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  #1  
Old 11-01-2001, 02:12 PM
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Location: Pittsburgh PA
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BACKFIRE..thru the carburetor

HI guys!

I have a 1969 model 250/8 sedan w114 m114 with dual Zenith carburetors. I have been trying to get this car to RUN for some time now. (Novice!)

The car will almost start now, but stalls on every attempt because there is a BACKFIRE through the REAR carburetor.
Is this a carburetor adjustment? If so, which ONE?? OR is it the timing?

Help!!
~Christy
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  #2  
Old 11-01-2001, 02:22 PM
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How are your valve clearances? Tight valves can cause this problem.
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1991 300-SEL - Model 126
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  #3  
Old 11-01-2001, 02:59 PM
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Valve clearances are CLOSE if not PERFECT. They were checked and adjusted a few months ago...

Could it be POINTS??? Dwell??

~Christy
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  #4  
Old 11-01-2001, 03:03 PM
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I seem to recall that faulty ign. timing may also be a factor. Can't recall whether it's a retarded or advanced situation that can cause this.

What do your points look like? Burnt? What's the gap? How does that compare to the factory spec?
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1991 300-SEL - Model 126
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  #5  
Old 11-01-2001, 03:11 PM
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Points are NEW! They WERE gapped correctly, but the distributor has since been moved a bit so I tried today to check dwell and the darn meter reads 00?!?!?!?!??!?

The points look TIGHT, but that's how they're supposed to be.
btw...can I get an accurate dwell reading from merely cranking the starter??

~Christy
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  #6  
Old 11-01-2001, 03:51 PM
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Make sure points are gapped (the old thickness of a matchbook cover will do for now) and turn distributor to RETARD your timing(same direction that the rotor moves when you crank the starter) CW, I think .
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  #7  
Old 11-01-2001, 04:50 PM
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My experience checking dwell via cranking only has always been flaky 'cuz the dwell meter needle has a tendency to bounce a bit. I believe you get a better reading with a running engine. You mention a tight setting. How tight is tight? What does the MB manual call for?
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  #8  
Old 11-01-2001, 04:57 PM
MIKE FREEMAN
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CHECK FOR CROSSED PLUG WIRES FIRST(BECAUSE IT'S EASY)
IF THE FUEL MIXTURE IS TOO LEAN ON THE REAR CARB IT WILL BACKFIRE.THIS COULD BE CAUSED BY INCORRECT CHOKE SETTING,LOW FLOAT LEVEL,OR STUCK NEEDLE VALVE.
MANY OF THESE OLDER CARS HAVE HAD THE WIRING MODIFIED(BOTH WIRES TO ELEMENTS WERE JOINED)
THE CORRECT HOOK UP IS,POWER TO THE FRONT CHOKE ANY TIME KEY IS ON AND POWER TO THE REAR CHOKE ONLY AFTER THE ENG. REACHES ABOUT 165 DEGREES F.
MF
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  #9  
Old 11-01-2001, 06:09 PM
LarryBible
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My Dad used to call this condition "starving for gas". It is a very good explanation of what's happening. Yes it's a lean condition, at least a momentary lean condition. Additionally excessively retarded ignition timing can have the same effect.

I would suggest that you check the ignition timing first. Once you are satisfied with that, take off the air cleaner and peer down the carburetor throat with the engine not running, but the fuel bowls full of fuel. Snap the throttle open and you should see a stream of raw fuel from the accelerator pump nozzle. If you don't see the raw fuel, then what is happening is a momentary "hole" of air that is not being richened by the accelerator pump, thereby causing a momentarily lean condition. I would expect the accelerator pump to be the problem. Since the car has not been driven on a regular basis, the accelerator pump diaphragm or piston, I don't remember which is on a Zenith, is probably dried out.

Good luck,
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  #10  
Old 11-01-2001, 10:16 PM
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Thanks guys!

The carbs are a-ok....meaning they have been rebuilt and recently (yesterday) reopened and filled with gas. Floats aren't stuck and I turned the mixture screws all the way in and backed them out 2 turns for a decent starting point.

I checked the dwell...it's 36 which I'm pretty sure is within range. I have a digital meter.

So...ignition timing. That COULD BE the culprit. What does that consist of? Could you give me a qucik refresher on that? Is that cranking the motor by hand? I forget.

Thanks sooo much!
~Christy
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  #11  
Old 11-02-2001, 07:45 AM
LarryBible
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The most common method for adjusting ignition timing is with a timing light. The light is a strobe which triggers from the number one ignition wire. The light is aimed at the harmonic balancer on the front of the engine. You will need to find the timing specification, which should be on an emissions sticker somewhere under the hood. Once you find the specification, examine the balancer for timing marks. It helps to mark the balancer with white paint or white out to help so the correct point. For example if it calls for 6 degrees before top dead center(BTDC) mark that point on the balancer with the white out so you can see it easily when it passes the pointer.

Connect the timing light, carefully route the wires so they don't get caught in the belts or fan. If the engine won't start and idle, have someone turn the ignition while you aim the light and watch the marks location. You will have to loosen the distributor lock nut just enough where you can turn the distributor to set the timing, but the distributor is not so loose that it will not stay where you point it. Once you have the timing set, tighten the locknut and then recheck the timing.

After the timing is set, and you can start the engine, you need to rev up the engine while using the timing light to see that the timing advances. As you rev up the engine the mark should move about 25 or 30 degrees or so. If it doesn't, this could also be your problem. If the timing advance mechanism is not advancing the timing as the engine speeds up, it will cause this backfire that you are experiencing. If that were the case, you would need to find what's wrong with the distributor.

Also, back to the backfire, if you had a vacuum leak somewhere, it can also cause your backfire. If this were the case, the engine would not want to idle either.

Good luck,
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