My timing is fine at idle and low RPMs, but jumps back and forth 5 or 10 degrees at 2000 - 3000 RPM. Timing is set at 4 degrees ATDC and now advances correctly to about 30 degrees BTDC at 3000 RPM.

I have eliminated the distributor as the cause by swapping it with a new one. I have tried both conventional points and a Crane electronic ignition. The engine is a Metric Motors rebuild with less than 1000 miles on it.

I am at a loss where else to look. Would a bad plug or cable cause timing problems such as this? What should I test?

Thanks,
Steve
1969 280SL

BTW - replacing the distributor (vac retard faulty) and adjusting the throttle linkage has fixed the engine surge I was previously experiencing - thanks for the help.

Plugs or wires won't affect timing.

Try testing timing on #6 cylinder and compare your readings. They should be the same. The usual reason for such deviations is loose springs in the centrifugal advance or some amount of vacuum fluctuating in the vac retard/advance unit.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Cylinder #6 is identical to cylinder #1. The timing deviation occurs with both the new and old distributor/vac retard. It also occurs with and without vac retard in place.

The timing does stabilize again above 3000 rpm. Are there any other conditions that could cause this?

Thanks,
Steve

>>The timing does stabilize again above 3000 rpm. Are there any other conditions that could cause this? <<

It stabilizes above 3K because the weights are fully extended to the stop.
So it is slop most likely at the spring ends or the tension spring on the slide ball bearing at the breaker plate is loose.
They tend to wear a slot where the bearing rest on the plate.

That makes sense, except this is a brand new distributor, and the old one had the same symptoms... could it be anything else?

Steve

Brand new or rebuilt?

Brand new in a sealed Bosch box.
Steve

OK

Things I can think of:

Rotor fits tight ?

Did new dist come with the drive dog assem. at the bottom shaft
end or did you have to use the old one ?

Dist clamp tie down is tight ?

You oiled the shaft under the rotor wick on new dist ?

As you can see where this is going, the variation is linked to mechanical slop from the input [ shaft drive] to the out [ rotor] of dist.

May also be chain/tensioner variation, but I doubt it.

Hi Steve,

You had stated in your earlier e-mail that the surging problem you had was gone. Is there any particular reason to keep checking the timing? Is there another running problem? What I'm getting at is the "fluctuation" may be just the timing advancing and the instrument/timing light showing the change. It depends on what type of timing light and the delay between the capacitor charge and discharge cycle. A timing light only shows a single cylinder every "other" revolution of the engine...the mark may not be stable while the timing is changing. If it runs fine now, it's fixed.

Regards,
Randy D.

With the distributor removed from the engine the rotor rotates and snaps back as expected.

However, when the distributor is installed on the engine, the rotor has an additional 1/8" play. I traced the additional movent to the dog at the bottom of the distributor socket (on the engine). It has about 1/8" rotational movement before it starts to lift and disengage from the drive underneath. Is this normal?

The engine actually runs great since I sorted out the vac retard and FI linkage problems. However, if there is a problem with the timing I'm concerned it may lead to the premature demise of my newly rebuilt motor, so I don't want to take any chances.

As always, your advice is greatly appreciated.

Steve
Vancouver, BC

Steve,
Whenever there is additional "slop" in the distributor, I have to ask....are you 100% sure that the drive is not 180 degrees out? The drive tang is offset, but it will "almost" work when installed wrong. You mentioned "lift and disengage from the drive underneath". No, that isn't normal and would also be an indication that the distributor or drive piece is 180 degrees out.
I have had people install rebuilt distributors that way and cause no end of confusion when it's just a simple mistake.
Regards,
Randy D.

I thought about mentioning this 180degree trip also. I have seen it done a number of times.

In cars that I have seen it, the distributor shaft swings an arc when watched with the cap off and engine cranking. This will also give quite a variation in dwell as the points will be openned further on one side. This will cause a timing variation but should be consistant through the RPM range and was the reason I mentioned checking the timing on number 6. It will show up in that fashon. In other words; you will get timing variations from cylinder to cylinder but not on the same cylinder.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

>>However, when the distributor is installed on the engine, the rotor has an additional 1/8" play. I traced the additional movent to the dog at the bottom of the distributor socket (on the engine). It has about 1/8" rotational movement before it starts to lift and disengage from the drive underneath.<<

I am reading this to mean the dog on the helical drive shaft for the dist in the engine, not the dist shaft.
In that case , this shaft has a gear assembly that is keyed and shimed . It is possible to have wear here.
A wide spade screwdriver [ or anything to insert in this drive slot ,
close to size, will give you a feel for this play.
You mentioned a rebuilt engine , so I am assuming this to have been set up , but maybe not.
Again, I would think this would show at all rpm's, but maybe not at higher as the gears would be it the drive force posiston.
[meaning tight]
If this dog is not the one you meant , forget what I am trying to say......

That's right - it's the dog at the bottom of the socket on the engine. There's about 1/8" slop when I rotate it with a slotted screwdriver. Is this difficult to repair?

Steve

I would not worry about it.
But if you want to look into it, the dist housing plate comes off the front of the engine and the dist gear and key can then be seen . A worn gear can be changed, but it is recommended to
change the mating shaft [ the one you put the screwdriver blade in] as a pair.
Not sure how much play they allow.