1970 911 T timing & distributor

[911Tfan]

hi, i have a 1970 911 T with a marelli distributor. i have been having some backfiring especially when i back off the throttle at highway speeds so i tried to set the static timing. when i loosen the distributor nut the distributor doesnt turn as much as i expcted compared to other cars. does this sound right? also i turned it clockwise all the way and it backfires less but still a little bit. i sort of wish i could turn it just a little more. what is the horizontal screw going into the distributor for?
thanks

[rw7810]

In my opinion, popping and backfiring at speed coming off the throttle may be an over lean or carburetor problem. There was a solenoid on the 70-71 carbs t o compensate for this.

[tharbert]

Setting the timing by ear is a little like adjusting your tire pressure without a gage. Do you have a timing light?

[Grady Clay]

911Tfan,

First,
Welcome to the Forum …
You will find a lot of help here.

I think your first mission is to get (buy, borrow) a quality and 12 V powered timing light. You cannot safely drive a 911 without the ability to measure and set the ignition timing at 6000 rpm. You don’t need the ‘fauncy’ adjustable timing versions. A plain but bright one works great.

Next you will benefit from some cleaning of the timing marks on the crankshaft pulley. I’m assuming an original pulley. Yours should have marks at TDC, 30°, 35°, 180° and 270°. The 180° and 270° marks are for valve adjustment (in addition to the TDC mark).

Once you have reset the timing at 6000 rpm to spec (’70 911T USA Marelli is 33-35° BTDC), you should record the timing at idle and static. If you want to check the condition of the distributor, you can measure every 500 rpm and compare to the curve specifications.

There is much more to the ignition.


Bob, Pelican rw7810, is correct, ‘popping’ on trailing throttle can be from too lean mixture. The reason can be from many sources. Most common is with carburetor adjustment and (now after almost 40 years) wear. Some wear can be compensated for – typically not all.

On USA versions, there is (usually was) a ‘shutoff’ solenoid that reduced the issue for emissions reasons.

A vacuum leak can be another cause. You can check for this with various liquids/gasses at the intake manifold gaskets (at the head and carb). Extreme caution if using something flammable).

Another is an exhaust leak. This can be a gasket or even a pinhole somewhere (typically muffler). It is very difficult to detect such a leak under power.

Of course there can be more esoteric reasons. I hear a story of an ignition wire pinched from the engine & transmission loose on the mounts.


SO … what to do?

Get the ignition in perfect condition with all new high voltage parts and new points. This is an area that has to me new annually regardless of mileage. Sure an old (time) set of parts will still function. You just don’t know if some simple part plug connector, rotor, etc.) just occasionally arcs. This is also the least expensive set of parts and are very ‘time sensitive’.

I like to regularly remove the exhaust system. It isn’t because I like to fight with the fasteners. It is to maintain new gaskets and (most important) to be able to easily remove it for some (other?) service.

Have someone experienced help you adjust the carburetors. There is a LOT of good advice here on Pelican but carburetor adjustment can be the ‘art of compromise’ with 40-year old carbs.

There are a bunch of other areas of your ‘70T that need regular attension.

Best,
Grady