Dwelling on Dwell; I have some questions.

[rwest]

Hi,

Car is a 77s. Last fall I had a weird problem with the short wire attached to the points so I replaced the points this spring and am suspicious that my dwell is wrong.

I started out with the .35mm feeler gauge and am not even close to it now, I had to really close the gap to get 35degrees of dwell.

The car takes a lot of cranking to start and the drivability under 3000rpm isn't great.

My dwell tester is an OTC automotive oscilloscope and I double checked that it is set to 6 cyl, I don't have another meter to double check with, although not opposed to purchasing another.

Any suggestions? Since it is a CDI(MSD 6A) should I just use the feeler gauge and then reset the timing, or?

Thanks in advance,
Rutager

[Bob Ashlock]

The points are really only used as a trigger to the CDI box. If the points are gapped at .015" or so, they will properly trigger the box. The actual gap of the points, within reason, has no effect on the duration or strength of the discharge the CDI applies to the Coil. Variances in the gap of the points will affect timing, but that is then separately adjusted by rotating the distributor accordingly. Your dwell checker is only responding to the on/off time of the points. I guess the MSD, like the Bosch has an internal pull-up resistor that the points apply a ground to. Depending on what that resistor is connected to (12V or ??) you might get varying results with your dwell tester. In a conventional Kettering ignition, the dwell/gap setting is important because it affects the 'charging' time of the coil. CDI ignition is different. The points are just a trigger signal to the CDI ('fire now!') when they open. So, to answer your last question, yes ... just set the points gap and then adjust the timing!

[T77911S]

correct.
the dwell is not a big deal.
with my 77s I NEVER gapped the points.
I pulled the dist and eyeballed them. make sure they are closed most of the time and that they open. its just a ground path.

you have MANY other things that will cause your problems.
air leaks
mixture (verify fuel pressures first).
timing. advance working.
good plugs, wires, cap rotor
did I say air leaks
injector Orings, sleeves, other vacuum lines.
are the vacuum lines routed correctly.
you should have the vacuum controlled WUR. is the vacuum line connected correctly and if you still have the thermo time valve (TTV) is it working. (fuel pressure should drop about .8bar with no vacuum).
this WILL have an effect on drivability, especially in the low RPM.
air leaks

I also had the MSD in my 77. MSD coil also. mount it upright if you can,. I had clewette make me a longer coil wire for my 930.

also injectors.
you can check the volume of all 6. they need to be equal. check at WOT and at what would be around idle or just above.

[rwest]

Thanks guys, I'll feeler gauge the thing and be happy!

I do suspect an air leak and haven't smoked it yet. I did a pretty complete refurb of the CIS last year and had checked the pressures and bench tested many of the components. I did the six bottles check for volume and set the mixture with an LM-1. Rotor, wires, cap and plugs are a less than a year to two years and only a couple thousand miles. Which doesn't mean that one of the above hasn't gone bad or out of adjustment since.

I started focusing on the ignition since that was where the problem started. I'll gap, re- time and then see where I'm at.

Removing the oil cap doesn't drop the idle, however it has a hose going to unmetered air and I thought I read that the '77 isn't affected by cap removal, is this true?

[rattlsnak]

Dwell certainly is a big thing.. Dwell will affect timing, but timing does not affect dwell, so always set the timing after setting the point gap.


edit... didnt see that he's talking about CDI vs standard igntion.. My comment is for standard ignition!

[Jonny H]

Originally Posted by Bob Ashlock

The points are really only used as a trigger to the CDI box. If the points are gapped at .015" or so, they will properly trigger the box. The actual gap of the points, within reason, has no effect on the duration or strength of the discharge the CDI applies to the Coil. Variances in the gap of the points will affect timing, but that is then separately adjusted by rotating the distributor accordingly. Your dwell checker is only responding to the on/off time of the points. I guess the MSD, like the Bosch has an internal pull-up resistor that the points apply a ground to. Depending on what that resistor is connected to (12V or ??) you might get varying results with your dwell tester. In a conventional Kettering ignition, the dwell/gap setting is important because it affects the 'charging' time of the coil. CDI ignition is different. The points are just a trigger signal to the CDI ('fire now!') when they open. So, to answer your last question, yes ... just set the points gap and then adjust the timing!

^ What he said!

But also...

1) Just a couple of weeks ago I saw a '69S which had a misfire at around 5000 RPM. I could tell by eye the points gap was way too small so -with no feeler gauges to hand- I set them by eye. Misfire gone.

