'71 250 Backfiring through the Carbs - Page 2 - Mercedes-Benz Forum

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post #11 of 17 (permalink) Old 07-25-2005, 06:04 PM
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RE: '71 250 Backfiring through the Carbs

you could have a vaccuum leak but I'll bet you checked everywhere. Try loosening the distributor hold down screw and turning the whole distributor counter clockwise about a half inch and see if it makes a difference. if it is a little better but still not good turn it another half inch counter clockwise. that will advance your timing. If it does no good put it back where it was. For some reason my car runs a whole lot better with the timing advanced. I can get pretty good power even up hill but I know something is still wrong and I will keep trying things but you should try this just to see what happens. Alot of people time their engines by ear turning the distributor and never put a timing light on it. Also you could try turning your mixture adjusting screw in a little and if the engine stumbles turn it back but do this after you try the timing. Just do one thing at a time so if the engine is affected you know what did it. good luck.Pat in NH.

Pat Mc.
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post #12 of 17 (permalink) Old 07-25-2005, 07:34 PM
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RE: '71 250 Backfiring through the Carbs

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MB250 - 7/25/2005 7:13 PM

I still haven't fixed it, I think i have figured out that it is not the carbs though because I rebuilt both of them and it still runs the same, I checked the timing and that was correct. Now the dwell, I have no idea what or how to play with that but I suspect it to be the root of my problem because it has done this in the past and looking through old receipts from the dealer the thing they did to fix it everytime was to set the dwell and adjust the timing. If anybody could tell me what dwell is and how to set it or advance it I would surely apprectiate it. By the way the ignition system is points.
Thanks
The screw that holds down the points assembly, loosen it and push the assembly towards the cam of the distributor. For mine (after I did the Weber conversion, it was backfiring under load, despite every possible carb adjustment), I had to push the assmebly until it was making contact with the cam on the distributor. After that, it cleared it up. I didn't have anything to measure the dwell, but I just kept adjusting in small increments until it ran without the snap, crackle, and pop.
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post #13 of 17 (permalink) Old 07-26-2005, 05:47 AM Thread Starter
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RE: '71 250 Backfiring through the Carbs

Thanks guys, I'll try it and report back in a day or so.
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post #14 of 17 (permalink) Old 07-31-2005, 04:58 PM
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RE: '71 250 Backfiring through the Carbs

my idle jets were loose. when I tightened them in the idle mixture screw startrd working

Pat Mc.
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post #15 of 17 (permalink) Old 08-05-2005, 06:24 AM Thread Starter
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RE: '71 250 Backfiring through the Carbs

Quote:
pm220b - 7/25/2005 8:04 PM

you could have a vaccuum leak but I'll bet you checked everywhere. Try loosening the distributor hold down screw and turning the whole distributor counter clockwise about a half inch and see if it makes a difference. if it is a little better but still not good turn it another half inch counter clockwise. that will advance your timing. If it does no good put it back where it was. For some reason my car runs a whole lot better with the timing advanced. I can get pretty good power even up hill but I know something is still wrong and I will keep trying things but you should try this just to see what happens. Alot of people time their engines by ear turning the distributor and never put a timing light on it. Also you could try turning your mixture adjusting screw in a little and if the engine stumbles turn it back but do this after you try the timing. Just do one thing at a time so if the engine is affected you know what did it. good luck.Pat in NH.
Ok just a quick question as I haven't gotten around to playing with the dwell when I advance it counter clockwise should the car be off and at TDC? or should the car be running? Does it not even matter, could I just go outside and turn it without doing anything else? I haven't really gotten around to doing this yet because the mechanic we took it to said he'd fixed it but he hadn't so he said he'd come over to our house this weekend and take a look at it.
Thanks
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post #16 of 17 (permalink) Old 08-06-2005, 08:32 PM
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RE: '71 250 Backfiring through the Carbs

You should loosen the hold down screw while the engine is off just for safety but keep the screw snug enough so you can just turn the distributor by hand but not too loose because you dont want it moving around on its own. Dont wory about Top dead or anything you can turn it while the engine is running at idle and as you slowly turn it you will notice the engine running better or worse. Counter clock wise advances and clockwise retards. If you do have a timing light put a white chalk mark on the zero on the Crank shaft damper (harmonic balancer)then attach the timing light to # 1 spark plug wire and as you turn the dist you will see the degrees that the white mark is from the pointer but if you dont have a light just time it by ear by turning the dist slowly as it idles. till it sounds better. Leave it just snug and road test. You might have to stop on side of road and turn it back a little or forward more. Best to move it a little at a time till you find what works best for you.

Pat Mc.
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post #17 of 17 (permalink) Old 08-07-2005, 08:00 AM Thread Starter
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RE: '71 250 Backfiring through the Carbs

Oh, ok everything is clear to me now. Thanks Pm220b
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