Hand tight until it doesn't leak is fine. The two crush washers that surround the banjo fitting can be reused if they are not overly damaged from the last guy overdoing the tightening.

-CTH

[JeffreyNMemphis]

Originally Posted by cth350

it means the bowls won't fill fast enough.

Carefully disassemble the valve and soak it a bit in something that removes gunk. (kerosene; simple green, etc).

The diaphragm is likely kaput and needs to be replaced. It's not cheap, so don't help it along by flexing it by hand.

Don't break the bronze housing by over tightening it on reassembly. Hand tight until it doesn't leak is fine. The two crush washers that surround the banjo fitting can be reused if they are not overly damaged from the last guy overdoing the tightening.

-CTH

The bronze housing is soft metal that breaks if over tightened. The replacement part was $259.00 five years ago! Luckily I had an extra one to replace the one I destroyed.

By wide open, you mean pushed all the way in? I would think that wide open would not effect your full throttle fuel delivery. The valve is intended to return fuel to the fuel tank or limit the amount of fuel flow to the carburetors when you let off the throttle and at idle. If it was stuck in the out position I could see fuel flow as an issue at wide open throttle. Another possibility is that valve is pushed in, but clogged so fuel is not flowing.

[mosu]

The pin is jammed in the fully lowered position. Had it disassembled and tried to free it with all manner of gentle persuasion (really just WD40 and carb cleaner) to no avail. I didn't insist further.

At around 2000 rpm, there's a steady stream of fuel coming out the return hose. So pin-out all the way is open valve, correct?

[JeffreyNMemphis]

Can you show a picture of the valve? I want to make sure we are talking about the same thing. I thought pin pushed in occurs when throttle is pressed on. Easing off of throttle would allow pin to go back out, restricting flow.

[mosu]

Here it is. The pin is stuck in that position. The little lever below it spins with difficulty. I couldn't find a position for it where it moved with the throttle.



Does that crudilicious carb need a rebuild or what?

[JeffreyNMemphis]

That pin should be pushed out with a small gap between pin and lever at idle. When you push throttle the lever should push pin inward opening fuel flow.

[mosu]

Interesting - so fuel flow is restricted with the pin down and the throttle pushes it up to close the return line. My pin is so stuck in the return position, it's not even funny.

I'm trying to source cores in better condition for a rebuild or have this one done professionally. Good to know that the fuel return affects running so much.

Is the ~4-5 psi inlet fuel pressure too high for Zeniths? I haven't found a good spec in the manual.

[JeffreyNMemphis]

I have a set of Zenith carbs in storage. I will see if the pin in my valve moves correctly. That valve is also used on the Solex twin carb system.

[n10]

From what I know that little "thingy-lever" circled in red should be under that metal pin, not above it. At least that's how mine was when I ran Zeniths. With it like it is there would be no leverage to push up the return button



But if your return button is stuck it doesn't matter

[cth350]

just checked the pair of cores sitting behind me (don't ask). yes. that flat spring needs to be below the pin on the linkage.

-CTH