Yet another Zenith SOS call

[B diddy]

Hey all,

After much frustration I am at a point where I need help from someone who knows these Zenith 35/40 carbs in depth.
Vehicle is a 1970 280s M130.920 automatic. Fresh rebuild on the engine and in the process, both carburetors were also rebuilt using Royze kits. Carbs were fully taken apart, submerged in carburetor cleaner, and all passages cleaned with throttle body cleaner and compressed air. New needle valves installed, floats checked for leakage and height set at 23mm (Adjustment washers per the manuals are NLA from MB)
I have looked at all available online Zenith resources and Haynes manuals.

Issue is the fuel mixture is too rich causing some white fuel rich smoke puffing from the tailpipe when idling that increases in intensity when driving/revving engine. No matter how much the idle adjustment screws are wound in the issue remains. Current baseline used is approx 1 & 1/2 turns out. Further investigation shows fuel seems to be dribbling down the throat at idle which may be the whole problem.

This issue has been brought to light on this forum (see link) before but not resolved as the owner converted to Webers which is not my goal.
http://www.peachparts.com/shopforum/vintage-mercedes-forum/190328-zenith-purgatory.html

I have attached a link to a video I uploaded that demonstrates the issue. Any advise would be great.
Thank you
https://youtube.com/shorts/z6aeRqqPLcc?feature=share

[Frank Reiner]

Originally Posted by B diddy

New needle valves installed, floats checked for leakage and height set at 23mm.
Further investigation shows fuel seems to be dribbling down the throat at idle which may be the whole problem.

If in fact there is fuel overflowing, two things may be involved:
1) Float level set too high.
2) Fuel pressure too high.
Is there a vacuum operated fuel return valve installed? If there is, check that it does open when idling, and blow thru the return line to tank. If the return line is blocked the fuel pressure will be too high.
If there is no return valve, install a fuel pressure regulator set to 2.5 PSI.

[cth350]

>> fuel seems to be dribbling down the throat at idle
Like Frank said, that should not be.
If it's not an over abundance of fuel as Frank described the other possibility is too much air being pulled in around the butterfly shafts (worn).

One of the tests for balance is plugging the little hole on the lid with a pencil eraser and measuring the RPM drop. It should be the same for either hole. Did you do that test? Any chance the RPMs went up instead of down?

-CTH

[rumb]

Make sure the brass fuel return valve on the front carb is adjusted correctly and actually works. These can be purchased new again, or I have see just the diaphragms for sale also.

My first thought also was floats or stuck needle.

What does the exhaust smell like? gas or sweet? Could be antifreeze or trans fluid.

" increases in intensity when driving/revving engine"

A small extra amount of gas would seem to diminish issues at higher rpms. I'd be examining other systems to rule them out.

I'd even do a compression test, and monitor trans fluid levels.

[B diddy]

Thanks for all the input guys.
As per Frank's suggestion I'll be checking the fuel pressure today and reporting back my findings. The fuel return valve is the brass variant and is not controlled by vacuum, rather a small pin on the bottom section that is pressed by the throttle linkage when at idle.

The exhaust smell is not sweet or coolant like at all and smells of fuel, another possibility could be some old oil build up in the muffler from before the rebuild.
With the float set to 23mm I'd assume that this is the "lowest" setting in the fuel bowl as a thinner washer would in turn allow more fuel to sit into the bowl by holding the needle valve higher. Needle valves for float level are brand new and were replaced, I'd like to hope they aren't sticking but I have to entertain all possibilities

Compression has not been measured since rebuild but have only approx 10 miles on it since re-assembly.

[rumb]

Just to be sure, you followed the setup procedure in specified order in the jamie copp zenith manuals?

You balanced the carbs with flow meter?


Are you positive thw lower throttle plates are right side up and close completely? The plates are tapered on the edges so they seal completely.

[aluminum]

You have to have some Clearance between the Idle setting and the Round stem coming down ( Not sure what you call it LOL ) but you need about at least 15 Thousands Clearance before you can really set the Air Mixture properly > is one on each Carb
*** Not sure if a 70 still has them but >>
If you have Heat risers still makes is Hard to set those Carbs > the Risers Shafts leak exhaust most of the time anyway and smell make noise > Remove the shaft and tap then Plug the manifold > You need to Make a Little Block off Plate to go between the Manifolds
or it will get to hot under the intake and over vaporize Fuel
hard part is to remove the bolts the connect the manifolds together >

They run great blocked off