912 timing...how to?

[bigandbald]

Bought a lovely, clean '68 912 and am going to change plugs/wires today. Dropped by my mechanic and he "thinks" I've got a bad valve, but rather than pay $1000 to be sure, I thought I'd do a tune-up. I can work on my Ducati, but a Porsche is a new thing.
How can I do the timing in a painless manner? I can do the rest, but is there a trick to setting the timing?

thanks...details will help!

[DarrylD]

This is an excellent 912 timing "how-to" write-up:

http://www.petry.org/markp/912tuneup.htm

[bigandbald]

thanks a ton

[DarrylD]

Glad to help!

[lep]

The valve (lifter) lash should be adjusted about every 2500 miles on a 912, and therefore this has a big impact on smooth operation and max performance. Timing's important as well, but if your valves are out of adjustment the engine will run poorly. There have been stories of owners who have pulled over on the side of the road (at 2500 miles since last adjustment) to adjust their valves -- so it's a major deal.

Ironically, I adjusted my valves today -- it takes about 30 minutes. 0.010 inches (ten thousandths of an inch) setting on intake valves and 0.015 inches on exhaust valves. You have to adjust each cylinder by going thru the timing sequence 1-3-4-2. Overall, for normal maintenance, you have to adjust the valves a lot more than you have to set the timing.

Also, if you have stock points in the distributor with a condensor, then these are very sensitive to wear and affect smooth operation/performance. A lot of people have switched to breakerless (pointless) ignition systems which require no maintenance as long as they are working correctly. Pertronix breakerless points uses a rotating magnet inside the distributor to switch the points, while the Crane optical ignition uses a laser switching device. Moving to breakerless points can reduce a lot of headaches caused by bad points, but you don't have to.

A lot of 911 owners use stock points with condensors and swear by them, so it's your choice on what you want to eventually do with your ignition system.

good luck,

lep

[DarrylD]

Originally posted by lep
There have been stories of owners who have pulled over on the side of the road (at 2500 miles since last adjustment) to adjust their valves -- so it's a major deal.

The engine has to be cold to do valve adjustments as metal expands when it's hot so the valve clearance changes... not something done at the side of the road like a pit stop for sure!

[lep]

Daryl: Good call - they must have waited for cool down or something -- I guess it's why they are stories.

On the 912, most tech manuals call for tune-ups every 6,000 miles, during which points are checked (and possibly replaced) and ignition timing set (among other things like replacing oil, filters, etc.). One manual said to check the valve lash at 3,000 mile intervals, but over the 26 years of owning my 912, and after speaking with so many Porsche mechanics and owners, I have always used a 2500-mile interval for adjusting the valves. Also, adjusting the valve lash has always seemed to result in a much better improvment in performance when compared with setting the ignition timing. Timing should not be set just for the sake of simply setting the timing. Timing is usually set after points are replaced or the distributor is removed or turned. Now that I have breakerless points, I don't need to set the timing as often -- but this is specific to my own car.

For the courtesy of bigandbald (the new 912 owner who asked if setting the timing will help), I wanted to suggest adjusting the valve lash more frequently than setting the ignition timing for improving performance, especially if the points or distributor were not touched.

cheers,

lep

[mpetry]

Hey, Lep, let's be sure about the measurement - the correct valve clearances are 0.004 " (inches) for intakes, and 0.006" for exhaust. I believe the metric equivalents are .0010 and 0.0015 mm respectively.

A stepped feeler guauge makes setting the gap much easier.

Pelican hosts a good illustrated procedure on valve adjustment:

http://www.pelicanparts.com/techarticles/356_adjust_valves/356_adjust_valves.htm

Mark Petry
Bainbridge Island, WA

[lep]

My apologies, I meant 0.010 and 0.015 cm instead of 0.010 and 0.015 inches.

However, your conversion from inches to mm is too low by a factor of 1000. Uisng the rule-of-thumb that you will always get a larger answer when going from inches to centimeters (2.54 cm/in) or mm (25.4 mm/in), your 0.004 inches should results in a larger number, but yours is 0.001, which is smaller. Correctly, your 0.004 inches converts to 0.10 mm (25.4 x 0.001) and 0.006 inches converts to 0.15 mm (25.4 x 0.006). 0.10 and 0.15 mm convert to 0.01 and 0.015 cm.

Given the above, readers are encouraged to select 0.004 inches for exhaust and 0.006 inches on a feeler gauge when setting valve lash on the 912. [I use a tool for the 912 simply marked with "0.1" and "0.15" (again, in units of mm)].

Thanks for picking up the wrong units.