1971 250C fuel return question - Mercedes-Benz Forum

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post #1 of 19 (permalink) Old 03-22-2008, 01:46 PM Thread Starter
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1971 250C fuel return question

Hello, I started my zenith rebuild today. When I took apart my fuel return valve I found that somebody had plugged all the holes with solder. It is plugged tight.
1) Why would somebody do this?
2) Is there a remedy besides buying a $200.00 valve?
3) Is this why my car died after about 10 miles when I bought it? It acted like it was out of gas. Vapor lock???
4) Why would somebody do this?
Also where does that little blue rubber cup go that came in my rebuid kit. I'm only part way done so maybe I haven't got there yet.
Thanks, Joe
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post #2 of 19 (permalink) Old 03-22-2008, 02:17 PM
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Fuel return valves...

since the mid-1980's were built so that you cannot re-build them, the exterior casing had radial "crimps" that distorted the internal threads to make it near impossible to open the valve for repair...

I have not seen one for a great while, but I suspect they are still built the same way. Back in my 250/8 and 250C days [long, long ago] I went to a Mercedes specialty wrecking yard and acquired a few older valves that were rebuild-able.

I suspect yours is soldered closed due to the lack of an operational toggle-lever on the carb, that is spring loaded to actuate the value; there are two leaf spring pieces on the toggle, a thin one and a thicker one. If not lubricated, the thinner spring piece wears through and breaks. On a properly adjusted set-up, you can see the lever rock back and forth in time with the pulses of the mechanical fuel pump.

The intent of the fuel return valve was so that a higher capacity fuel pump could maintain a stream of cool fuel to the carbs, with the excess fuel being returned to the tank to re-mix with cooler fuel and prevent vapor lock.

I would scrounge around and see what can be found. Hemmings might be a source for a NOS part. I suspect the dealer would have to special order the valve, as most clear their stocks of older parts after a certain point.

Good luck,

JR
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post #3 of 19 (permalink) Old 03-22-2008, 07:45 PM Thread Starter
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O.K. the leaf springs are there, but I couldn't find where they should rest for tension. The toggle just flopped around. Finished the first carb & had a few parts left over, is this normal? Still not sure about the little blue rubber cup, does it maybe go on the accelerator pump? It kind of looks like the seal on the bottom of it only not as thick. Any help is appreciated. Thanks, Joe
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post #4 of 19 (permalink) Old 03-22-2008, 09:10 PM
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The toggle...

works against a pin on the reverse of the front carb lever as I recall. The "cup" may well be a lip seal for the accelerator pump piston; it is rather difficult to tell from your description. [They were not blue in my day.]

Remove the cir-clip and throttle plate linkage on the front carb and take the lever off, you should be able to see the pin on the reverse, unless the peening has come loose and the pin has fallen out. You may also be able to check this out by "feel" or of the top section of the front carb is removed, by looking down the backside of the lever. If the pin is loose, I used to fix them by silver soldering the pin in place. [with the lever removed from the carb, of course...] Also, sometimes the carbs are repaired/re-built via cannibalizing old units, and I have seen front carbs fitted with rear unit levers, minus the pin...

Yes, it is not unusual to have a few bits and pieces left over, as most kits cover a range of Zenith units.

The carbs can be frustrating at times, but can be made to work; it just requires time and patience.

Good luck,

JR
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post #5 of 19 (permalink) Old 03-29-2008, 12:35 PM Thread Starter
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O.K. I finished my carb rebuild. It started right up, all good there. I'm looking for a used fuel return valve, if anybody has a lead on one please let me know. When I was putting the top plate on the rear carb I found 3 of the holes were stripped out. Does anybody have an easy fix for this? I was thinking maybe crossthread a SAE thread bolt into the holes. Please stop me before I screw something up. Thanks, Joe
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post #6 of 19 (permalink) Old 03-29-2008, 01:54 PM
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Hmmm....

A new middle section from a used carb is one thought... Depending on thickness of material available, a heli-coil is another, or drill/tap the hole for a slightly larger SAE thread.

Back when I owned a couple of these cars, I had half a dozen used carbs [from wrecking yards] in the garage to use as "donors" for mis'c. parts. Even at that time [back in the mid to late 1980s] a new pair of carbs would set you back close to $2K. Those carbs are now long gone, along with the 250/8 and 250Cs that I owned.

For the fuel return valve, a WTB post here on BenzWorld, or a WTB ad in Hemmings would be a good place to start.

Good luck,

JR
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post #7 of 19 (permalink) Old 03-29-2008, 09:39 PM
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Is it #174 you're looking for?
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"If spending money you don't have is the height of stupidity, borrowing money to give it away is the height of insanity." -- anon
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post #8 of 19 (permalink) Old 03-29-2008, 11:34 PM Thread Starter
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Yes, #174 and whatever parts go inside of it. When I opened mine up I found that somebody filled the whole thing with solder. Thank's Joe
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post #9 of 19 (permalink) Old 03-30-2008, 10:02 AM
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German Star:

Are the current valves, #174 in the diagram, still factory crimped to make them non-repairable? I think the OP has two issues: 1) the price point on the new part, and 2) the issue of rebuild-ability, so to speak.

The needle/diaphragm insert for the older valves used to be available separately and at minimal cost; whereas, the newer [non rebuild-able] valves were pretty pricey considering the relatively simple nature of their construction and function.

Just curious...

JR
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post #10 of 19 (permalink) Old 03-30-2008, 10:45 AM Thread Starter
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While studing up on vapor lock issue's I've read that some people put a T-fitting in their incoming fuel line just before the carb, restricted it some, and then returned it to the tank. Does this sound like a good option, or am I likely to starve the carbs for fuel.
Thank's Joe
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