BenzWorld Junior Member
Date registered: Oct 2013
Vehicle: 1969 250C
Location: Ankara, Turkey
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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).
"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!