W114 250 Carb Issues - Head against a brick wall!

[NewKidOnTheBlock]

Hi All, I'm new here, got a 1969 W114 Mercedes 250 with dual zeniths and I feel like i'm hitting my head against a brick wall. I've come across some good information here but after months of rebuilding again and again and adjusting one way and the other I'm asking for help!

I've read every thread on this forum and others that I can find but I find myself stuck and not sure what is the cause of my issues. I've got twin Zenith INAT 35/40s fitted, which I have stripped and rebuilt, I found various issues along the way, all of which have been rectified as I went, I bought another 35/40 new old stock card and used this as a donor for various parts to get the best pair I could. Even down to swapping one of the secondary throttle plates as one of my originals had damage and was allowing air by when closed.

I've read the JamieKop Zenith guide and followed that through step by step, multiple times but still I find myself struggling.

The state the car is in now, it now starts beautifully on choke/high idle. But as it settles down off the high idle it starts to run rough and eventually it will stall. I just cannot find a setting where it will hold an idle without me physically holding the linkages to keep it running fast.

I've got the vacuum hose that sits between the carbs plugged

I've fitted all new vacuum hoses and fitted clips to try and ensure no vacuum leaks

I have removed the pushrod from the dashpot so that's out of the system currently and also the link bar between the 2 carbs is disconnected so that shouldn't be coming into play.

I'm currently at about 1.5 turns out on the idle mixture screws, at 2 turns out I had a VERY smokey exhaust.

I do notice that at least once it stalls there seems to be moisture - condensation I think as it doesn't smell like fuel around the idle bleed holes, I'm not sure if this is normal or an indication that somehow fuel is being blown out of them!?!

Is there an easy way to set a 'base' idle setting that's fast i.e. what size feeler gauge to fit between the throttle plate and the wall or a similar trick? Stupidly when I stripped the carbs I lost the original position for threaded 'hard' stop on the linkage so I re-set that to the (from memory 2.8mm gap?)

Any help is hugely appreciated!

[NewKidOnTheBlock]

No ideas, anyone?

Playing around yesterday a friend spotted that there was a plug missing from a bore in one carb, I don't think it was critical but I replaced it anyway, it seemed to just block off an already blanked off section that went into the float bowl. Once fitted I started the car but no luck, it did seem run for longer than it has done but once it works off the choke it stumbles and misfires, with some mild popping sound from the exhaust until it dies. I don't seem to be able to get it to hold a fast idle even by fully shortening the throttle link screws, this is what baffles me.

I did wonder if maybe I've switched the vacuum hoses for auto advance, going to try swapping them the other way or disconnecting and plugging them but I'm not convinced.

[Benz Dr.]

Have you looked at your spark plugs? What heat range are you using and do you have any carbon core in your plug or coil wires?

[Number400]

Back to basics. Test your fuel pump. Get a gas can and some extra hose and verify it is squirting out when cranking. Change the tank and other fuel filter. Hang in there, I was in this state myself. Sounds like you are getting enough fuel to start but not enough to actually run.

[rumb]

The throttle plate is tapered on the edges so that it will shut completely. Did you put it in correctly? It should completely seal the bore when closed.

Q how old is your gas?

Did you reuse the old leather accelerator pumps? The blue plastic ones that come in the kits can just fall off. paper gaskets on both sides of phenolic carb base spacers and metal drip plate?

The Kopp manual works, you have to do everything in the order specified. You should be able to set them up pretty close on the bench.

What do the plugs look like after it stalls? wet? dry?

[barry12345]

What about intake air leaks so common on these setups.

[cth350]

what kind of carb synchronizer are you using and what do you have for a hood over the carbs?

-CTH

[kavadarci1]

I have rebuild my engine and did the carbs.
They were like yours.
They were rebuild by a professional one of the best around.
I never managed to sync them properly or adjust.
He asked me to bring them back to rebuild them again.
I never did.
Tried everything and they never worked as expected, I sold the car in that condition.
I wish I listened to the recommendation to swap them for Webers.

Might not be too late for you.

[aluminum]

The 35/40 Carbs are one of the simplest Carbs to rebuild . However its simple to go wrong like the primary and secondary jet size easy to mix them up if not careful .
To sink the carbs is Easy but the motor must be completely warmed up and air cleaner removed of course . Sounds like perhaps the Gaskets are on wrong unless you have other Vacuum Issues Perhaps . Just turn up the idle until it keeps running Evan if to high then run water from a garden hose on low about the Motor ( not down the Carbs ) to look for vacuum leaks . The motor well Slow when You find a Leak if that's the Problem .
Don't know if this helps ? you might check the Distributor Points and such as well

[801mbz]

valve adjustment and compression test if low compression you can't adjust the carbs I have a bench for those carbs maybe 8 carbs I could never get to be happy