A newbie with some Zenith issues. My first Mercedes in about 15 years. A very nice 71 250C, but has some running issues. From reading this forum I have learned quite a bit already, but now need a parts source. Basically the car runs well but will start to run rough and stall once warm. Especially in gear (automatic) From what I can tell I am missing some important (I think) pieces of the carb setup. The vacuum throttle control is missing all of the adjustable parts as well as the spring. Basically just the vacuum module and shaft is all that is left. It moves correctley, but does not contact the linkage. From what I can tell the float bowl vent valves are also gone. Not sure if these are that important to running as the car is a 71. Pretty sure I need the vacuum throttle control to work correctley for good running. Does anybody have an old throttle control for sale? Or know of a parts source for these? I think that mine is good and will work if I have the missing adjustment pieces. Thanks

Ron
(I'm sure that I will have many more questions in the future)

Please visit the 'Useful Vintage Links' post at the top of the forum. In there is a link to rebuilding Zenith carbs:

http://jaimekop.com/CarbManual/Page01/toc.html

Someone might have some replacement carbs.

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Regards

Warren

Currently 1965 220Sb, 2002 FORD Crown Vic Police Interceptor

Had 1965 220SEb, 1967 230S, 280SE 4.5, 300SE (W126), 420SEL

ENTER > = (HP RPN)

Not part of the in-crowd since 1952.

I have a couple of Zenith-INATs left from my M114 70 250, when I converted to Webers, and they're slightly different from the ones on my M130 72 250C.
Shoot me email pics of what you're needing from the other carb, if there - while I think about breaking them down...
I found Zenith-INAT 35/40 kits locally at BAP-GEON, but also available from Phil - click "Buy Parts", above.
Stan

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stanbabu@3bubbas.com
59 300d
65 250SE/Cpe
70 250/8
72 250C
78 280CE

My 71 doesn't have the bowl vent valves either. Basically all the throttle control valve does is give it a little more gas under load at idle (like engaging the transmission). You could turn the idle up a bit and it should run fine. I would start by looking at your chokes, mine was doing the same type of things and I unplugged the electric element on each choke so they open according to the engine temp and it seems to have fixed my warm running problem.

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Adam Lumsden
(83) 300D
Vice-President of the MBCA International Stars Section

Thanks for the replys. I have rebuild kits on the way, Royze major kits. I also have the manuals for the carbs. The chokes seem fine, close when cold, open completely in about 15 minutes, front before the rear. It will idle and run smooth when on the cold circuit? but once the chokes are fully open wants to stumble and stall. pumps etc seem fine strong squirt both front and rear. All of the emmissions stuff is present and seems to work. (vacuum switches, temp sensors etc) The last zenith/solex carbs I played with were on Opel GT's years ago. Replace with a DGV Weber. I may do that here as well but do not want to spend $1K right now. Tristar, I will try and send a picture, Basically what is missing is the spring, nut, and the adjustable bolt and lock nut that threads into the shaft of the vacuum module. The module itsself is there, but the parst that set the adjustment are gone.

Ron
71 250C
70 MGB
69 MGCGT

The vent valves were discontinued, sometime around 1971, I think.

The car should idle in gear without the vacuum assist. That is supposed to kick in when you turn the AC on or use the power steering at idle.

If you have the idle solenoids, unplug them and be sure that they click when you reconnect them (ignition needs to be on). These fail and cause idle problems.

I would also check point gap and timing and look for vacuum leaks.

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Chuck Taylor
Falls Church VA
'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

I will try and reset the idle for warm running after I rebuild the carbs, they are quite dirty. The points cap rotor all look new but I will check the dwell and timing before I take the carbs off. Would like to get it running on the original Zenith's before considering the Webers. One other question, where is the vacuum tank on a 71 250C? It's not in the trunk perhaps behind the rear seat? The locks are not working, suspect a vacuum leak somewhere. I always was confused by the choice of vacuum locks.. I will be draining and changing the tranny fluid and filter as well. Nothing on what type of fluid to use in either the owners manual or the Haynes manual. From the forum I have read the Dextron III wold be correct. Is that OK? Or what do you all use in your Automatics?

Thanks,

Ron

As far as the points go, you might want to consider a Pertronix electronic system. I spent $70 on mine and I can tell you there was a significant improvement in running.

