Fuel Return Valve Daiphragm Failure
 

Hi All,

My original post did not go through. So giving this a try.

Have a Zenith 35/40 dual carb setup in my 1967 250S. I know that the diaphragm in the fuel return valve assembly is leaking because of pinholes because I pressure tested the diaphragm.

I have not seen any posts here on how to either seal the pinholes? Or, how disable the fuel return function by forcing the needle/piston in the closed (pushed up).

The pinholes cause gas to leak from the assembly anytime the engine is on.

Thanks in advance.

Permatex 85420 sealant
 

Am now considering loading the bottom chamber full of Permatex 85420 fuel resistant sealant. This would force the needle/piston in the closed position and keep fuel from dribbling out the bottom of the assembly. And this sealant could be removed in future (unlike plugging it with JBweld, for example).

I can't imagine that a failed diaphragm is that uncommon.

Any ideas, anyone?

karshack, although I am unfamiliar with the fuel return valve on your automobile the only repair that makes sense would be to replace the faulty diaphragm, or the entire fuel return valve if the diaphragm is not available separately.

When it comes to the fuel system it is always better to not redesign it.The return is there maybe to keep fuel cooler and not perculate on hot days.Benz should have those available.
Rich

Thanks Rich. Indeed, from my readings avoiding fuel vapor issues is the principal reason for the fuel return valve. Fortunately, I'm in a part of the SF Bay area that does not get wicked hot often. Maybe 10 days tops, if that. And, it's been ages since I've been in frightfully long lines to get into Laguna Seca or Sears Point.

No, in the short-term I need to stop this fuel leaking. If I need to get a new/old carb (about $200-300) that can wait.

Unless, I hear better ideas I'll go the plug-n-plod route.

Still open to ideas. Thanks!

Hi Ferdman,

I've looked at a lot of carb rebuild/maintenance kits for the 35/40. None of them include this needle/piston with diaphragm. Nearly everything else but this.

I think I would have to buy an entire primary carb. Or, hope to find some pne who is selling individual parts of a primary carb. I do not see how the carb can possibly function without the valve assembly in place. But it can, from what I've read run even if the return feature is rendered inoperable.

Thanks

Update -- Success fixing leak
 

Hi folks,

I see that there have been some more eyeballs on this thread. So, here is the latest.

I used Permatex 85420 sealant to fill the bottom of the valve assembly. Let that set for a little over an hour. Perhaps I should have let it cure overnight, but I was in a hurry. Reassembled the valve. Checked for leaks while idling for 10mins. Then took it for a 26 mile drive, checking it at 13 miles and at return. No leak, no dribble, no hint of any gasoline exiting from the valve at all.

At the end of the drive while the engine was still piping hot, I used a spray bottle filled with water to do a very cheap steam clean of the carb bodies, carb catch plate, and manifolds. Then re-filled the bottle. Trying to get gasoline residue off of these and other areas that have been showered with gas splatter.

I could not smell any gasoline during my test drive.

So far, so very good. I will look into if and how I can get a new needle/diaphragm, in the not too distant future -- without buying a new carb. This repair was $10USD.

I hope this has been of some help.

See a couple of pix in my gallery.

Zenith Inat 35/40 Fuel Return Valve photo
 

So, it's still driving nice and dry -- no gas leaking! Thought I'd post a pic since I have yet to see one on this forum.

Upadate!! Got rebuilt needle/piston!!
 

Hey everyone,

Sorry for the delay in getting this update to you.

My Permatex sealant solution only lasted about 3-weeks/200+ miles. Could have lasted longer; but it was not going to be a permanent fix. A fellow at work had very good results at a carburetor shop that rebuilt his 4-barrel for a 1968 Mustang. The good news is that the shop is local. That was really important to me because this is my daily driver - and I hate "down" time in my transportation.

I spoke to Merrill, the boss of The Carburetor Refactory and told him my tale of woe. No problem, he said. He gets this request every now and then. Hopefully he will has some available on hand when you need them. Otherwise he will have to send out his cores to get re-gasketed.

Fortunately for me, he had one on hand. I went over, gave him my needle/piston with holed gasket as a core exchange, and I got a beautiful specimen in return -- I did have to pay for it but it was under $45 inc tax (I don't have the receipt near me now). I installed the new piece right there and ---- oh, happy day -- no drippy, leaky gas anywhere. Bone dry. Been that way ever since.

Drove it around, to get the engine good and hot, parked, then began spritzing the carbs, manifolds, headers, etc on that side with water from a spray bottle. Really cheap steam clean. Went to a self-serve car wash the next day to do it right - but nice and gentle so you don't get water where it should not be.

The Carburetor Refactory
815 Harbour Way South #5
Richmond, CA
(510) 237-1277
Carb / Carburetor Kits | Rebuild Kits | Holley, Mikuni, Weber, and More

Merrill does nothing but carbs. Highly recommended.

All the best! :)

Any new leads?
 

As of Nov. 2021, has anyone been able to find a rebuild kit for the return valve? Or replacement diaphram?
R/
OTB

OTB: Later today I'll try and find my engine parts catalog to see if there is a parts number. Even if there is no number, you could still try to contact The Refactory or Classic Mercedes in southern Cal. I know the Zenith 3t/40 was used in some BMW so you could check some of those forums.

As regards disabling the fuel return valve, look at more info on that on the forum. Obviously some folks don't need it because they don't drive longish distances or time in hot enough weather to prove heat related gas issues in their carbs.

OTB: Sorry for the delay. Took ages yo find that parts catalog.
Catalog Number 10-107
Mercedes-Benz Type M108
Edition A

Table 5, Groupe 7, Page 28 only shows the whole carburetor no exploded views of parts.
However Page 28 just "Fuel by-pass valve" as part 00-070-11-46.

I would suggest that you contact the Refactory and see if they can help you. If not, contact the folks at Mercedes-Benz Classic in Southern California 1-866-622-5277 or email them at classicparts@mbusa.com.

The reason to not diy this job is that failure probably results in fuel dripping from the carb to hot headers. Pennywise and pound foolish.

Good luck

Thank you, karshack.
I appreciate and accept your advise and as I feel, "A word to the wise should be sufficient"
R/
OTB