W114 250 backfire problem - Mercedes-Benz Forum

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post #1 of 5 (permalink) Old 12-05-2014, 01:44 PM Thread Starter
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Date registered: Mar 2010
Vehicle: 1973 W114 250 / 1988 W126 420SE
Location: San Jose, Costa Rica
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W114 250 backfire problem

Hey guys,

I have a problem with my 1975 250 euro model. The carbs were in bad shape and the car had been sitting for about 2 years, purchased the full set of gaskets and rebuild both of them, problem now is that when the car revs the #1 carb spits out or back fires when revving specially when hot.

We checked the ignition cables, and replaced the points and condenser but the car is still having troubles. Can anyone shine a light on this? is it possible that the #1 carb is not supplying enough fuel? If you disconnect any of the first 3 spark plug leads with the car running theres isn't that much of a change on the engine specially on number 3 the change is almost unnoticeable. Because of this we also checked compression but the lowest one was 120.

Will welcome any suggestions or ideas.
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post #2 of 5 (permalink) Old 12-08-2014, 07:00 PM
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Vehicle: 1970 250c, 300e amg
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anyone... anyone... lol, seriously though im kind of curious about this now. let us know if you figure it out
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post #3 of 5 (permalink) Old 12-08-2014, 10:17 PM
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I had fire spitting out the carb on my white m130 sedan when the spark was erratic. After sorting out the ignition, the car ran really well (except for light acceleration, as it would "carb fart"). But I think that the strong spark helped. Why? I'm not sure.

Just some stock wiring, non-resistor plugs and a good coil made things much better.


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post #4 of 5 (permalink) Old 12-09-2014, 08:49 AM
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(1) Motor warmed up spray water over intake ( outside naturally ) parts , Intake Manifold , Carb base , all vacuum lines .
Just use a garden hose with low flow and direct the water. If the motor slows its sucking water in a Vacuum leak .
Vacuum Leaks cause poor running and backfire
(2)check the valve clearance . If to tight on intake can cause this backfire
(3) Re Check Compression with a good gauge motor warmed up Dry Test first then a wet test . ( wet test add a little oil to cylinders that are low to wet test wait for oil to spread before testing )
(4) Check for blue spark from spark plug wires to ground when disconnected from plug running . bad change wires .
(5) Distributor Cap
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post #5 of 5 (permalink) Old 12-10-2014, 01:15 PM Thread Starter
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Date registered: Mar 2010
Vehicle: 1973 W114 250 / 1988 W126 420SE
Location: San Jose, Costa Rica
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Hey guys thanks for all the feedback, turns out it was #1 carb going dry there was not enough fuel in the float chamber. Hope it helps if anyone else bumps into the same problem.
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