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Adam Lumsden
(83) 300D
Vice-President of the MBCA International Stars Section

Check out the idle circuit on each carb by locating the idle air hole on the front top of the carb, and plug it with a pencil eraser. If the warm idle falls, then the idle circuit is OK.

If the idle doesn't change, then try a "hand job" to unplug the idle air passges. Bring the engine up to 2500 RPM or so and put your hand over the primaries. You will fee the suction on your hand as the engine starts to die. But it will force air through the idle circuit and may unplug it. Repeat a couple of times.

If the car is otherwise running well and warming up without backfiring, I would avoid the temptation to rebuild the carbs or adjust them. About the only wear parts are the float valve and the accelerator pump and they are easy to check.

I would leave the carbs alone for now. We have seen a number of Zenith problems start with a rebuild or adjustment!

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Chuck Taylor
Falls Church VA
'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

Ron:

You really need to get the rear carb properly setup for the vacuum control valve; this should greatly aide with your driveability issues. The vacuum valve kicks in significant amounts of additional throttle so the engine will not stall when it is in gear or idling with the AC on. Find one of those scrap Zeniths and get the whole setup.

As to giving your carb a "hand job", with all due respect to Chuck, I believe this merely mimics the choking action and draws additional fuel through the main fuel discharge ports in the venturi. If you suspect the idle may be plugged try three things:
1. Remove the idle jets and spray cleaner into the passages.
2. Check and clear the idle air bleed holes in the carb top. These are very small drilled holes that you can see in the top of the carb very near the primary air jets. Your engine speed should change if you plug them with a pencil tip.
3. I believe your carbs are 32/40 INATs with no bowl vents and solenoid idle control. Make sure this works by disconnecting the solenoid; the engine should stall.

FWIW,

230/8

p.s.: Don't give up on your Zeniths. Webers do not perform any better. Once you have sound Zeniths properly setup they are reliable and trouble free. But, there are a lot of adjustments that must be made properly. Once you do that, you can forget about them. Based on your description it sounds like your car is suffering from bad work by mechanics who did not understand the car. As Chuck said, many Zenith problems are caused by improperly done work during a rebuild.

No, it doesn't mimic the choking action. You don't want to run it with you hand over the primary. You want to cut the air flow to the main venturie drastically and only for a second or two, and transfer some of the vacuum to the idle circuit, where it can pull out a blockage if its not too bad.

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Chuck Taylor
Falls Church VA
'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

Got the parts in today. I am going to do a "partial" rebuild, I will take the tops off and look at the float jet assemblys. Will check the top plates to see if they are warped. I have most if not all of the service history on the car, somebody has been in the carbs before, but other than the vacuum throttle and float vent valves all seems OK. I am not afraid of carbs, very good with SU's and Weber sidedrafts on British cars. The Zeniths are a bit more complicated but still similiar to American carbs. Should get to it this weekend. I will report back on how it goes. The shutoff solinoids are working fine. Carbs are 35/40 INAT types.

Still one more question, is Dextron III correct AT fluid for this car?

Thanks,
Ron

Hey guys I have a big problem with my carbs and maybe someone can point me in the right direction and help me fix this. I have a 1967 Mercedes 250s. When idling from cold it runs rough. It will warm up but I have to help it some. It will fire up right away and time though. If I give it any gas and start to rev up the motor the rear carb shoots gas out of the float bowl vent hole. What could be causing this and what is the best way to fix this? I have feeling this is part of the reason it is running rough on idle. Past idle it runs strong and doesn't backfire. Very weird. Any help or suggestions on fixing this problem would be great. I want to go out and drive the car some more but I am afraid that I will start a fire with the way the gas is shooting out. I hope to hear back from someone..

