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#1
05-30-2007, 12:05 PM
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1973 Mercedes - Back Fire
Model: 280C
Year: 1973 I have replaced the carburetor, set new points, but it's still back firing, I don't know what to do anymore, any suggestions? I have also created a video on the problem if any care to take a look, maybe that will better help you help me. http://vids.myspace.com/index.cfm?fuseaction=vids.individual&videoid=1594026895 
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#2
05-30-2007, 12:26 PM
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Saw the vid...and the backfire that almost lifted the air cleaner off the car!
Two other possibilities to consider: 1) Bad plug wire(s)...worn or exposed insulation causing one or more wires to "arc" onto the metal on the engine. This however would cause the engine to sputter at all speeds, and your engine sounds strong otherwise. 2) Intake or exhaust manifold leak...more likely intake...at the gasket seal between the manifold and the head.
__________________
2009 ML350 (106K) - Family vehicle 2001 CLK430 Cabriolet (80K) - Wife's car 2005 BMW 645CI (138K) - My daily driver 2016 Mustang (32K) - Daughter's car
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#4
05-30-2007, 03:09 PM
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Look at the wires in the dark. Sometimes you can see a bad wire arcing.
Have you changed the distributor cap?
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#6
05-31-2007, 11:06 AM
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I know that you said that you set the points, but did you also set the timing? You need to set points first and then set the timing because any small amount of change in points is a big change in timing.
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#8
05-31-2007, 05:50 PM
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not quite sure what you mean. The "timing" that I am refering to is the relationship between spark and location of the piston in the corresponding cyl. Your timing chain controls the relationship between piston location and valve location. You need a timing light to check the timing that I am talking about. You loosen the clamp on the distributor and rotate it to set the proper timing (this procedure is located in a basic car manual. For the timing chain to be "froze" you car would not run at all. From the video it looks like the car is running good absent the backfire.
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#9
06-05-2007, 10:43 PM
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Late valve timing? Large vacuum leak? (vacuum leaks are not unheard of on those solex carbs - usually between the carb and the manifold)
__________________
It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein 09 Jetta TDI 1985 300D
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#11
06-19-2007, 08:40 PM
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Carb issue?
A 1973 280 would have a Solex 4A1 carb. Other than a stuck float, the power valve in the carb could be spitting gas.
What conditon was the carburetor replacement? Rebuilt? Holley 350 cfm 2 barrel, replace the Solex
__________________
1970 280S M130 engine- good runner 1971 250 M130 engine- #2 rod bearing, gone 1971 280SE (blown engine,parts car) 1977 German 280S W116-only 33 years old
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#13
06-22-2007, 09:24 PM
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If it fires out the carb, it's either getting spark when the intake valve is open or seriously pre-igniting.
Check to make sure you have the plug wires going to the correct cylinders (the order is marked on the valve cover as a series of numbers). A pair swapped will do this. So will a flat cam lobe on an exhaust valve. Both will cause rough running all the time, though. However, if it only bangs on deceleration, you have a massive vacuum leak somewhere and/or the ignition timing is way off (this later is most likely). You need a timing light that triggers off the #1 spark plug wire, it then fires a xenon bulb -- the light flash "freezes" the rotation of the front crank pulley so you can see the pointer and the degree wheel there. You loosen the distritbutor clamp bolt and rotate the dizzy to set the timing where it belongs. You must also use the correct procedure to set the timing on this beast, it's not a Ford -- I don't know which permutation of the emissions controls you have, and don't know if it should be set at idle or at 3000 rpm, or with out without some vacuum lines attached. Without any other data, I think setting it to 35 degrees BTDC at 3000 rpm (be carefull, only a few seconds at that rpm with no load!) should get you close, whatever it is supposed to be at idle, likely 10 degrees ATDC. If you set it at the "normal" 6-8 degrees BTDC with the vac line(s) off the distributor, likely it will be way off. You also need to verify the setting and operation of the decel valve/throttle positioner, any switch activated vacuum swithover valves, and basic mixture -- over lean on decel will bang, but usually in the exhaust as the mixture won't burn with a spark in the cylinders. Double-check the valve timing (a pain on that car if it's got the M110 twin cam, you have to check BOTH of them) -- a tooth off will give you fits (the engine will not rotate it it's that late). Need to check chain stretch too. Peter I suspect either ignition timing or carb setup, personally.
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1972 220D ?? miles 1988 300E 200,012 1987 300D Turbo killed 9/25/07, 275,000 miles 1985 Volvo 740 GLE Turobodiesel 218,000 1972 280 SE 4.5 165, 000 - It runs!
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#15
07-05-2007, 12:16 PM
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This sounds like a lean mixture, probably caused by a vacuum problem, to me. These carbs are famous for leaking air around the top plate. Rebuilding the carb doesn't usually help this. Try getting and installing two top gaskets from MB (the thick ones).
Another trouble spot is the carb base gasket, espcially if you re-used the old one after the rebuild. If you did, replace it and make sure you have even torque on the manifold. Do you get the backfire all the time, or does engine temp make a difference?
__________________
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
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