I have a 1974 280C that the timing fluctuates. It is rock solid at idle but when you hold it at 2500 rpm the timing fluctuates from 30 degrees to 20. It seems to change at a pretty consistent rate. Almost like something is switching back and forth. Any thoughts would be appreciated.

Disconnect and plug the vacuum lines/line to the distributor advance mechanism and recheck.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Ok. I hooked the timing light and dwell/tach back up. I let the engine warm up and ran two tests. The first with the vacuum advance hooked up. The timing did not jump until right around 2700 rpm. Then the timing would jump from around 25 to 35. You could see the rpm jump about 100 when the timing advanced and then drop back down when the timing retarded. The odd thing above or below that rpm it stayed pretty steady. Test 2 was with the vacuum line to the advance off and plugged. I saw no big jumps in the timing throughout the range. At the above mentioned 2700 rpm it seemed what I would call twitchy. Fluctuating a couple of degrees back and forth.

Does your '74 still have the original ignition points set-up? A 2 or 3 degree fluctuation can be caused by "point float", a bit of wear in the distributor bushings, a slight binding of the advance mechanism plate or within the timing light itself.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Yes everything is original. I'm no too worried about the 2 or 3 degree movement it's the 10 degree jump that I would like to figure out.

In the interests of protecting us from ourselves, the EPA and the CARB saw fit to require a supposed reduction of exhaust emissions in the early '70s. Mother Benz included RPM triggered vacuum switchover valves to control timing advance. What is being observed is:
1) "Normal"
2) Unnecessary
In '75 timing control reverted to centrifugal only; no vacuum.

Frank, the switch-over valves are also RPM controlled? Hunh, you learn something new every day. I always thought they were activated by the 70C temperature sensor and the A/C solenoid. Talk about redundancy.

I believe '75 and later models went to the VR ignition system which eliminated the ignition points removing the need for ignition retard.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

The way the retard system works is:

At idle or "no-load" (cruise speed) vacuum is high. This pulls the lever on the mechanism offsetting the centrifugal effect of the weights resulting in a 10 degree reduction of advance. This causes a later ignition event of the fuel mixture resulting in a more complete "burn" leading to fewer un-burnt hydrocarbons and a lower CO level exiting the exhaust.

Under acceleration, vacuum drops and the weights advance the timing for performance.

As vacuum increases (accelerator pressure is reduced) the lever is pulled and advance is again retarded.

The RPM control adds another layer of complexity to the mix.

The hemispherical combustion chamber design and the carburated M110 tend to run a bit "fat" in the fuel mixture at lower air flow so MB engineers had to jump through all kinds of hoops to get the emissions to pass the EPA standards.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

'75-'76 still had points; '77 VR

Ahh, it must have been a later '76 I had worked on or someone had changed out the distributor.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

So what you saying is this is normal even at the higher rpm? I can kind of understand retarding the timing at idle but to fluctuate the timing at driving rom's seems counter productive.

Easy-peasy solution. If your state doesn't require emissions testing on your model year, disconnect and plug the line. You can always re-connect the line if your state does require emissions for the duration of the test.

You did NOT hear this from me or any members of THIS site!

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

"Productive" and Federal regulations? Those terms have no common relationship. I don't know if the "six degrees of separation" theory can be stretched enough to form a connection.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

So if I do this then I would need to set the timing so overall advance is what? I deal with race cars without vacuum advance and we normally set total advance around 40 degrees. What would be the total advance required (35)?

At idle, 10 degrees BTDC. At "full advance" (I believe at 3000 RPM's but 2700 is close enough) roughly 30-42 degrees depending on distributor number.

You're probably going to have to split the two to get a good average. I'd try to keep to the lower end of the scale for "normal" everyday driving.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now