Does low vacuum mean engine rebuild?

[jedwards]

Gentlemen,
I have a 1967 250SL Auto with 60K miles on the clock, which I believe is a genuine figure.
I have a vacuum reading of around 15”- 16"  at 750-800 rpm. Although this figure is similar to the numbers reported by many others in this this forum, the experts seem to be unanimous that this is way too low and indicates something wrong with the engine.

The car starts well and  runs well but is a bit sluggish up hill compared with modern traffic but it is 45 years old.  I do have some issues with getting a reliable super-smooth auto shifts and I believe that can be caused by low vacuum.

I have done everything other than a smoke test to locate any vacuum leaks but cannot find any evidence of one.  I have used the propane, carby cleaner and soapy water methods on separate occassions but  found nothing.  My WRD closes off well and the vacuum advance and timing are exactly as Dr Benz suggests.  Ignition is new/good and I have set the valve clearances as per the tech Manual and believe they are close to spec.  I don’t have a compression gauge so I don’t know the actual compression, but that means a day off work and a few hundred dollars just to get it checked.

Should I planning for a costly engine rebuild based on those vacuum numbers or is there something else I can do before I bankrupt myself?

[Cees Klumper]

I suggest you get a decent compression meter (probably less than $50) and check that first. As I understand it, on some engines ignition timing is better set at values different from standard, perhaps due to stretch in the timing chain? Worth a try; ignitin timing does affect vacuum. That's my 2 cents, good luck and curious to hear what others can offer.

[GGR]

Camshaft profile and its timing does affect it too. Chain stretch induces some camshaft retard which lowers vacuum. First check that your camshaft aligns on the mark when the crankshaft is at TDC. Then make sure you adjusted the valve clearances properly as they can affect compression values (0.1 at intake, 0.2 at exhaust, engine cold, measured between the rocker arm friction area and the bottom of the camshaft lobe, with the top of the lobe pointing the opposite direction). Then measure your compression. A leak down test will also give you good information on the condition of your engine. It is worth doing all this before considering a rebuild in order to take an informed decision, given the cost involved.

[stickandrudderman]

A compression test, cylinder leak test, smoke test, timing check (cam and ign), valve clearance check should all be done 1st. However, I have had engines that pass all this scrutiny but still lack power and give poor shift quality.
A vacuum check then reveals that an engine rebuild is required, probably due to ovality in the bores.

[ja17]

Still sounds like an ignition timing problem.

[jedwards]

Ja17, you have always been 100% correct with your past advice, so am happy to follow your guide.

My understanding of troubleshooting timing is limited to simply setting the timing at in accordance with Dr Benz suggestions of 5 BTDC at idle and then check that I have around 35-37 degree at 2,500-3,000 rpm. I have fitted a new dizzy cap and have stripped the distributor down and oiled everything so it all moves nicely.
I have ditched the points for a Powerspark Ignition kit, which have performed perfectly on all my other cars.

Is there anything else I can do?

[Cees Klumper]

What I do, also because Dr Benz has referred to this approach, is to set the timing 'by ear' meaning I adjust the timing with the engine running and just checking to see at what setting the engine seems to run the smoothest, rev the easiest and so on. To do this, I slightly loosen the distributor clamp, put on a glove, pray I won't get electrocuted, and start very slowly turning the dizzy in both directions until I find the sweetest spot. Then I check to see just how far off I am from the standard timing. It's been a while since I adjusted my timing but I know I am not at standard spec. I also know from having tried this out once, with a vacuum meter hooked up, I was able to significantly affect the vacuum by changing timing.

[jedwards]

Hi Cees
I have tried this and it always seems to be happiest at 10BTDC at idle and over 40 degrees at 3K. I worry that this just too much advance even though it never pings.
Is there some potential for damage by using too much advance?

Jeff

[GGR]

Those pointers are quite flimsy and get easily bent. When that's the case the reading is thrown off. Make sure yours is straight. You can also check if your piston No. 1 is reaching TDC right when the "0" on the damper is passing the pointer (with a screw driver through the spark plug hole, or even better, a comparator gage of some sort).

[stickandrudderman]

A little knowledge can be a bad thing.
The problem with setting things "by ear" is that no-one can say how good your ear is and setting too much advance can easily lead to major engine damage.
Setting the timing by vacuum is a very bad idea unless you really know what you are doing.
Now, if you are 100% sure that you know how to recognise pinking then go ahead and set the timing as far advanced as you like but if you don't I would prefer to recommend the factory settings.
If your engine is burning more oil than it should then pinking will start a lot sooner.

[Cees Klumper]

Good points Colin. Not to worry, I hear pinging a mile away (figuratively speaking). I wasn't suggesting to set ignition timing to obtain a certain vacuum level, I was noting the relationship between the two. So if vacuum is not what it should be, off timing could be the cause.

[ja17]

If your down on power and the engine seems to run well, check to make sure you are getting full throttle when your accelerator pedal is all the way down.  Have someone press it all the way down while you look to make sure the intake venturi is open all the way.

[Benz Dr.]

Total advance is the only number you need to worry about. Set to at least 38 degrees full and the idle timing will set itself.

[ja17]

Good advice from Dan and Stick! I agree.