280S backfires at highway speeds

[tivoklr]

I have a 1970 280 S that has recently started to backfire occasionally only when at highway speeds. It seems a little surgy when cruising too.

If I floor it it proceeds up to 70 no problem, no hiccups, but if I then let off the accelerator and maintain speed, it will backfire once or twice when I reapply the throttle to say, go uphill.

The timing is set to 30 degrees advanced at idle with all the vacuum lines hooked up to the distributor. The distributor has had pertronix parts with the pertronix coil for a year or so, and has worked great.

Of course, now we are looking to sell this car and it starts acting up.

Any suggestions would be welcome, as I know selling it with this problem will be difficult...

[flacollect]

I am assuming it has carbs. Last time I said something the guy who seems to know a lot about benz' said "it's fuel injected there's no fuel pump" --. Wow, I didnt know it was fuel injected. -- Anway, on the carbs is a diaphram that helps keep the cars fuel at correct levels when you let off on the gas. If they go bad, that could cause your problem. Carbs been rebuilt lately? If not kits are about 40 bucks. Just my 2 cents. Good luck.

1967 - 250S - 4 Door - Power Windows and Air

[tivoklr]

It has dual Zeniths. Maybe that is the problem...my problem is I don't have time to fiddle with this car right now. Last semester of school, blah blah blah...

I'll keep that in mind though.

Thanks.

[Phill]

I have never even seen a Mercedes gas engine, however, in the past I have worked on a few carburetors. I have found that a lean condition will cause backfiring, possibly a timing problem too. I would check that both carburetors are synchronized first. If the carbs have accel pumps them they must both flow a nice stream with no hesitation. Carbs are probably dirty and the high speed circuitry not flowing as well as it should. You may have a vacuum leak in the manifold gaskets or vacuum hoses. This will lean out a cylinder(s). Try squirting some WD40 around every joint you can find and listen for a change in engine idle speed. Sometimes a can of carburetor cleaner squirted into a carburetor will open up the air bleeds and make a difference. Surging is a sign of a lean condition I think. Phil

[MunichTaxi]

If the backfire is from the exhaust, the fuel mixture is too rich.

If the backfire is from the carbs, the fuel mixture is too lean.

[mak]

clean the carbs with a spray cleaner , the top section has two brass jets visible ,inject spray here to clean . Most likely its a air leak somewhere ,the vaccuam pipe near the dashpot or the the unit itself .
mak