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  #1  
Old 12-31-2008, 03:12 PM
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67' 250S Vent hole on Carb Question

Ok, I keep getting closer. I plugged the "VENT hole" on the rear carb,and the car seems to run better. When I let off of it, it wants to stumble, and die.. Any ideas why? Thanks.

1967 - 250S - Zenith 35/40's

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Old 12-31-2008, 08:36 PM
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The manual says that when you plug that little hole on the top, there should be an RPM drop. What the manual doesn't say, is what it means if you get an RPM rise.

From my experiences, it means take the middle section of the carb out and clean it. There's something plugged. But maybe that's just me.

-CTH
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Old 12-31-2008, 10:41 PM
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Thanks. The carbs have been cleaned and soaked, but who knows....more dirt got in maybe.......thanks

Dave
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Old 01-01-2009, 09:10 AM
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Hello,

Try separating the linkages when you do this test. Make sure that both carbs are backed all the way down. Make sure that the secondaries are not partially open.

What happens when you adjust the idle mixture screw at this point? The rpm rise may just mean that you have the mixture screw adjusted too lean. What happens when you block the other vent hole? The mixture screws should be adjusted at the maximum rpm threashold. If the idle is too fast at this point adjust the small links to lower the idle. Carbs must be kept in sync. Re-adjust the idle mixture. etc.

There is a hidden horizontal passage in the idle circuit which when plugged is difficult to diagnose, I will try to post a pic here shorly.
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Joe Alexander
Blacklick, Ohio
1964 220SE Rally (La Carrera Panamericana someday)
1966 Unimog 404s (Swedish Army)
1969 300SEL 6.3 (sold)
1969 280SL Pagoda
1973 280SEL 4.5
1974 450SLC FIA Rally car (standard trans)
1982 300D turbo (winter driver)
1986 560SEC
1989 Unimog FLU419 (US Army)
1991 300TE (wife)
2002 SLK 32 AMG (350 hp)
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Old 01-01-2009, 09:12 PM
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It could also be that there is a vacuum leak (probably between the top plate and the mid section), so that when you shut off the air to the idle circuit, the mixture improves. There is a great post by dantrcav on how to check and correct top plate problems.

But as Joe alludes, this could also be a synchronization problem.
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'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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Old 01-06-2009, 02:57 AM
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Hello,

Here is a picture of the second stage of the Zenith carb (upside down). The red arrow indicates the "hidden horizontal passage" It is directly below the vent hole. It is a low spot in the idle circuit. Crud can build up in here. If it is plugged, the idle in this carb will not function. You will see a plug peened in place, where the manufacuturer drilled the passage. In severe instances I have had to drill out the plug, clear the passage and re-plg the hole. Just make sure the passage is clear by blowing air through it. If not clear it.
Attached Thumbnails
67' 250S Vent hole on Carb Question-carb.jpg  
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Joe Alexander
Blacklick, Ohio
1964 220SE Rally (La Carrera Panamericana someday)
1966 Unimog 404s (Swedish Army)
1969 300SEL 6.3 (sold)
1969 280SL Pagoda
1973 280SEL 4.5
1974 450SLC FIA Rally car (standard trans)
1982 300D turbo (winter driver)
1986 560SEC
1989 Unimog FLU419 (US Army)
1991 300TE (wife)
2002 SLK 32 AMG (350 hp)
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  #7  
Old 01-06-2009, 04:29 PM
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I have to check that. Thanks.

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