Sincerely,
Paul Kepner IV

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1984 Euro Spec Mercedes 500 SEL
1967 Mercedes 250S
1985 Jaguar XJ6
1979 Fiat Spider
1994 GMC Safari

This is just a for what it is worth kind of thing. Take it as you wish.
When I was faced years ago with zenith carbs that had gas dry out in them several times sitting. The idle passages were obstructed. I thought in unpleasant words that I would have to soak them out and kit them. Then I remembered laquer thinner dissolves gas residue. So I filled the bowls through the fuel line and let them soak until the thinner flowed normally out the idle jet ports. Actually let the engine pump them full of thinner by separation of the fuel line under the car. Remarkable the resultant change afterwards.
I let the car run out of fuel first as the thinner then primarily fills the bowls rather than going down the return line. They were even far better than the last time I had used the car after treatment.
Cars run and idle well on laquer thinner. Took two soaks of about twenty minutes each time until I was satisfied. As far as I can determine there is no downside when trying to clean the carbs of dried out gas residue by this method. Or I guess I should say I found none.
This is helpful mostly for idle problems needless to say but does seem somewhat benificial overall again in just my opinion. I checked around afterwards to see if anyone used this approach. Could find nobody that did but felt it beat carb kitting as a result of needing just a basic cleaning.

Paul:

Your Zenith carburetors are basically fine pieces of equipment. While I have no evidence to support it, I suspect they are only suffering from either years of neglect and/or uninformed tinkering. Here is what I would do.

First, after the engine is warmed and running well, look into the air horns and see if there is any fuel dribbling from the center of the venturi. This can also cause the backfire. If so, the float in that carburetor may be too high. Go to this site for information on the Zenith rebuild process:

http://www.jaimekop.com/

Click on the carb manual link.

If the car runs pretty well after warm-up, I would check the timing to ensure it is at the factory specification; 35 degrees BTDC at 4,500 rpm with an initial setting between 3 and 13 degrees BTDC at 800 rpm. The condition of your points and plugs is immaterial at this point if it seems to otherwise run well.

If your timing checks out pretty close I think your next step should be to establish a good balance and idle speed setting for the car. This will probably fix the problem.

Remove the air filter assembly and make a temporary connection to the intake manifold from the cam breather hose if your engine is equipped with this PVC system. A length of 3/8 inch diameter rubber fuel hose will do the job so there are no vacuum leaks. Next, snug up the four base screws on each carburetor to reduce the chance of a vacuum leak. Now you are ready to balance the carburetors. Here is what I have written in the past. I think it is fairly easy to follow:

ZENITH ADJUSTMENT

1: Set the idle mixture screws at 1 1/2 turns. Fine adjusting the idle mixture can come later. The 1 1/2 turns is a good start setting for you.

2. Disconnect the linkage between both carburetors, but do not change its length, yet. It may never need to be adjusted after you get the carburetors set up, or it may need some tweaking. Later for it.

3. Make sure there is a hose connection from the breather tube to the inlet on the manifold otherwise the car won't run or idle properly. Mentioned this before, don't forget it.

4. Check the manifold vacuum line for the power brake to ensure it is tight and not leaking.

5. Identify the idle speed control dashpot on the rear carburetor. Locate the screw on the dashpot that contacts the little pad on the linkage and run in this screw until it does not touch the linkage at all. Get some clearance; try for 1/8 inch if possible. You will have to properly adjust this later to set idle speed with the auto transmission, if you have an automatic. You may not get clearance between the screw and linkage pad with the engine off, it may need to be running so vacuum will pull it back away from the linkage. This is a good time to replace the rubber vacuum hose.

6. At this point, try to get a vacuum gage on the vacuum line to the dashpot and see if you get a reading. Hopefully you should get 15”-18" of vacuum that will indicate no major vacuum leaks. (The vacuum reading will depend on your elevation; i.e. you get a lower vacuum reading in Denver than for the same engine in LA.)

Now you can concentrate on your next task which is to properly set the basic idle speed and balance. You will need some proper tools to balance Zeniths; a bonnet adaptor and a UniSyn flow meter. You can get them here:

http://www.baumtools.com/

Do a search on Zenith to locate them. I cannot say what they cost, I got mine cheap years ago in Vienna. If you have a UniSyn you can try to use a Cool Whip or similar food container to fit over the carburetor tops. There is usually a very close size that can be made to work in a pinch.

7. Attach a tachometer to the engine to measure idle speed.

8. The adaptor hood is placed on top of one carburetor and the UniSyn tool is place atop the adaptor. There is a rubber gasket on each carburetor where the air filter seals, leave it in place.

