Backfire and Run-ON. - PeachParts Mercedes-Benz Forum

johan01 · #1 10-15-2001, 12:41 AM

What causes sneezing/backfire through the carbs? Most of the time, the backfire is so violent that the engine just stops running.

And the engine keeps on running for a few seconds after turning the ignition key off -- ending with a big wheeeeze.

What's going on?

BTW, I was playing around with the carbs and ignition timing -- I think I've lost my baseline.

Could someone please post the tune-up specs and procedure for a '68 280S?

Thanks in advance,

MikeTangas · #2 10-15-2001, 01:37 AM

I'm guessing you have a non-US version since you're in the PI.

Looks like you want 8 degrees BTDC without the vacuum advance connected. That should get you in the ballpark. You'll need a timing light which will connect to #1 spark plug wire. There is a pin and corresponding notches on the harmonic balancer (crankshaft pully). Highlight 8 BTDC with chalk or whiteout or yellow crayon so it will stand out (if your pully is clean you may still have the original highlighting visible). There is a 5mm allen capscrew which retains the distributor. Loosen that screw a tad and turn the distributor slowly until the highlight aligns with the pin. Once timing is set, secure the distributor and recheck to be sure nothing moved.

Reconnect the vacuum advance and hit it with the light, timing should be in the ballpark of 2 degrees BTDC.

Incorrect ignition timing will cause the problems you described.

Hope this helps.

johan01 · #3 10-15-2001, 01:50 AM

Thanks Mike!

The car is US-spec, but I don't see any emmissions equipment on it. Is it because its a '68?

Does the US-spec engines require different timing specs?

Thanks in advance,

MikeTangas · #4 10-15-2001, 01:57 AM

For 1968 with US emmissions control it calls for TDC +/- 3 degrees without vacuum advance connected.

It's possible your car is US spec, but without US emmissions controls. Try both settings, see what works best. My money is the first setting will be best, but try them both. If you haven't done so yet, you might also check into adjusting your valves. You will of course have to reset ignition timing afterwards, but it sure did smooth out the idle when I did it.

johan01 · #5 10-15-2001, 03:59 AM

I'll check on the ignition timing and get back to you.

Just a quick question as to WHY it runs on when the ignition is out of spec?

Thanks in advance,

#6 10-15-2001, 12:38 PM

[Hi guys, I'm new here and will be learning a lot from you all so I'll try to make myself useful and contribute too.]

Setting the timing to the correct spec is a good idea since it is quite possibly the cause of your backfiring, and an incorrect ignition timing leads to unburnt fuel which causes more deposits (see below) and can also cause the engine to be a lot hotter than it should be (see below) but as far as I understand cars, the ignition system is not causing the "dieseling" itself.

One common causes of run-on is that the throttle plates are sticking open slightly and allowing fuel in, or your idle circuit is letting fuel in. If you've adjusted your idle speed a lot when you were adjusting the carbs, maybe they aren't coming to a complete close anymore. Or your float levels are way off and the fuel is overflowing. Somehow fuel is getting into the engine. Then carbon that has built up in the combustion chamber gets hot and ignites the fuel (i.e. it isn't a spark from your spark plug that ignites the fuel, but simply combustion chamber temperatures). If the car runs-on more when you shut it off when the engine is really hot then the leaky carb and carbon cause is likey it. I'd check the fuel system and also run a product through the engine that claims to disolve carbon buildup (there are many on the market such as STP or Marvels Mystery Oil).

Some cars have a run-on valve that is supposed to shut off air/fuel when the ignition is cut. This sometimes goes bad and leads to dieseling.

I'd also make sure that you've got a good tank of gas. Some parts of the world switch to "winter gas". If you run this stuff on a hot day, it can be trouble because it vaporizes easier than "summer gas". Same thing goes for gas that is too low an octane rating (remember that carbon deposits in your combustion chamber basically increase your compression ratio, requiring higher octane - another good reason to try to disolve the deposits. In fact, this is what most of the over the counter "octane boosters" actually do - they don't add much more than .2 or .3 to the octane rating of the fuel, but they contain additives to clear out carbon reducing your engines need for higher octane and so eliminate pinging that way), and gas with a high alcohol content (unfortunately we are stuck with this stuff in the states now).

Sharp edges have more hot spots than smooth edges so cars that have a slightly burnt valve or two that has reshaped itself to have sharp edges also have more problems with dieseling and pre-ignition.

Anyway, ignition can take place without the spark plug - but it can't really take place without fuel. So I'd try to figure out how fuel is getting in.

-Rich

johan01 · #7 10-17-2001, 02:35 AM

I set the ignition to about 10-degrees btdc with the vacuum advance disconnected and plugged up. Cleaned the points too while I was at it.

With this igntion setting, I was able to back off the idle speed adjustment on the carb.

No more backfires nor dieseling.

Runs great, climbs my steep driveway without stalling, pulls away from stop without drama.

I think I've got it guys. Thanks!

PS: I'll have to make a carb hood adapter for my synchrometer to check if the carbs are in synch. I just used a vacuum cleaner hose to my ear to check for pitch and volume -- it should be close enough.

regards,

MikeTangas · #8 10-17-2001, 10:14 AM

Glad to hear it's running good now. Enjoy the beast

BTW: What does the timing show to be at idle with the vacuum advance connected - somewhere in the area of 2 dgrees ATDC?

johan01 · #9 10-18-2001, 04:50 AM

It actually reads about 14-degrees btdc with the vacuum advancer hooked up. I thought that was correct -- so do I have my vacuum lines hooked-up wrong?

There are two vacuum ports near the front carb. One on the base of the manifold, and another right on the carb itself -- about same plane as the throttle valve.

I plugged the one on the manifold -- and hooked up the vacuum advance line to the one on the carb. My don't think my distributor has vacuum retard.

BTW, the breaker plate (is that what it called -- the flat round "platform" where the contact point is screwed on and where the vacuum element tugs on) is loose. With the engine off, I can rattle it around -- and can visibly change the gap of the contact point. There's a spring-loaded ball bearing holding it in place. Is that really the only thing holding it down? Should I re-tension the spring so it pushes harder on the plate? Oh boy, it doesn't look right -- but if it is, I'd like to hear about it.

thanks in advance,

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