2) If the dwell isn't right, the tacho can misbehave, particularly on early cars as the tacho is sensitive to the mark/space ratio of the signal that comes from the points.

[Bob Ashlock]

Jonny makes a good point on this. In fact, the tachometer itself can be a useful troubleshooting device... if it is acting 'whacky' because the signal coming to it is erratic, then you can bet that the car is also bucking and missing!

[rwest]

Hi Guys,

Appreciate all the responses; three day weekend, so I'll get to the garage and get the points set by feeler gauge and then adjust the timing (I did remember that "point").

Just to be clear, I'm not getting a terrible feel or any backfires, just a less than smooth, but no where near bucking acceleration up until about 2500rpm. It also takes a bit of cranking to get it to start, whereas when the gap was bigger she fired right up. I drove it to Iowa and back to MN- 10 hours last Saturday, so not dealing with a big issue, just wanting it to sing.

Seems my trying to get 38degrees +- 3 has caused the problem, we'll see what a slightly bigger gap gets me.

Best,
Rutager

[dicklague]

Originally Posted by rwest

Hi Guys,

Appreciate all the responses; three day weekend, so I'll get to the garage and get the points set by feeler gauge and then adjust the timing (I did remember that "point").

Just to be clear, I'm not getting a terrible feel or any backfires, just a less than smooth, but no where near bucking acceleration up until about 2500rpm. It also takes a bit of cranking to get it to start, whereas when the gap was bigger she fired right up. I drove it to Iowa and back to MN- 10 hours last Saturday, so not dealing with a big issue, just wanting it to sing.

Seems my trying to get 38degrees +- 3 has caused the problem, we'll see what a slightly bigger gap gets me.

Best,
Rutager

Why don't you replace the points with a Pertronix unit? I did years ago and it is absolutely reliable.....never worry about points and dwell again

[rwest]

Originally Posted by dicklague

Why don't you replace the points with a Pertronix unit? I did years ago and it is absolutely reliable.....never worry about points and dwell again

Something to seriously ponder- never had much issue with the points until now and even that might be my own over thinking things and highly suspect a bad dwell meter.

[rwest]

Just went out in the garage and "felt" things out. Set the gap with the feeler gauge; left the dwell meter in its case.

I reset the timing, adjusted idle and took it for a couple runs around the block and it seemed to be running better.

The car has always had a flat acceleration up until 2500rpms and a very, very mild bucking sensation while holding a slow rpm. I've had it for 27 years and many replacement parts and tune ups. I'll get the LM-1 on it soon and double check my AFR to see where I'm sitting.

[Trackrash]

How does the timing look from idle to higher RPM?

If the advance mechanism is sticking or not working smoothly that could be the cause of your symptoms.

[Walt Fricke]

RW - you are right about the oil tank cap. The tank breather through 1977 on the CIS just goes into the filtered side of the air filter box. On the later models it goes into the rubber boot between the two sides, so removing the oil cap lets unmetered air past the air sensor flap, which leans the mixture and causes an RPM rise.

[rwest]

Originally Posted by Trackrash

How does the timing look from idle to higher RPM?

If the advance mechanism is sticking or not working smoothly that could be the cause of your symptoms.

I've operated the throttle linkage by hand with the timing light on and it sure seems like a smooth advance as the revs go up.

The '77s don't have a factory spec that I know of for max advance setting, just an idle spec. I did set my idle at more like 3 degrees before rather than the 5 degrees.

[rwest]

Originally Posted by Walt Fricke

RW - you are right about the oil tank cap. The tank breather through 1977 on the CIS just goes into the filtered side of the air filter box. On the later models it goes into the rubber boot between the two sides, so removing the oil cap lets unmetered air past the air sensor flap, which leans the mixture and causes an RPM rise.

Thanks for confirming! I wonder if I'm not the only one who is thinking they might have a big vac leak because of the oil cap not making the revs drop?

[T77911S]

Originally Posted by Walt Fricke

RW - you are right about the oil tank cap. The tank breather through 1977 on the CIS just goes into the filtered side of the air filter box. On the later models it goes into the rubber boot between the two sides, so removing the oil cap lets unmetered air past the air sensor flap, which leans the mixture and causes an RPM rise.

I think the tank hose on my 77s went to the rubber boot on the AFM. so it was metered air. perhaps a change over month or someone put a later boot on my car.

stay with the points. I am NEVER a fan of making mod changes when things don't work right with factory parts because then you may be trying to fix 2 problems.
plus I LOVE the reliability of points.

are you using the vac retard?

one thing I did with my car was to remove the dist and clean all the ground contact points starting with where the points are bolted to the plate. also cleaned the body of the dist so it grounded to the case better.
I noticed it fixed misses when I would go to set the points and it did make it a little smoother.

try going richer on the mixture.
blip the throttle and let the RPM's drop. keep going richer until the RPM dips down then comes back up then back off to the point it no longer does that. let it idle for a few minutes. if it does not start to surge, leave it there.
IE, set the mixture as rich as you can and still have a good idle.

timing:
after setting mixture as rich as you can-
there are several things you can try.
check the max advance for where you have it set at now. if less than 30 or even 35, advance it a little. don't go past 35 and go back if you get spark knock.
connect the vac retard with current timing setting and up the idle.
if you connect it, you should be able to go a little richer with the mixture.
trust me, it LIKES to be on the rich side.
try advancing timing again to see if that helps.

I have spent years playing with timing and mixture on the 77. if you retard and drive it, then keep advancing it, you can feel the point where the advance gets right.

you also need to make sure the vacuum control on the WUR works. this WILL cause drivability problems.

[dicklague]

Originally Posted by T77911S

I think the tank hose on my 77s went to the rubber boot on the AFM. so it was metered air. perhaps a change over month or someone put a later boot on my car.

stay with the points. I am NEVER a fan of making mod changes when things don't work right with factory parts because then you may be trying to fix 2 problems.
plus I LOVE the reliability of points...........................................
.................................................. ................

The following is my opinion:

RELIABILITY of points??? Did I hear that right.

All debatable.

Do you think Porsche would have installed points if Hall effect ignition was available back then???




I drive my 1973 2.7 MFI A LOT..... 5k to 10 K miles a year. I could tell when the points were closing and the dwell was going away. You could feel it in the power band. I even added a small stub of wire with an insulated spade connector at the end and labeled it DWELL so I could check it easily....and of course adjust it. The distributor is in very good shape and the rubbing block always lubed.

Last straw was when I broke down on a very busy 101 freeway and had it towed to TRE. Dave called me the next day and said "you should check your dwell.....the points were closed."

THAT was it.......and I spent all of $100 for a Pertronix......that was 12 years ago and 100,000 miles ago. Now I have to remember to lube the distributor once in a while.

I plug into my dwell reading wire once in a while and the reading never changes.

I have a fresh set of points in my tool bag under the hood.....and a spare Pertronix in there too.....both are 12 years old.

I could put points back in at any time.....no mods to the distributor needed for Pertronix install.

Best $100 I ever spent on this 911.

[mysocal911]

Originally Posted by dicklague

RELIABILITY of points??? Did I hear that right.

All debatable.

Not really! Points can ALWAYS be cleaned (nail file) and re-adjusted easily on a trip.

Originally Posted by dicklague

Do you think Porsche would have installed points if Hall effect ignition was available back then???

No! Given the then unreliability of a Hall device in the hot engine environment of a 911.
That's why they used a variable reluctance pickup in the 911SC for its CDI.

[Goth]

Originally Posted by Trackrash

How does the timing look from idle to higher RPM?

If the advance mechanism is sticking or not working smoothly that could be the cause of your symptoms.

Also; The timing at idle should be rock solid. If not; the mechanical advance springs are too loose. The timing should be super stable at idle and start to advance just off idle.

Loose/worn mechanical advance springs will cause a few issues:
1. Unstable idle/low RPM
2. Loss of top end power
3. Runs hotter than normal when just cruzing down the road

I've seen this on quite a few cars with 100K+ miles (not specific to P cars).

I hope this helps!

[dicklague]

Originally Posted by mysocal911

Not really! Points can ALWAYS be cleaned (nail file) and re-adjusted easily on a trip.



No! Given the then unreliability of a Hall device in the hot engine environment of a 911.
That's why they used a variable reluctance pickup in the 911SC for its CDI.

1. I would rather not be using a nail file on the points on the side of the 101 during rush hour.

2. That is the point......it is a screw in mod now with MODERN, reliable Hall effect.......and all you add is switched 12vdc to the red wire.

Loren, I know you will always cheer for the old way of doing Porsche. For us guys who put lots of miles on our cars we want reliability and performance......and always make it reversible.....IF I ever want to sell it, we will go for improvements.