Zenith Carb Issue - What would you do? - PeachParts Mercedes-Benz Forum

Mint2Me · #1 06-11-2013, 08:08 AM

Trying to get the Zeniths on my 230S to run right has been a daunting task. Symptoms: Slight stumbling with part throttle while accelerating. Not fun. Full throttle and cruising seems fine.

The front carb seems to be ok. The plugs in the cylinder that it feeds look good. The rear carb runs too lean and nothing we do can get it to run any richer. Those 3 plugs are really light in color. The fuel tank has been boiled and re-lined, new fuel filter and the carbs have been cleaned twice. My mechanic suggests getting rid of them for a pair of new Webers at $1000 plus install. Yikes. What would you do? Rebuild just the rear carb? What do you think is wrong with it? And don't say get a new mechanic. He's one of the best.

JMela · #2 06-11-2013, 08:21 AM

Assuming they are balanced (which is essential), perhaps they're an issue with your linkage. Are you getting full throw at speed? Have you changed your linkage ball joint cups? They can wear over time and get sloppy. Just an idea.

Squiggle Dog · #3 06-11-2013, 11:03 AM

My roommate's 1965 W111 220S had the Zeniths replaced with Webers. They seem to work fine, other than they are taller than the Zeniths, so the air cleaner duct and breather tube have to stretch quite a bit. Whoever did the conversion also didn't hook up the wiring that goes from the carburetors to the automatic transmission, so it slams harshly when downshifting, and I have no idea where to hook up the wires on these new carburetors.

pedrocamp · #4 06-11-2013, 05:29 PM

I have been having the same issues balancing the front and rear carb mixtures on my 68 230 W110, it runs quite smoothly for the most part though I do have a slight stumbling off idle till warm. I have removed the return valve and modified the system as a constant return, added a jet with a 0.020" hole to the return fitting. I have set the front carb float level a bit lower than the rear. I have added a Malpassi fuel pressure regulator, this really smoothed out the fuel pressure, I set it to 2.5-3.0 PSI. My linkage has been properly adjusted, carbs freshly rebuilt and balanced with a Unisyn. I still would like to move the fuel supply to a T between the carbs rather than feeding the front carb then the rear. I am assuming that the fuel pressure pulses are bouncing the front float open, thus richening the front 3 cyl... why I have to set the front lower(?). The pulses are dampened by the front float so the rear carb float is not as affected. I think by moving the fuel feed to the center this will even out the pulses between carbs. I am interested to know what you find and if any of these suggestions may help your Zeniths.

This is an example of the Malpassi I used:

Malpassi Filter King Adjustable Fuel Pressure Regulator Carb Reg 6 8mm Glass | eBay

I placed it after the pump to control the pulses. I have a standard filter before the pump to protect it from debris.

ctaylor738 · #5 06-13-2013, 05:27 PM

Have you checked for vacuum leaks around the base of the carb and intake manifold? Is there a good squirt from the accelerator pump tube when you open the throttle?

Mint2Me · #6 06-13-2013, 05:58 PM

Quote:

Originally Posted by ctaylor738

Have you checked for vacuum leaks around the base of the carb and intake manifold? Is there a good squirt from the accelerator pump tube when you open the throttle?

That's another good lead. I'll check that! Thanks.

ja17 · #7 06-14-2013, 01:48 AM

Yes, as Chuck indicates, the carb mountings can be leaking air. The paper gaskets under the carbs shrink, deform then leak air.

pedrocamp · #8 06-14-2013, 06:04 AM

Check the carb bodies for flatness, they can warp from the exhaust heat. Make sure the flapper valves aren't stuck open, I had to block mine off.

strelnik · #9 06-14-2013, 08:30 AM

Quote:

Originally Posted by Mint2Me

Trying to get the Zeniths on my 230S to run right has been a daunting task. Symptoms: Slight stumbling with part throttle while accelerating. Not fun. Full throttle and cruising seems fine.

The front carb seems to be ok. The plugs in the cylinder that it feeds look good. The rear carb runs too lean and nothing we do can get it to run any richer. Those 3 plugs are really light in color. The fuel tank has been boiled and re-lined, new fuel filter and the carbs have been cleaned twice. My mechanic suggests getting rid of them for a pair of new Webers at $1000 plus install. Yikes. What would you do? Rebuild just the rear carb? What do you think is wrong with it? And don't say get a new mechanic. He's one of the best.

The rebuild kits are 18.00, Jam Engineering sells the Webers for the amount you specified

tcb323 · #10 06-14-2013, 07:18 PM

The Zeniths are often misunderstood and chucked out for Webers, but they are reliable and good carbs if set up right.

