My 68 Mercedes 250S backfires - Mercedes-Benz Forum

 
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post #1 of 7 (permalink) Old 05-11-2011, 03:19 PM Thread Starter
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Vehicle: 1968 Mercedes 250S 6 cyl gas
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My 68 Mercedes 250S backfires

I just bought a 1968 250S 6 cyl gas 4 door that had been in dry storage for 5 years and can't seem to get it to run without backfiring. It is a beautiful car and has been well taken care of. The prior owner bought it in 1976 with 96K miles on it. It now has 175K miles, the engine was completly rebuilt at 151K miles at a Mercedes dealership in NJ in 1984, the transmission was also rebuilt at about the same time and also had a new paint job. The owner said the car ran perfect prior to storing it. I had a local mechanic who has worked on Mercedes for years check it. He changed the points, tightned the plugs because they were loose and checked the distributer cap, checked and adjusted the timing, adjusted the carburators, did a valve compression test. He said one valve dropped in compression and felt a ring was stuck and not seating and he couldn't get get it to stop backfiring. It will idle fine but when you give it gas it backfires out of the exhaust and conks out. He said it is not backfiring at the carburator but backfiring popping came out of the carburator at the same time. One plug is rusted in and he couldn't get it out for fear of breaking it off, all the other plugs look fine. Should I add mystery oil to the gas or oil, any ideas on what might be wrong with this. I really like the car and want to keep it. Thank You for any help with this. Tony
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post #2 of 7 (permalink) Old 05-11-2011, 11:06 PM
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I'd add penetrating oil to the plug and let it sit for a few days, and try again. Perhaps the plug is fouled? Backfiring through the carb isn't likely to be caused by bad rings.

Have you purged the fuel system of the old gas? And have you cleaned out the carb?
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post #3 of 7 (permalink) Old 05-11-2011, 11:27 PM
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Sound's like a gummy situation. I have heard of people adding two qts of diesel to the oil letting it run for an hour just at idle, changing the oil and it running fine after.

I would use a pair of insulated pliers and take the wires off (while running) until I found the dead cylinder(s), and make sure he did not put a few wires in the wrong place. I find most people make small little mistakes that carry them off to OZ. Then Im the A-hole that has to point it out.
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post #4 of 7 (permalink) Old 05-12-2011, 03:43 PM Thread Starter
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my mechanic drained the old fuel out and replaced with new, it only has 1/4 tank so will add more gas plus fuel stabilizer. I don't believe he replaced filters and oil because he thought they were ok. He adjusted carb and timing. I will probably change all filters and new oil and I have some mystery oil that I thought I would add. I bought pb blaster to try to loosen the rusted plug. Bought all new plugs and probably should buy new wires and distrubutor cap. My neighbor has all the tools and had a mercedes so he will be helping me out. Not a lot of people in my area that work on Mercedes. Going to work on it tomorrow. Thanks, Tony
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post #5 of 7 (permalink) Old 05-12-2011, 04:11 PM Thread Starter
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Will try that tomorrow, have a few friends that will help me out. I had it idleing yesterday and will try again tomorrow and add fuel stabilizer after the battery has charged. Know a lot of small mechanics but not many like to work of mercedes, but will be checking them out. Most of the mercedes mechanics in my area are out of town at dealerships and if all elso fails will have it brought there. Thanks for the info, Tony
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post #6 of 7 (permalink) Old 05-24-2011, 05:59 AM
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you should give it a valve clearance, as for backfiring, check for wear of the dist, shaft, you will be able to see this if someone can connect it to an oscilloscope, or check it manually. Once I found someone had overhauled the carbys and incorrectly installed one of the butterflys. also check that the choke butterfly is not flapping about.
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post #7 of 7 (permalink) Old 05-24-2011, 07:58 AM Thread Starter
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Thanks for the information, plan on checking these things as soon as it stops raining, I have to work on it outside, it has rained constantly here in Maine. Will post an update when I get a change to work on it.
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