71 250 Backfires doesn't start - Mercedes-Benz Forum

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post #1 of 4 (permalink) Old 03-24-2009, 11:11 PM Thread Starter
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Date registered: Jun 2008
Vehicle: 1971 250 1988 420SEL
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71 250 Backfires doesn't start


MY 1971 250 has been back firing latly specially when its cold sometimes between 30-40 mph and other times between 50-60 when it warms up it runs preety decent... Any ideas? Also if the car is sitting for a more then a couple of hours I need to spray starting fluid into the carb to get it going. I am preety sure that the car is going to need a vacume advance anybody know where the part number is located so I can order it?

Thank you
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post #2 of 4 (permalink) Old 03-25-2009, 07:32 AM
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How is your contact breaker points and other ignition components? Sure they are 100% OK?
Good luck!
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post #3 of 4 (permalink) Old 04-08-2009, 06:03 PM
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Is it backfiring up through the carb. or out the exhaust? As stated above, check the ignition completely...plugs, points and point setting/condenser, wires, cap, rotor and ignition timing. While you have the plugs out, you may also want to do a compression check, just for reference info. sake. A bad vacuum advance canister would not likely cause this, unless you are extremely lean for some reason to begin with. (You can check the canister with a hand vacuum pump.) Then it would "pop" back through the carb. Also, since most of the "backfiring" happens when cold, check for proper choke setting and freedom of operation. No, or too little choke when cold could cause stumbling, stalling, or popping back through the carb. Could also be a valve (intake or exhaust) that's bad or out of adjustment. Having to spray the carb. after sitting a while is a little more interesting??? When it finally starts, is there black smoke from the exhaust? Does it "chug, chug" when it first fires. Both these indicate richness, perhaps due to internal leak in carb., improper float adjustment, bad float, leaking needle and seat, etc. Also, if this were the case, you normally would not have to use starting fluid to start, just hold accelerator to the floor (do not pump) while cranking. IMO, try this first and see if it will start when doing this. If your starting fluid is ether or lacquer thinner based stuff, it might aid the hot start, even if flooded. Another thing to check for is other vacuum leaks. This could cause leaness, but in this case you should still be able to start it without fluid...just a couple pumps of the accelerator. I think I'd verify and make sure the ignition is working properly first, and then see where you're at after that. Good luck and keep us posted.

Last edited by mbbuff; 04-08-2009 at 06:06 PM.
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post #4 of 4 (permalink) Old 04-09-2009, 03:50 AM
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How is the timing set? Has anyone removed and re-installed the distributor?
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