Trouble setting timing - Mercedes-Benz Forum

 
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post #1 of 6 (permalink) Old 07-30-2005, 12:21 PM Thread Starter
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Trouble setting timing

Hi all,

As I'm trying to set the timing and I'm turning the distributor and every thing is fine at about 1000 RPM & 10 BTDC.

I turn the distributor clockwise just a hair more, and the rpms drop and the RPM's stumble and stalls.

What am I doing wrong.

This is all on my 72 250 W114 M130.

Any help would be greatly appriciated.

Thanks
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post #2 of 6 (permalink) Old 07-31-2005, 10:20 AM
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RE: Trouble setting timing


I don't know the 250 except I believe it is a straight six and it looks alot like mine. My book calls for four BTDC but it runs better at ten or fifteen. I dont have a problem with that. If it runs better it runs better regardless of what the book says. Can you leave it at ten and bring the idle down a little with your Idle screws.

Pat Mc.
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post #3 of 6 (permalink) Old 11-15-2005, 05:40 PM
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RE: Trouble setting timing

I take it when you say you are turning the distributor a hair more that you mean advancing it more than 10 degrees btdc. When you ignite the fuel too early the engine begins working against itself; that is, if ignition takes place at 11 btdc, that last 11 degrees of the compression stroke is being worked against. Hard on the pistons, rings, rods to advance too much, plus loss of power. In general, if timing is retarded relative to what it should be (but not retarded), the engine will start and idle great, but big loss in power. Too advanced (but not too advanced) and they'll start hard but have more power. So, it's the happy medium you're looking for. The engineer's who say xx degrees btdc should be the ignition point, they have good reason for saying so. A too advanced (but not too advanced) ignition will give you better top end power. In my opinion either use factory specs or cheat by a degree or two. Ten degrees is way too far advanced if it faulter's and dies. Keep in mind that this is distributor advance, the counter weights bring in more advance with higher rpm's. Vacuum advance, if you have that also cranks in more advance. So, for example, I'm making these number's up, but they're in the ballpark) if you have your distributor advance set to four degrees you might end up with 25+ degrees at the crank at higher rpms. But, from you description of stalling beyond ten, then ten is way too much initial distributor advance. Personally, I'd go with factory specs.
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post #4 of 6 (permalink) Old 11-15-2005, 07:25 PM
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RE: Trouble setting timing

Actually, the distributor rotation is clockwise, so rotating the cap clockwise will retard the timing, not advance it. I have the same engine in my 1972 250C, and the factory spec for the idle is 800-900 RPM in neutral. Also, believe it or not, the factory timing spec is four degrees AFTER top-dead-center at idle. 1972 was when the pollution-control era was just getting into full swing, and the excessively-retarded timing was a result. If power and/or fuel economy is your goal, you can safely vary the timing a bit.

If your engine is idling at 1000 RPM and 10 degrees BTDC, and retarding the timing even a little bit makes the engine stumble, then you may have a vacuum leak. Mine will continue to idle smoothly down to 650 RPM or so.

On the stock ignition, there are two vacuum hoses and a coolant-temperature sensor that will affect ignition timing, so you have to make sure you have the appropriate hoses and wires connected or disconnected when you're checking the timing.

The settings on my car right now are 7 degrees BTDC and an idle of 850 RPM in neutral. The vacuum reading is 15" at idle, which I initially thought was low, but talking to other people, I think it's pretty typical. I've got a Mallory electronic distributor in place of the stock one, so I only have one vacuum hose and no temperature sensor.

At 7 degrees BTDC, cold-starting and idling are perfect and there's plenty of power, but hot-starting is a little hard, so I may retard the timing a few degrees as an experiment.

If you adjust your idle, try to adjust the linkage that controls BOTH carbs, rather than using the individual idle controls on each carb. The last thing you want to do is mess up your carb synchronization while you're trying to get the timing set.

Scott Gardner
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post #5 of 6 (permalink) Old 11-16-2005, 07:18 AM
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RE: Trouble setting timing

All this talk of clockwise and counterclockwise is confusing.. but if your car runs better with a setting far off (more than 3deg..?) I believe you might have a stretched timing chain.

Can anyone confirm this?

Anil
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post #6 of 6 (permalink) Old 11-16-2005, 10:26 AM
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RE: Trouble setting timing

No, my timing chain is new, tight, and correctly-indexed.

Most cars work well with 7-15 degrees of initial advance at idle, depending on things like camshaft profile and engine size. The overly-retarded stock settings were simply for emissions control. I could run at 4 degrees ATDC instead of my current 7 degrees BTDC and everything would work fine. The engine would be easier to start when hot, and I might pick up some fuel economy, but power would be far-from-optimal.

I still think that if his car won't idle at 1000 RPM with any less than 10 degrees of advance, he's probably got a vacuum leak or some other carburetion/ignition problems.

Scott Gardner
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