Need help with 1969 250C Zenith carb rebuild problem - Mercedes-Benz Forum

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post #1 of 16 (permalink) Old 11-03-2013, 11:55 AM Thread Starter
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Need help with 1969 250C Zenith carb rebuild problem

Hi all, just signed up for this forum and appreciate any help you can provide. This looks like a great resource!

I am helping a friend who owns a 1969 250C rebuild the Zenith carburetors. He complained of rough running idle and poor acceleration among other problems. The car has been nicely restored but the carburetors were never serviced.
I have rebuilt several SU and weber carbs years ago, and have some experience with tune-up and repair so I volunteered. We are in Turkey, so getting parts is almost impossible, but the owner ordered a simple rebuild kit from Germany. It contained just the gaskets and washers.
(BTW, I can't find any reference to the model number of these carbs)
I got a lot of documentation from this site (thank you!).

I did a simple teardown, clean, replace gaskets, measure and adjust, and reassemble without doing a total disassembly. I did find several problems. Second stage throttle valve was frozen shut, debris in the other 2nd stage throat, etc. Got it all working. This seems to have solved most of the problems.

Upon reassembly and testing, everything seems to work except the darn idle. The engine runs "wobbly" at idle. Not rough, but erratic. Of course I tried to set the idle and sync them, but that's hard given the erratic idle.
I think I traced the problem to the rear carb that is doing something strange. I notice at idle that there is a LOT of gas dribbling out of the "outlet arm" into the peratomizer (what I would call the full injector into the venturi). The front carb doesn't do this. You don't see any visible gas dripping out.
The same amount seems to dribble out even if I open the throttle to about 2000 rpm. Definitely too much gas.

I checked the main jets to make sure I reassembled correctly and even tried swapping them with the front carb. No change. Idle mixture should not have any effect on this, and it doesn't.

The only thing I can think of that might cause this is way too much gas in the float bowl (or float chamber), that is leaking when there is just enough vacuum to start it flowing or just enough fuel pressure. I checked the float and the float position when I cleaned it and it seemed fine. The float valve might be leaking of course.
The float chamber venting valves seem rather "flakey". They move but not too well. I didn't take them apart since the manual says they were "factory adjusted". I tried playing with the one on the rear carb while running to see if that wold have any effect. Nope.
Anyway, I thought I would try to get some advice from this group before I tear it apart again. I am out of ideas.
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post #2 of 16 (permalink) Old 11-03-2013, 03:09 PM
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You MUST adjust these in the correct sequence or they will not work properly. I can't emphasize this enough.

Jamie, a good friend of mine, has all of the shop manuals for these carbs on his web site at:

Mercedes Benz Zenith Carb Manual | JaimeKop.com

Read through the manuals and follow the setup and adjustment steps EXACTLY as described.

I love the Zeniths, but unfortunately they have a bad reputation. I believe much of that is due to people simply not following the procedures for setting them up. Unlike the common domestic carbs that you could just randomly tweak individual settings on, the Zeniths rely on a sequential adjustment procedure that has each setting based on the previous one. Skip a step and you'll have problems for sure.

Good luck,

Dan
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post #3 of 16 (permalink) Old 11-03-2013, 03:15 PM
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Hi. Did you clean out the inside of the idle jet and the other jets before putting them back? If you know German, below is a good webside. Look at the section called >Know How< And then at the subsection Engine. Hope this will get you there.
Harald.
www.strichtacht-forum.de
/8-Forum :: /8 Technik
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Last edited by Over 75; 11-03-2013 at 03:21 PM.
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post #4 of 16 (permalink) Old 11-03-2013, 08:21 PM
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Yes, if the rear idle circuit is plugged, the engine vacuum will pull gas out of the main circuit.

The carbs may also be out of sync, meaning that the throttle plate on the rear carb is open more than the front. So the engine vacuum is demanding more fuel from the rear carb.

I assume you don't have a carb synchronizer, so try this. Unhook the linkage between the two carbs. Get a piece of heater hose, put it to your ear, and listen to the air noise at the base of each carb at idle. Then adjust the individual throttle plate openings to equalize the air noise. Sounds like you may need to open the front and close the rear a bit to stop the extra gas from being sucked. Then reconnect the two carbs, loosening the ball sockets on the rod so that the position of the throttle levers does not change when you reconnect them.

Good luck,

Chuck Taylor
Falls Church VA
1998 SL500
1959 220S
1970 280SL
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post #5 of 16 (permalink) Old 11-03-2013, 11:45 PM
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zenith idle issues .

Clean the idle tube and check for a clogged fuel intake to idle tube.

