1970 911 T Ignition Questions

[911Tfan]

hi,
i have a 1970 911 T 2.2L with the original marelli distributor. my car backfires a little when i let off t highway speeds, is that normal for this model? i have read that they are known for strange noises out the exhaust. i tried to set the timing and the distributor doesnt move much when i loosen the hold down nut, is this normal? i cant time it at 5000 rpm practically so i am just doing an approximate job for now. seems you have to remove the distributor to replace the points. so for now i just cleaned them with newspaper. does anyone know what the horizontal machine screw is for attached to the underside of the marelli distributor? my chiltons and clymer books and 101 projects book dont mention it?
thanks

[tharbert]

We must have had a time warp here... I was reading Grady Clay's answer to 911Tfan on a separate tab in my browser. I went back to the board and it seems his answer was truncated. I copied Grady's answer from my other browser tab:

Originally Posted by 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

[911Tfan]

Thanks for doin that tharbert. i got a lot of info in that one answer. if it backfires because its lean is that out the pipe or would that be from the intake?

[tharbert]

I believe leanness would cause popping out the intake. Excess/unburnt fuel would cause backfiring out the exhaust/muffler. Then again, I'm a novice. Experts please chime in if I got it wrong.

[kidrock]

Originally Posted by tharbert

I believe leanness would cause popping out the intake. Excess/unburnt fuel would cause backfiring out the exhaust/muffler. Then again, I'm a novice. Experts please chime in if I got it wrong.

I'm curious to this as well...I always believed that a backfire through the exhaust was due to a rich mixture. Please explain?

[tharbert]

I checked my history and found RW 7810 had also posted prior to the time warp:

Originally Posted by 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.

As for the rich mixture as a source of backfiring, I guess that's one way but I can think of a few others. If it's just a rummble and the occasional pop on deceleration, I'm with R dub.

[911Tfan]

Grady can you tell me how to set timing at 6000 rpm? do youneed a dyno for that or do you do it on the road somehow, or do you have someone sit in car and hold accelerator down? isnt revving that high in neutral bad for the engine? especially an old one?

[Grady Clay]

EVERY 911 engine can easily free-rev to 6000 rpm to set the timing. If it can’t, it should not be driven and should be immediately repaired.

The reason it is important to set the timing at 6000 rpm is the rist of too much advance. Too much advance at high rpm can lead to detonation and engine failure. The engine noise at high rpm masks any detonation noise.

The best of all worlds solution to servicing the distributor is to set the engine to TDC compression (rotor pointing to #1 cylinder) and remove the distributor for bench testing.

The good old Sun Electric distributor machine is available at many shops. This allows you to set the points and run them in while at bench height, not your head stuck in the engine compartment. You can also see that there isn’t excess cam lube ready to get on the points. You can confirm (or accurately measure) your actual distributor advance curve.

When doing this it is also important to note the static timing compared to idle and 6000 rpm. This lets you accurately static time the engine and usually only a 6000 rpm check is necessary.

As a reverse procedure, after you have set the timing at 6000 rpm, record the static timing for the next time you need to remove the distributor.


Is a dyno necessary? No, a 911 engine will free-rev just fine with no damage. It is working a lot harder under full load. With a dyno, I would give the engine about 10% load for timing procedures. It just sounds better.

Best,
Grady

[911Tfan]

thanks grady. i will see if i can locate the sun electric machine somewhere local. is there a type of shop (like the ones that specialize in rewinding alternators , etc..) that specialize in working on distributors? also is it possible to do any cleaning with carb cleaner of the air passages or jets of zenith carbs like i have on my 1970 911 T? i will eventually have them rebuilt i guess but id like to clean them as good as i can for now.