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  #1  
Old 06-16-2011, 08:56 PM
muleears's Avatar
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Join Date: May 2006
Location: Windsor, VA
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Zenith carbs hunting for idle and stalling in gear

These are the problems I'm having on my '67 250S "barn find" car. It starts right up, then when warm will begin an up and down idle cycle. Down to maybe 500 rpms then up to almost 1000 (I'm guessing, no tach). When placed in gear it will stall within 10 seconds. The attached pic shows the vacuum solenoid which seems to kick the idle up when it slows.

What is in the black circle and does it have anything to do with the idle? The duct tape in the white circle needs to go away but I don't know what is under it. I'm afraid I wont be able to replace it.

The car has: new plugs, points (properly gapped) wires and condensor. Timing is unknown. Fuel tank boiled out and painted, fuel lines and pump replaced. The car has been driven only about 200 miles since awakening from its 20+ year slumber.

Any advice is appreciated.
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Zenith carbs hunting for idle and stalling in gear-sam_1705.jpg  
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Muleears
'07 E320 Bluetec 133K my DD
'04 Jaguar XJ8 VDP, 34K
'10 Hyundai Accent 60K Grocery Getter
'02 VW Golf soon to be on the road again
'97 E300 Diesel Son's DD
'61 VERY tolerant wife

Hampton Roads, VA USA

Gone but not forgotten:
'67 250S 95K
'86 300SDL
'87 300D Turbo, 364K! R.I.P.
'98 E300 Turbodiesel, 213K
'02 S420, 164K
'01 Prius 138K
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  #2  
Old 06-18-2011, 12:34 AM
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It's been a while since I've messed with my carbs, but the circled (black circle) item looks like the accelerator pump plunger. Not likely to be your problem. Mine doesn't have the vacuum solenoid, but I'm pretty sure you've circled a leak with your white circle. There's an excellent chance that's your problem.
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Anyone who thinks a 300D is fast drives too slow.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 357,xxx miles
08 Triumph Street Triple 24,xxx miles, lowered 10mm in front, Pirelli Angel GT tires, EBC HH brake pads, otherwise stock.
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  #3  
Old 06-18-2011, 09:11 AM
muleears's Avatar
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Quote:
Originally Posted by Skippy View Post
It's been a while since I've messed with my carbs, but the circled (black circle) item looks like the accelerator pump plunger. Not likely to be your problem. Mine doesn't have the vacuum solenoid, but I'm pretty sure you've circled a leak with your white circle. There's an excellent chance that's your problem.
I'll take the tape off, hopefully its just a missing vacuum connector, and see if I can replace it with the proper fitting. I assume it is leaking vacuum there. I'll report back what I find out. Thanks for the help.
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Muleears
'07 E320 Bluetec 133K my DD
'04 Jaguar XJ8 VDP, 34K
'10 Hyundai Accent 60K Grocery Getter
'02 VW Golf soon to be on the road again
'97 E300 Diesel Son's DD
'61 VERY tolerant wife

Hampton Roads, VA USA

Gone but not forgotten:
'67 250S 95K
'86 300SDL
'87 300D Turbo, 364K! R.I.P.
'98 E300 Turbodiesel, 213K
'02 S420, 164K
'01 Prius 138K
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  #4  
Old 06-18-2011, 09:54 AM
muleears's Avatar
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Well, I pulled off the tape. All it was, was a missing vacuum connector, so I cut one leg off of one of the 4 ways I have for my diesels and stuck it on. Fit like it was made for it. There was some improvement but still hunts and stalls. Only hunts now between 450-500 and 800. I read somewhere that this car is supposed to have a fast idle, around 800. Is this true? I am going to try to video and post the car running so you can see/hear what its doing. I don't know how large a file I can post though.
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Muleears
'07 E320 Bluetec 133K my DD
'04 Jaguar XJ8 VDP, 34K
'10 Hyundai Accent 60K Grocery Getter
'02 VW Golf soon to be on the road again
'97 E300 Diesel Son's DD
'61 VERY tolerant wife

Hampton Roads, VA USA

Gone but not forgotten:
'67 250S 95K
'86 300SDL
'87 300D Turbo, 364K! R.I.P.
'98 E300 Turbodiesel, 213K
'02 S420, 164K
'01 Prius 138K
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  #5  
Old 06-18-2011, 11:00 AM
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Location: Brooklyn, NY/ Minneapolis, MN
Posts: 985
Carbs need to be balanced or else they'll make you nuts. Also check for vacuum leaks around the carb base plates.

http://www.jaimekop.com/CarbManual/index.html

Last edited by JMela; 06-18-2011 at 06:39 PM.
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  #6  
Old 06-18-2011, 04:41 PM
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Location: Carson City, NV
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My Haynes manual doesn't cover your model, but the idle specs I do have for the straight sixes with a pair of Zeniths run from 650 to 900 rpm in neutral.
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Anyone who thinks a 300D is fast drives too slow.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 357,xxx miles
08 Triumph Street Triple 24,xxx miles, lowered 10mm in front, Pirelli Angel GT tires, EBC HH brake pads, otherwise stock.
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  #7  
Old 06-18-2011, 09:57 PM
muleears's Avatar
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Join Date: May 2006
Location: Windsor, VA
Posts: 1,435
Thanks Skippy and JMela, That is all useful information.