In Above posted photos by 'over seventy , photo #3 shows the main jets and the left one is the main jet stage 1 with a side well attached from which the fuel is delivered to the idle tube. Most get clogged with a hard deposit if the engine has been standing for a long time. Requires removal of main air correction jet stage 1 for access to check.

Warpage is also very common in Zenith's and a good idea to check at the parting faces .
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post #6 of 16 (permalink) Old 11-04-2013, 11:46 AM
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I do many zenith restoring and never had a problem with them with any client. Just be sure that all the nozzles are really decarbonized and no obstruction in any one. If you have leaking problem then install a new fuel valve, that will close the fuel inlet in the float chamber. Did you check the fuel cut off valve on the rear carb? the one on the bronze screw?
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post #7 of 16 (permalink) Old 11-04-2013, 12:17 PM Thread Starter
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Follow up on Zenith Carb rebuild problems

Thanks to everyone for the great advice. I can respond to most of this, which brings up a few more questions:

LWB250 - I did get the manual from JaimeKop prior to the rebuild. The manual does not say anything about the order that the adjustment must be done in, but I followed the sections in the order that they are listed; accelerator pumps, float level, second stage diaphragms, etc.

Over 75 - The photos are great1 I did blow out (compressed air) the idle jet hole in the body.
The german site you listed is great. There were excellent photos there. There is a great article in german that I tried to get google translate to translate, but the translated version is still not comprehensible. But I notice that in one of the photos, a guy looks like he is blowing air through the idling jet. The thing looked solid to me. Maybe I will try this again and pay more attention to the jet.
I did get an EXCELLENT document (Zenith Carburetor 35/40 INAT) that had detailed drawings of the interior fuel flow. This looks super helpful, got to study it in detail to see if I can determine where the gas is coming from that's leaking in or where the vacuum is coming from thats sucking it in.

ctaylor738 - see notes
"Yes, if the rear idle circuit is plugged, the engine vacuum will pull gas out of the main circuit."
That makes sense. I am going to focus on the rear idle circuit.

"The carbs may also be out of sync, meaning that the throttle plate on the rear carb is open more than the front..."
That is what I thought, but I carefully observed the openings of both plates and they look identical, I can't see any difference.

"I assume you don't have a carb synchronizer, so try this. Unhook the linkage between the two carbs. Get a piece of heater hose, put it to your ear, and listen to the air noise at the base of each carb at idle..."
Your are correct. Forget it in Turkey (I could order one, but would take months to get here and then customs would kill me). I know the heater hose trick from my days working on SU's. I did this to try to get em in sync, but I can actually hear the gas squirting out of the rear carb and it makes quite a noise which makes syncing hard. Got to fix the "leak" first.

alder - Sounds like a plan.
"Requires removal of main air correction jet stage 1 for access to check."
What is the "main air correction jet"? (I'll check my drawings).

javimx5 -
"If you have leaking problem then install a new fuel valve, that will close the fuel inlet in the float chamber. Did you check the fuel cut off valve on the rear carb? the one on the bronze screw?"
I am a little confused on this. by "fuel cut off valve" do you mean the "needle valve" that the float opens and closes?
The only thing I checked was whether it was free, that it moved. I didn't check to see if it actually shut off the fuel. Did not see a way to check this. Do you know a way?

Conclusions - I am leaning to a problem in the ideal fuel path from the float to the throat, including the fuel cut off valve. It does make sense that if it was clogged, fuel would be sucked through the normal speed fuel path. I will go back (later this week) and take another shot at cleaning it out and I will swap the fuel valves between the two carbs to see if the problem moves with it.

In the meantime, I have a couple of other questions I will post shortly.
THANKS GUYS, really appreciate the help!
I wish I had found this forum (and the documents) BEFORE I did the rebuild!
Grayson
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post #8 of 16 (permalink) Old 11-04-2013, 12:26 PM
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I mean this one, the bronze screw. This is from a new carb that arrived to me broken, yep, the mail guys did it again but after getting a new Pagoda cylinder head broken in half I´m not surprised

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post #9 of 16 (permalink) Old 11-04-2013, 12:33 PM Thread Starter
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THANKS for the photo!
That looks like the fuel return valve on the front carb. My rear carb does not have one. I don't see one in other photos.
I tried holding it open and closed when idling but it didn't make a difference.
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post #10 of 16 (permalink) Old 11-04-2013, 09:28 PM
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You are right ,there is only one fuel return valve on the front carb and none on the rear.
It's function is to supply only new, fresh and cool fuel to the system to prevent a vapor lock in a hot engine. Also check your small injection nozzles and see if fuel is coming out during idle. New ones are $14.50 at uncle Benz part# 000-070-16-73
http://www.benzworld.org/forums/vint...ml#post6046626
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