280 SL problem

What might be the cause of a lumpiness / bucking when driving slowly in first or second gear? These are the symptoms:

-slightly lumpy acceleration from 1000 to 2000 RPMs
-bucks when maintaining speed 1000 to 2000 RPM on a slight incline (1st/2nd gear)
-downshift "burbles" from 2000 to 1000 RPM
-idle no problem
-higher RPMs are fine
-throttle linkages are adjusted properly

Does this sound like an ignition problem? Fuel injection? Fuel pump? Vacuum advance?

Thanks in advance for your help,

Steve
1969 280SL 4 speed

[stevebfl]

I would suspect a lean misfire or a timing variation (getting too advanced).

BTW, I would take all the electric wires off the solenoids and tape them for diagnostic purposes. If you have two solenoids on the back, one is for decel cut-off. An intermittant application can cause severe bucking. The other is a starting solenoid (extra fuel). You don't want it involved after starting either.

Solonoids and timing are good, but the fuel pump has been noisy lately (screeches). Would low fuel pressure to the fuel injection cause this type of problem?

Thanks,
Steve

[stevebfl]

Not very likely. That is because you stated that the car ran fine at higher speeds. Higher speeds would require more fuel so it should be worse if you had a fuel delivery problem.

I have written some long posts on the diagnosing and set-up of mixtures on your car. You might look through the archives on this.

Have you removed all vacuum from the distributor and verified the condition still exists. There are ways to get vacuum attached wrong that will give fluctuating timing in lower load conditions.

I checked the archives, and Steve Brotherton's posts on vacuum advance and timing, and I think mine may be out of specification:

Timing is set to 4 degrees ATDC at 800RPM with the vacuum advance hose in place. When I remove and cap the vacuum hose, the timing changes to 23 degrees BTDC at idle, and 45 degrees BTDC @3000 RPM. Idle speed increases considerably with the hose removed.

Is this OK or should I investigate further?

Thanks,

Steve
1969 280SL

[stevebfl]

You seem to have way too much change due to vac retard. I would try backing it off 5-10 degrees. Leave the vacuum off for testing.

My car has no vac retard - only vac advance. Are these readings out of spec?

Thanks,
Steve

[stevebfl]

You stated:

"Timing is set to 4 degrees ATDC at 800RPM with the vacuum advance hose in place. When I remove and cap the vacuum hose, the timing changes to 23 degrees BTDC at idle"

Do you understand what vac retard means? Look at your statement. You removed the vacuum and the timing advanced 27 degrees. That is called vacuum retard. With vacuum the timing is retarded from that which it is, without vacuum.

[Arthur Dalton]

Your car only has mechanical advance.
The vac is Retard only
The correct timing procedure is to time the car without vac hooked up at 30 degrees btdc at 4500 rpm.

The vac retard works when the throttle plate is completely closed at idle. This is a retard/deceleration system.

The difference between vac and no vac at no load/idle calls for 8-14 degrees. This is adjustable on some distributors [ don't know which # you have], by shorten/lenghten the vac control rod
to the dist base plate. I find they run best with a short [ 8 degrees] adjustment.
If the difference at idle is beyond these specs, i would look into the base plate support bearing, mounts and also check the mechanical springs under the advance plate. These are known to wear and break off the tabs and a broken one[ there are 2] will over- advance the plate as soon as you remove the vac line at idle because
there is no tension to hold the centrifical wieghts. { That is what the springs do]

Recommend taking a good look at the dist. , from mounts and adjustments to wear and broken /worn parts.

Thanks Steve and Arthur, for assisting me with my distributor problem. It looks like it needs a rebuild. In addition to the excess vac retard, the timing becomes erratic (+/- 5 degrees) when I rev the engine to 3000.

Steve

[Arthur Dalton]

I might add that the reason these dist. become "Sloppy"
is bushing wear.
They have 2 oil res. that get over-looked by many owners.
One is a wick under the rotor. That oils the hollow advance
shaft. The other is an oil cup at the bottom, outside that does the main bushing.
Being a point ignition system, many are converting to electronic
ignition with pleasing results.
There are several on the market and they are probably the best short $$$ you can spend to improve the running of these Mechanically Injected engines.

Has the distributor ever been replaced? The reason I ask is that the correct distributor for that car shouldn't be capable of that much advance. If one were substituted from a carbureted car that would explain it. That car should have either a 0 231 116 051(cast iron) or a 0 231 116 062 (aluminum) distributor in it.
Regards,
Randy D.

My distributor is #0231185010. Randy, do you have the correct rebuilt distributor in stock?

Also, I have converted the points to the crane 3000 ignition. Which coil should I use? There should be no internal ballast resistance with the 3000.

Steve

Hi Steve,
That distributor number comes up on the Bosch price list...but not in the application guide. It is a replacement, but for what I don't know. They have a tendency to replace several part numbers with a single compromise part. The "full in" advance for an injected car shouldn't be more than 30 degrees or so.
I'm the wrong guy to talk to about the optical trigger systems (like Crane). Once upon a time, I "had to" work on British cars that were converted to Luminition or Allison systems. I'm not what you would call a big fan of Crane/Allison, etc. I've used PerTronix systems for some applications, but I've forgotten everything I knew about the optical trigger/chopper wheel kits.
Regards,
Randy D.