Another Zenith Post
 

I just did a partial rebuild both of my Zenith 35/40 Carbs also.
My car idled very well but it would
stumble if given any gas is gas at all.
I changed the accelerator pump cups, Cleaned the sludge out of the float chamber And gave it a general spritzing with carb cleaner.
While going through it, I noticed that the fuel return valve was seized. I soaked it in seafoam, began tapping on it and freed it up.
After reassembling, I started the car and it ran great. I checked for leaks and fuel was pouring out of the bottom of the fuel return valve. I disassembled the valve and it has holes in the diaphram.
I have since found out these things aren't just lying around everywhere, Neither are the parts.

Are you talking about the brass valve on the front carb? You can buy them new now and I have seen just the diaphragm part for sale. Sorry dont remember where, but search for "zenith inat parts"

you want "mercedes 0000751107". The catalog here lists it as a dealer item they would sell you, but it out of stock. -CTH

Fuel Return Valve
 

Yes, I have found the complete valve at Niemoller, but it is listed as a W108 part. Now I am pretty sure it is the same part as on my M130 engine, but emailed to confirm. It appears they are manufacturing them or having them private labled:
https://www.niemoeller.de/en/w111/w111-mercedes-benz-220sb/B012/070109/d07018-fuel-return-valve

I just purchased the diaphragm/needle from here, $51 USD shipped. Would rather have a complete valve and will buy one if found and keep the spare part as a spare part.
https://classiccarbs.co.uk/product/flash-sale-zenith-inat-carburettor-fuel-return-needle-valve

I have found the complete valve in Australian, aftermarket also, $100 USD + ship.
https://www.mcstarparts.com/product-page/m108-m114-m121-m130-m180-carburettor-nozzle

I have made many searches and I am surprised I have seen zero vendors for the part in the US.

Thanks
 

Yes, I found that part number yesterday and did a google search which all ended on Ebay at closed auctions or at dealers as NLA. Again, vendors for the diaphragm, but all items originate in europe.

Thanks.. Gary

Jaime Kopchinski replied to an email I sent to his website, a very nice guy.
He suggested another supplier for the fuel return valve and I bought it this morning.
In a couple of days I should have a new reproduction valve and a spare diaphragm.
Then I will find out if the car actually revved without stumbling, or if it was my imagination.

https://www.mbclassics.de/carburator-nozzle-M108114121130180

Here is another source:
https://www.dellorto.co.uk/shop/other-carburettor-injection-parts/kadron-keihin-nikki-pierburg-solex-stromberg-su-zenith/zenith-carburettor-spares/zenith-inat-fuel-return-valve-needle/

Rumb, I had that site bookmarked, but couldn't find it this week.

Aside from having that replacement diaphram, they've got the adapter hood for synchronizing...
https://www.dellorto.co.uk/shop/other-carburettor-injection-parts/kadron-keihin-nikki-pierburg-solex-stromberg-su-zenith/zenith-carburettor-spares/synchrometer-adaptor-zenith-inat-carburettor-01/

And they have the BK sync tool that I like...
https://www.dellorto.co.uk/shop/car-accessories/balancing-tools/synchrometer-airflow-suitable-for-sus-icts/

-CTH

If you read the previous posts, you will see what I work I've done on the carbs. I purchased and installed the new Fuel Return Valve and took it for my first drive. I went two full blocks and that convinced me the car was far from ready to go any further.
The car starts and idles just great, but hesitates badly when giving it fuel. If I give it too much pedal, the car will hesitate and backfire.
It could still be the carbs, this started with a rusty gas tank (resolved, see previous posts).
I am going to pull the valve cover, and the plugs. Mostly to check the timing chain, but will adjust the valves while I'm in there.
The way I understand it, I should check tension the complete length of the chain. Which pully do I use to turn the engine over, water pump?
If the chain is not stretched, I will time it.
Thanks. Gary Moore

Have you synched the carbs? Did you adjust them following the manual instructions?
Try small adjustments to the idle screw. - the brass one at base of carbs.

Also have you checked engine timing?

Use a big socket on the crank bolt to turn engine. Line up cam marks and look at crank damper to see what you chain stretch is. Sometimes you can put pressure on the fan belt and turn the bolt on PS pump.

BTW what car/engine are we talking about?

The car is a 1971 250C - /8 engine.
I cleaned the sludge out of the float chamber. Made sure float was not sticking.
I replaced accellerator cups in the carbs and changed the fuel return valve due to a leak.
Didn't go thru the carbs completely because winter is looming and wanted to get the new antifreeze circulating. I was able to get that done.
I may be done for the year if Temps stay below freezing.