1969 280S Running on 3 Cylinders... - Mercedes-Benz Forum

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post #1 of 15 (permalink) Old 04-25-2016, 10:01 PM Thread Starter
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Question 1969 280S Running on 3 Cylinders...

So as you can tell by the title my car is running on only 3 cylinders. I just got the thing running after sitting for months and after managing to get an idle, I noticed that only some of the cylinders were firing. I began unplugging the distributor lines and saw that cylinders 1,2,3 were firing but 4,5,6 were not. After talking to some buddies they concluded that the back carburetor must be clogged and not sending fuel to the back 3 cylinders. I just wanted some confirmation that this is a possibility before paying the shop that "fixed" my carbs a little visit to talk over my 500 dollars not well spent.

Any and all feedback is welcome. Stories of of how your car has been a pain in the ass also welcome. Lord knows mine has been for past few months.
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post #2 of 15 (permalink) Old 04-25-2016, 11:07 PM
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I walked away for dual zeniths and will never look back. It was a great learning experience trying to tune an m130 with tons of blowby while trying to learn the dual zenith system. Fun, but not my ideal transportation.

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post #3 of 15 (permalink) Old 04-25-2016, 11:15 PM Thread Starter
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I was considering ditching the zeniths for a single weber but I figured I would attempt to keep the originality of the car. I'm guessing that was the wrong decision.
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post #4 of 15 (permalink) Old 04-26-2016, 06:06 AM
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1969 280S Running on 3 Cylinders...

Quote:
Originally Posted by turbodiesel4life View Post
I was considering ditching the zeniths for a single weber but I figured I would attempt to keep the originality of the car. I'm guessing that was the wrong decision.

The standard zenith-to-Weber conversion requires two webers. It's apparently a solid conversion, and people apparently know how to adjust webers more commonly than zeniths. The problems I believe I was fighting were an old tired engine and the electric chokes that would adjust at different times, and assume that any time the key was in the on position that the car was running and warming up.

This method is supposed to be tried and true for adjusting zeniths, and zeniths are apparently very good carbs if you know how to adjust them.
http://www.jaimekop.com/mb-zenith-carb-manual/

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post #5 of 15 (permalink) Old 04-26-2016, 10:59 AM
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Are 4-6 spark plugs firing or not? That would be an entirely different problem.

When the engine is running look down the carb throat you should see gas going in. if you blip throttle you should see extra gas pumped in.

If you see neither, then revisit your mechanic.
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post #6 of 15 (permalink) Old 04-26-2016, 11:05 AM
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"I just wanted some confirmation that this is a possibility before paying the shop that "fixed" my carbs a little visit to talk over my 500 dollars not well spent? "

As no one took the time to answer your question............I will try. Yes your Carb is causing your problem. Simply stated. your motor is two three cylinder engines coupled with a single crank and cam. The three front cyl. have their own intake and exhaust system and the three rear cyl. have their own. That is why it is so difficult to get the carbs sync'ed properly. Fuel wise it is trying to get two separate motors sync'ed. Hope this didn't confuse you. Good luck with your car.

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post #7 of 15 (permalink) Old 04-26-2016, 11:25 AM
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"I began unplugging the distributor lines and saw that cylinders 1,2,3 were firing but 4,5,6 were not."

This could be interpreted 2 different ways. The literal interpretation is that the spark plugs are not firing.

The other is that there was no noticeable difference in how the engine ran when disconnected, but that is not definitive as to the cause.

We all know the first 2 thing when diagnosing are 1. is there spark, 2. is there gas.

At this point we dont know either.
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post #8 of 15 (permalink) Old 04-26-2016, 05:34 PM Thread Starter
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Quote:
Originally Posted by snebcb@embarqmail.com View Post
"I just wanted some confirmation that this is a possibility before paying the shop that "fixed" my carbs a little visit to talk over my 500 dollars not well spent? "

As no one took the time to answer your question............I will try. Yes your Carb is causing your problem. Simply stated. your motor is two three cylinder engines coupled with a single crank and cam. The three front cyl. have their own intake and exhaust system and the three rear cyl. have their own. That is why it is so difficult to get the carbs sync'ed properly. Fuel wise it is trying to get two separate motors sync'ed. Hope this didn't confuse you. Good luck with your car.
This is exactly the kind of answer I was looking for snebcb. Thank you so much! I will definitely have to go raise some questions (and possibly fists) over at that shop.


As for everyone that has commented about checking the spark plugs and distributor lines, I checked them and am receiving spark from each set of 6. Thank you for helping me rule that out.

Thank you all!
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post #9 of 15 (permalink) Old 04-26-2016, 05:46 PM
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One of the most important things about adjusting dual carbs is that the valve adjustment is done correctly. If all cylinders do not have the same vac to pull fuel in it will be very hard to make it run correctly. yes it does sound like the rear carb is not working correctly

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post #10 of 15 (permalink) Old 04-26-2016, 08:00 PM Thread Starter
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Hmm... I will check valves if the carburetor turns out to be running ok. Thanks for the tip.
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