Make sure you have no leaks as others have suggested. ANY false air will give you problems.

Make sure linkage is set up right and carbs are synced

The fuel pressure check is mandatory. The T junction suggested here is a good idea as well.

there is a blind passage inside these carbs that becomes plugged, I always drill out and clean this passage, then replace the welsh plug (lead) and this always works for me, I have fixed 2 'incurable' cars this way, which had the driveability issues you state. Do a search and you'll find a thread on this.

Your Mechanic may be 'one of the best' but if he's giving up and throwing $1000 worth of parts at your problem, without knowing WHAT is the issue with your Zeniths, I would seek another opinion.

Benz Dr. · #11 06-15-2013, 11:08 PM

I had this same basic problem on a 220S with a 250S engine it it. One carb ( the rear one ) worked great but the front was running super rich. I tried everything and eventually we changed it to another carb body. This seemed to work better but I had a very pronounced flat spot right off idle. I fixed that by bending the small arm that pushes on the accelerator pump piston. There should be no free play between the arm and the piston should move instantly.

Well....... this same car showed up again last week after winter storage. I had it running OK when it left last summer but it was now flooding the front carb, and of course, all the plugs were fouled out.

I looked it over and removed the top of the carb. The needle and seat wasn't tight enough so I found a new seal and installed it tightly - it doesn't leak now.
After some minor adjustments and a fresh set of plugs it now runs better that it ever has. Starts right up and idles smoothly. Pulls well and doesn't stall anymore.

Not easy to get right, that's for sure.

250 Coupe · #12 06-17-2013, 01:25 PM

My first ever Mercedes was a 1968 250/8 It had thrown a rod through the block so I sourced what was supposed to be a replacement engine. Things like the oil cooler lines didn't fit but I was able to get the head from the bad engine on the replacement engine and get it running. It never did run right so I threw a set of brand new Webers on it and still had trouble.

Only thing I found that might have been an issue was that the timing chain could not be indexed properly It was about a half tooth off no matter how much I fiddled with it.

Here's a link to an online manual for the Zeniths,
Mercedes Benz Zenith Carb Manual | JaimeKop.com

Michael

kenny33012 · #13 06-20-2013, 02:15 AM

I'm having trouble with my '72 250C's carbs. The problem developed recently. She starts up fine but dies when you try putting her into gear. She also dies at stoplights. She's fine on the open road. I suspect vacuum leaks and/or carb trouble.
That said, trying to find a shop experienced in this matter is very frustrating where I live. The place I usually take the car for work has what you might call an "itinerant" carb expert. He is summoned only for this type of problem. Problem is, the shop hasn't been able to locate him for the last three weeks. (This must be an ok shop...After all, they're restoring a 59 300SL!)
I went ahead and ordered rebuild kits. Each kit seems to have enough gaskets to deal with different variations of the Zenith. If I find someone to work on this, I'll give him the kits to use. If not, how hard can it be to rebuild a carb? I've got the Mercedes manual, a Haynes, and jaimekop's manual. There's also a very good series on YouTube (in Spanish) "bmw zenith inat 35/40 montaje" which was well produced and informative. It seems attention to detail and extreme cleanliness are key.
Can you tell I'm frustrated? Is there anyone near Hollywood (Florida) I can trust and depend on to do this work?
Any help will be greatly appreciated.
Ken

Palolo · #14 06-20-2013, 04:41 AM

Quote:

Originally Posted by pedrocamp

Check the carb bodies for flatness, they can warp from the exhaust heat.

And if you do discover a warped body and want to mend it, there is a solution. Sorry, can't do anything about the warped mind.

Land Rover FAQ - Repair & Maintenance - Series - Engine - Zenith Fix

Mint2Me · #15 06-20-2013, 07:13 AM

Thanks to all those who helped. Here is an update.

Ever since we received the 230S it was a problem runner. Bad fuel, bad fuel tank isn't the best way to start a relationship. For the 1st few months, it ran okay but we had cold start problem this winter. We got that ironed out but it was running lean on 3 cylinders. We had a slight roughness and I felt it struggled a bit to rev. Then it got worse which led to this ultimatum – let's get this fixed and buying a set of Webers is not a solution.

Turns out the carbs were leaking air in a few places. A good cleaning off the car and rebuild kit solved that along with some careful tuning. Not sure why it took so long but your suggestions helped. But the big news came when we decided to baseline the valves since we didn't adjust them when we got the car in December. The valves were adjusted by Ray Charles on a drinking and drug induced high. That my friends made all the difference. It now revs freely and races to 5000 rpms like a teenager. I knew there was a Mercedes in there somewhere.