Check that Distributor!, Chapter 120 of the famous automotive text, "Duh!!" Options

[pbanders]

OK, had a lot on my plate for several years, and wasn't driving my 914 very much. Decided to get back to it last year. Fixed some transmission issues, FINALLY diagnosed an incredibly pesky stumbling problem (loose trigger contact point harness connector) and fixed some other stuff. Got the car polished and ceramic coated, yada yada yada...

But the thing I did to it that really improved things was.... I fixed a major problem with the distributor, and problem that I would bet a LOT of 914 owners have and they don't know it. Here's what was up...

The symptom of the problem I would characterize as "it's running really weird". Strange idle performance. working great one day, not the next. Some part load stumbles, some acceleration stumbles. Everything looks good in the ignition, couldn't figure out what was up.

What was it? My mechanical advance was JAMMED. How the hell did that happen? OK, stay with me here. The issue is that the mechanical advance mechanism is below the points plate, which is secured to the body of the distributor. The way the mechanical advance works is that there's a lower part and an upper part which can move relative to each other. They're connected to each other by two flyweights and springs. As the distributor shaft is spun up by the motor, centrifugal force causes the flyweights to swing out, changing the relative position of the upper and lower shafts - advancing the timing up to 27 deg BTDC when set correctly. The "advance curve" is determined by the specific weights and springs.

The top part of the mechanism is what you put the rotor on to. You'll notice that if you remove your rotor, there's a felt pad under it, that you're supposed to put a couple drops of engine oil on to, so that it seeps through and lubricates the shaft that the top part slides on to. So, what's holding the top part on to the bottom part? It's our favorite part, the infamous "C" clip, or better known as a "Jesus clip" as in when you try to put one on or take it off, it goes flying off like a bullet with you saying, "Jesus Christ, where the hell did that go?".

So, here's how you jam your mechanical advance. The rev-limiting rotor on the 914 is often a rather tight fit onto the shaft of the top part of the mechanical advance. Putting it on isn't the problem, it's when you take it off. If it's really tight, and you have to give it a pretty good pull or yank, IT MAKES THE C-CLIP POP OFF under the felt pad. As you're probably twisting it while you're doing this, it's causes the mechanical advance to get "messed up". How "messed up"? Depends on how hard you're pulling and twisting. If you're lucky, you just popped the C-clip and it sorta works the same. If you're not lucky, it's jammed up and you don't have proper mechanical advance.

I'd suspected something was wrong with my mechanical advance, so when my Crane ignition failed, I decided to finally check it all out. Yep, C-clip was popped, and advance mechanism was jammed. I had a back-up distributor with a perfect mechanical advance, so I swapped it in, and all of those "weird running" problems went away.

Could I have figured this out earlier? Yeah, I wasn't paying attention to the jerky behavior of the timing mark when I was setting the timing, in response to my throttle input. Should have told me something was amiss. Totally smooth response now.

Anyway, be careful about this when changing your rotor. It's very easy to do, and easy to check if there's a problem. After you pull the rotor, use a pick to remove the felt pad and look down into the bore. If the C-clip is loose, you'll see it. If it's popped off, I suggest removing the points plate to check on the condition of the mechanical advance. If it looks good, put the points plate back on, push the C-clip back on (I used two very small screwdrivers to do this), lube the felt pad and put it back in, and put your new rotor on. Check the timing, watch the mark advance as you bring the engine speed up, make sure it's smooth.

[r_towle]

Yup

[r_towle]

First time I discovered the issues with the advance plates makes me look there first before anything else now…because

Sadly, the 911 distributor has similar issues when pulling the rotor off the shaft.

[rhodyguy]

Great post.

[Root_Werks]

Good write up! I've probably done exactly that more than once in my life. A 914-6 would be even more susceptible I would think due to location of dist and difficulty tugging, twisting and not easily being able to verify things are okay.

[ClayPerrine]

Good write up! I've probably done exactly that more than once in my life. A 914-6 would be even more susceptible I would think due to location of dist and difficulty tugging, twisting and not easily being able to verify things are okay.

Only if you have replaced the distributor on a factory six. The Marelli distributor that came stock in a 914-6 has the rotor attached by screws. If you swapped it for a Bosch distributor, or you have a conversion with a Bosch distributor, then you may have this problem.

Clay

[StarBear]

Thanks for the heads up tip. I’ve been sure to oil that pad but never lifted it to peek below. Dare I? (IMG:style_emoticons/default/dry.gif)

[pbanders]

FYI, here's a pic of the C-clip I'm talking about, and how I found it when I removed the felt oiling pad. You can see the clip lying on top of the shaft.

You can also see the jammed mechanical advance mechanism. One thing that's interesting to note is that the two springs are different, one heavy and one light. This allows the advance to progress faster at lower RPM's (lighter spring) and more slowly as it approaches full advance (heavy spring).

[Lockwodo]

FYI, here's a pic of the C-clip I'm talking about, and how I found it when I removed the felt oiling pad. You can see the clip lying on top of the shaft.

You can also see the jammed mechanical advance mechanism. One thing that's interesting to note is that the two springs are different, one heavy and one light. This allows the advance to progress faster at lower RPM's (lighter spring) and more slowly as it approaches full advance (heavy spring).

Can you share some tips on cleaning and lubricating the mechanical advance mechanism?

[Jett]

Discovered our 77 911 2.7 distributor had the same issue, but was running ok. The downside is that the shaft can now travel up/down almost a half inch. Distributor is being rebuilt this week.

[worn]

Ironic that the world leader for our FI systems has an ignition problem, and typical that he explains it to all of us. Thanks!