I know there is a tool for balancing the carbs, I can probably find one on here somewhere. How about instruction on doing so? I've done a search and not come up with much. I've read you can do it with a tube or a shopvac hose, just by listening but I have no idea how to do it.

How would I know if the carbs were out of balance?
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Muleears
'07 E320 Bluetec 133K my DD
'04 Jaguar XJ8 VDP, 34K
'10 Hyundai Accent 60K Grocery Getter
'02 VW Golf soon to be on the road again
'97 E300 Diesel Son's DD
'61 VERY tolerant wife

Hampton Roads, VA USA

Gone but not forgotten:
'67 250S 95K
'86 300SDL
'87 300D Turbo, 364K! R.I.P.
'98 E300 Turbodiesel, 213K
'02 S420, 164K
'01 Prius 138K
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  #8  
Old 06-18-2011, 10:10 PM
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Location: beautiful Bucks Co, PA
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Now you have a tight connection to the idle stabilizer, observe the motion of the operating rod protruding from it as you open and close the throttle. If it moves when you do this, then the stabilizer is functional. If there is no movement, then the device is kaput. If it works then you need to balance the carbs. A good ear can hear the difference in air flow from one carb to the other. I have always used a "Uni-syn" with a home made adaptor to balance idle air flow.
Once you get all that done, assuming the ignition timing is correct and the advance parts in the distributor are free and working, the stabilizer might work just fine. If not get back to us.
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  #9  
Old 06-19-2011, 04:43 PM
muleears's Avatar
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Join Date: May 2006
Location: Windsor, VA
Posts: 1,435
Quote:
Originally Posted by Chas H View Post
I have always used a "Uni-syn" with a home made adaptor to balance idle air flow.
Can you expand on the "home made adapter"? I have found a uni-syn and am planing to buy it, but need to know what the adapter involves.

One other thing, how am I going to balance the carbs when they don't currently hold a constant idle?

Thanks.
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Muleears
'07 E320 Bluetec 133K my DD
'04 Jaguar XJ8 VDP, 34K
'10 Hyundai Accent 60K Grocery Getter
'02 VW Golf soon to be on the road again
'97 E300 Diesel Son's DD
'61 VERY tolerant wife

Hampton Roads, VA USA

Gone but not forgotten:
'67 250S 95K
'86 300SDL
'87 300D Turbo, 364K! R.I.P.
'98 E300 Turbodiesel, 213K
'02 S420, 164K
'01 Prius 138K

Last edited by muleears; 06-19-2011 at 08:16 PM. Reason: add clarification
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  #10  
Old 06-19-2011, 09:39 PM
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Location: beautiful Bucks Co, PA
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Quote:
Originally Posted by muleears View Post
Can you expand on the "home made adapter"? I have found a uni-syn and am planing to buy it, but need to know what the adapter involves.

One other thing, how am I going to balance the carbs when they don't currently hold a constant idle?

Thanks.
I whittled a small block of wood to fit the carb top then glued some soft rubber to it.
I guess you're gonna have to quick if the idle isn't constant. Have you gone over the distributor to make sure the advance weights are free and the point plate moves in a strictly circular manner? It may help to plug the vacuum line to the distributor.
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  #11  
Old 06-19-2011, 10:26 PM
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Inverted cool whip container fits the Zenith tops perfectly.
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  #12  
Old 06-19-2011, 11:49 PM
mak mak is offline
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Location: Westfeld .
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idle air

The zenith idle air tube can get clogged leading to a varying idle speed. A Quick clean is by inserting the tip of carb-cleaner nozzle on the small hole for the air bleed . This is on the right side of the choke flap.
JC Whitney among others carries a carb synchronizer . German make ,and excellent in use , use adapter as mentioned by Jmela.
regards
mak
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  #13  
Old 06-20-2011, 12:38 AM
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Quote:
Originally Posted by JMela View Post
Inverted cool whip container fits the Zenith tops perfectly.
What size Cool-Whip?
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Anyone who thinks a 300D is fast drives too slow.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 357,xxx miles
08 Triumph Street Triple 24,xxx miles, lowered 10mm in front, Pirelli Angel GT tires, EBC HH brake pads, otherwise stock.
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  #14  
Old 06-20-2011, 09:04 AM
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16oz, I think, but eyeball it to be certain.

This is an Arthur Dalton trick.
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  #15  
Old 06-20-2011, 10:11 AM
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Quote:
Originally Posted by Chas H View Post
Once you get all that done, assuming the ignition timing is correct and the advance parts in the distributor are free and working, the stabilizer might work just fine. If not get back to us.
I don't know if the distributor is the same type as the one on my '71, but if it is, the cam mechanism can wear to the point where it has a little wobble in it. That makes it pretty much impossible to get a consistent dwell angle. That'll certainly make idle unsteady.
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