9. Press down firmly on the UniSyn/adaptor assembly to establish a seal against the gasket so the air can only flow into the carburetor through the UniSyn. While pressing down, adjust the UniSyn so the little gage ball is floating about midway in the column. Be sure the column is vertical and you adjust the UniSyn so the ball is floating near a marker line on the column. Check idle speed and adjust this carburetor to near the spec speed. This should be about 750 - 800 rpm for your car. Note the position of the gage ball in the UniSyn column. Mark it with a grease pencil if necessary.

10. Once done, move the whole adaptor/UniSyn as a unit to the adjacent carburetor. Do not readjust the UniSyn. Press down firmly, again, and observe where the ball is floating. A balanced pair of carburetors will have the ball floating at the same position on the second carburetor. If it is different, adjust the second carburetor to match the first.

11. Move back to the first carburetor. Check idle speed and position of gage ball in the UniSyn column. If idle speed changed out of spec or the gage ball is not in the same position as the adjacent carburetor, re-adjust by tweaking the carburetor adjustment slightly, up or down Do not adjust the UniSyn to alter the gage ball position.

This is a trial and error process, but once you make the first round of adjustment it all falls into place quickly at the second round of adjustment.

You now have balanced Zeniths and an idle speed of 750 - 800 rpm, and can now fine tune the idle mixture screws slightly, and evenly.

12. Slowly turn in one idle mixture screw until the engine falters and slowly turn it back out until it smoothes out again. Do the same on the adjacent carburetor. They should be pretty evenly adjusted, within 1/8 turn when you are done, and the engine idle speed should still be within the 750 - 800 rpm range. Try to keep track of the turns. That's all there is to it; nothing fancy, no secret handshake required.

13. Now re-connect the linkage between the carburetors; loosen the ball-end and adjust to fit if necessary. Once done, check the balance again to ensure it did not change with the cross linkage fitment. If so, quickly go through a final balance step with the UniSyn and you are done with balancing, no need to remove that piece of connecting linkage for this.

Next you need to set the curb idle and this is done with the dashpot you disabled at the start of the process. You need to initially adjust this dashpot to gain operating clearance between the screw head and the carburetor linkage when the engine is running at idle. You adjust this dashpot in two ways: running the screw in or out and adjusting tension on the big spring using the large nut or knurled adjusting wheel.

14. With the engine running and transmission in neutral, the dashpot screw needs to be set within .004" of the contact pad on the rear linkage. At this point you should transfer your interest to the adjustable spring on the dashpot. The spring tension needs to be set so that under vacuum the dashpot plunger assembly will withdraw the screw away from the linkage pad, increasing the clearance between the screw and the pad on the rear carburetor linkage; you want the spring just tight enough to pull back and bottom-out the plunger assembly, if possible. It may not be possible to get this piece to retract completely; it depends on your engine vacuum at idle. Your goal is to set spring tension that allows the plunger assembly to be pulled in completely with your available engine idle vacuum. But, you can get the spring too loose. If it looks like loosening it does not cause it to pull back, try tightening it a bit to the point the spring begins to extend. Again, this is somewhat trial and error. Once you have adjusted the spring so the dashpot plunger assembly has pulled back its maximum amount, go back to the screw and re-check the .004” clearance and re-set it if necessary. Use a thickness gauge to measure this.

15. Disconnect the dashpot vacuum line. This should cause the dashpot plunger and screw to extend and push against the contact pad raising the idle speed a bunch. If it passes this test, reconnect the vacuum line and then with the wheels blocked and emergency brake set, put the car in gear. The dashpot should extend enough to keep the idle speed at 600-700 rpm in gear. This prevents you from stalling the engine at stop lights and when you have the AC on. Check this further by turning on the AC (if equipped) and see if the dashpot boosts the idle up a bit more to keep the car from stalling. YOu might need to tinker this dashpot adjustment to get it optimum for your engine, but this should get you real close to perfect.

16. When you are done with this adjustment there should still be .004" clearance between the dashpot screw and the linkage pad with the engine idling in neutral. If not, re-set the clearance and re-check the idle speed in gear.

After all that you should have well balanced Zeniths that will remain so for a very long time. Install the air filter and then resist any urges to tinker with the carburetors.

Once you have reached this point you can adjust timing and replace points or plugs anytime without worrying about the carburetors.

Hope this helps,

230/8