Using a vacuum gauge for tuning

[Cees Klumper]

Thought I would try that out. I got me one of those old-fashioned vacuum gauges, hooked it up to the intake manifold, and it read too low. I changed the ignition timing to where the the vacuum reads "normal" and noticed the following:

- the automatic transmission seems to shift up at higher RPM's now
- the car seems just a bit less peppy (although this may not be the case, now that I have grown used to my new Volvo's 240 HP ooomph over my past two weeks of ownership)
- the idle is up some 150 RPM's (I did not have the time to go back and adjust the air/fuel mixture at idle)

I had used a timing light to set the ignition timing according to factory specs, so changing it to get 'normal' vacuum readings means it is not in spec anymore.

I figure I will drive with this ignition setting for a while to see how I like it, then try moving it back for a while, etc.

Others have experiences with using a vacuum gauge to tune the engine that they would like to share?

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[A Dalton]

I do use vac gauge sometimes , but I always use it for a/f after I hav spec. ignition.
 I believe , [ from what you mention] you have simply advanced the timing .The gauge only reflects that change , vs actually using the gauge to set a/f mix..

[Cees Klumper]

That's correct. So rather than adjusting the timing, I should try to get 'normal' vacuum readings by altering the a/f mixture? That I set during my regular service using a CO meter to factory spec. I.e. I had previously set both ignition (using a timing light) and a/f mixture (using a CO meter) to factory spec, and the cars drives nicely on those settings, it's just that the vacuum readings then came out 'too low'.

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[A Dalton]

<<'too low'.>>

 What did you get???

[Tom]

Cees,

I've had the same issue-low reading (14) and had trouble geting the timing, fuel, air and vacuum to come into factory specs.  You've peaked my curiousity-will check it now after the mini-resto.

Best,

Tom


1971 280sl Tunis Beige Metallic (restored & enhanced)
1971 280sl Tobacco Brown (low mileage stock)
1970 280sl Deep Red (Project Car)

[Cees Klumper]

Arthur - I don't recall offhand, and my garage is a couple of miles from my home. In any event, the reading is about one notch below the 'normal' range, in the area where (on my gauge at least) it says "ignition incorrect".

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[Benz Dr.]

I use a mighty vac - probably one of the most useful tools I own. On later cars with vacuum door locks and heating systems you have to use one to find leaks. Not only will it give you a vacuum reading,  you can also produce a vacuum by pumping the handle. This is very useful for testing distributor advance.

I set ingition with a timming light or by ear - I wouldn't depend too much on a vacuum guage which can be affeted by a number of different inputs such as low compression, leaking valves, late cam timming, vacuum leaks, etc.

Late ignition timing will have a smooth but very deep exhaust note. Too much advance will have a roughness in the idle that can't be tuned out by fuel or air adjustments.
Late timming will also have a lack of performance
while too much advance should produce pinging. Either will give you performance loss but too much advance will harm your engine.
Better put it back where it was, Cees.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org

[George Davis]

Cees and all,

I think you should not rely on what the vacuum gauge calls "normal" for tuning, you should rely on factory specs.  I've never seen a factory spec for normal vacuum at idle, but normal vacuum is a function of engine design specs, such as valve timing, overlap, and ignition timing.  These engines run relatively retarded timing at idle compared to a lot of other engines (for reasons we haven't puzzled out yet), and retarded timing at idle gives lower vacuum.  But retarded timing is what the MB engineers came up with and I think we should trust them.

My car reads about 14 to 15 inches of vacuum at idle.  From the dial on my gauge this is low, but the car runs great, idles great, has good scampitude.  Use the gauge as a tool, not as a guide to how it should be tuned.

The vacuum gauge is useful for those of us who do not have CO meters, as it provides an alternate method of adjusting the idle air/fuel ratio.  The CO meter is the better method, though.

Last, running too much ignition advance is very dangerous.  It may idle wonderfully, but there is potential for detonation under high speed/high load conditions.  Detonation can turn an engine into junk in a few minutes.  No more driving until you've put it back right, Cees!

George Davis
'69 280 SL Euro manual

[Cees Klumper]

Ok ok! To put things in perspective, I did not change the ignition that much. But I will put it back to where it was for sure.

I am glad I posted this question, learned something again.

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[mbzse]

quote:Originally posted by George Davis

These engines [for our W113's] run relatively retarded timing at idle compared to a lot of other engines (for reasons we haven't puzzled out yet), and retarded timing at idle gives lower vacuum.  ../..My car reads about 14 to 15 inches of vacuum at idle.  From the dial on my gauge this is low, but the car runs great, idles great, has good scampitude../..  The CO meter is the better method, though

Group, I am slighly puzzled over my M129 engine. The last couple of weeks I have experimented some with my ignition, inspired by your views and tips in threads here on the list. Now, my ignition is set according to spec (it's a 051 distributor fitted with a PerTronix 1864L unit), i.e 32 degrees BTDC at high rev's (vacuum removed) and about 0 degrees with vacuum attached and idle at 800r/m (it's an automatic). I have a CO meter, and CO value is about 3,5 percent at idle. I have also measured the vacuum, and yes it is low some 14"Hg.

Engine idles well and revvs well during tune-up, but here comes the strange part  :?:  : When I run the car, put it in gear, it is totally listless. No power, and the engine stumbles when I depress the accelerator.

It has run well (with the PerTronix) before (especially since I changed the faulty magnetic ring over to the "white dot" correct kind! [:p])  Really, what I have now varied is the length of the arm on the vacuum actuator unit, and thereafter re-set the basic high-revv distributor setting.

I am looking forward to a continued tuning session in my garage tomorrow, so any advise and tips from you guys is most welcome! Perhaps someone has experienced similar troubles? Seems like I should vary the distributor setting, but, then I alter the set basic distributor angle (32 degrees) which is according to spec (98 octane fuel).

I should mention that injection nozzles as well as compression are good.

/Hans

[Cees Klumper]

Hans - try advancing your ignition - just play with it, turn it counterclockwise with the engine running until it starts to miss, then turn it back a bit and test-drive the car. I had previously set my timing to what I believe to be factory setting, and it did not run well: lack of power, hesitating when accelerating etc. Next thing I will do is check whether `0´ on the flywheel is actually TDC or not.


Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[hands_aus]

When I run the car, put it in gear, it is totally listless. No power, and the engine stumbles when I depress the accelerator.

Hans,

3.5% CO could be just a bit too lean for the engine.

Sounds impossible doesn't it?

The fuel Inj pump of the 250sl was designed to run richer than the 230sl.

When I installed my 1864LA Pertronix (white dot) unit I had to adjust the timing and it ended up at 2 ATDC.

Before that the engine was like yours is now. A small adjustment gave it new life with no pinking.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto