Motor Mystery Solved!! 78SC Sag and Stumble Gone! But Distibutor Help Needed Now.

MotoSook · 08-06-2002, 06:57 PM

OH Yes!! Thanks to joeycatt I have figured out why my transplanted motor sags and stumbles at low throttle input. I have a faulty advance on the distributor. I plugged the vacuum line to the dist diaphragm and voila! problem gone.

Thank you soooo much John.

Now, who has a GOOD used 78SC distributor, or how do I repair the one I have? Is there a shop that can fix this? I'll do the research, but I wanted to throw it out there for comments or suggestions.

A good source on things 911 suggested replacing the POD on the distributor. Anyone done this R&R before?

Thanks Souk.

P.S. 27230 lives....

Superman · 08-06-2002, 07:56 PM

Disassemble, clean, reassemble using new vacuum advance unit.

stlrj · 08-06-2002, 09:10 PM

When you disconnected the vacuum line, was vacuum present at idle? If so, that may have been your problem all along since there should have been no vacuum at idle, only at mid throttle and above.

This could have happened if you used the 2.7 throttle valve instead of the 3.0 valve because the vacuum ports are located either above the throttle plate on the 3.0 or below as in the 2.7 causing the advance to retard as soon as you cracked the throttle making your engine sag and stumble as you described.

Just a thought,

Joe

MotoSook · 08-06-2002, 10:12 PM

No vacuum at idle, and vacuum starts at throttle opening (have vacuum gauge will travel). I have a complete 3.0, top to bottom.

I think Superman is right, I need a new vacuum pod.

Keep the suggestions coming. I have learned that the obvious is only good for the obvious.

Thanks all for the input.

Souk

MotoSook · 08-07-2002, 04:45 AM

So how does the vacuum unit work? What does it pull back on when there is vacuum? The unit I have holds vacuum and when I apply a constant vacuum source then suddenly remove vacuum, I can hear it pop back out. Is this a sign of a good unit, but a bad installation?

Thanks.

Jdub · 05:32 AM

That '78 distributor is something of a one-off so be very careful replacing it. I would recommend you strip and clean it - not very hard to do. See Rebuilt the distributor on my '78SC. for a procedure.

The '78 distributor used a pinion gear that is of a different pitch then later (?'79?) cars, as well as having a different rotation than the earlier cars. If you replace the dist. with a pinion gear of a different pitch there will be internal damage over time.

The vacuum acts on the diaphragm in the pod. When the diaphragm moves, an arm attached to the diaphragm moves the advance plate. It does this because the arm is connected to the advance plate internally by way of a hole in the arm that fits on a pin on the underside of the advance plate.

Your problem *may* be that the arm is no longer attached to the advance plate pin, the plate is stuck, or the swing weights on the advance plate pins are seized or gummed up or you have lost one of the springs on the plate. You can check the post on the pin theory simply by unscrewing the pod screws and seeing if the pod pulls straight out: if so, the post is not attached. If you feel resistance (and see a reciprocal reaction inside the dist.) then you know the post is attached.

Do check the link above. It was written for you!

John

RazorRacer · 08-07-2002, 05:21 AM

As soon as I get my problem fixed, we can both take a drive and meet half way for a big HIGH FIVE!!!

Humm, come to think of it I may be driving out during turkey-day weekend. Hope I have something to celebrate by then.

MotoSook · 08-07-2002, 06:01 AM

Jdub, thanks for the info. I did a search and I now have a lot to read and do. The link you gave is one of the good ones.

As for the connected or not of the arm to plate, the car runs fine with vacuum blocked and crappy on start up with the vacuum connected. I'll use your testing tip. Will report.

Razor...when I get this thing sorted out...it's road trip time. I think both our problems are spark relate. I spent the last number of monts going after air and fuel. The symptoms all indicate air and fuel..then Joeycatt took the challenge up and voila!

What a great place this is. Pelican is greatt!

Good luck Razor...I'll concentrate on your problem as soon as I get mined solved. Have you access to another motor (friends motor) to swap out distributor or???

Souk H

Jdub · 08-07-2002, 06:14 AM

Souk:

So if, with vacuum connected, the car runs badly, is there any connection to how you've set the timing on the car? I do see your point though: something is happening in there so the arm-to-pin connection is probably intact.

But really there is no point in messing with peripheral settings if the dist. is not working freely. Get that straightened out (it really is not that hard) and then work from there.

By the way, the new pin I put into the shaft/pinion is a section of a chain link of all things. Perfect fit. I have many leftover sections should you need one. My alternative to this was a roll pin that, when set in the dist shaft, could in turn receive a smaller pin inside it. VERY secure but I went with malleable over hard roll pins.

John

MotoSook · 08-07-2002, 12:11 PM

Thanks John. I'm going to give it my best shot this weekend. Could it be that I have the wrong vacuum unit that is providing too much travel thus the over advancing? I set the timing 3 or 4 times prior to this discovery and it all checked out according to all the books that I have.

Is there a number for low RPM timing advance with the vacuum attached? Is this the curve that is in the Factory Manual? I think I printed it out once. Anyways, this is all speculation until I point the dist. and look at the parts.

Thanks. Update forthcoming.

Souk H