What did I break?

[Don Z.]

See, the problem is, I only know enough to be dangerous.

Car: '74 US Spec Carrera, 2.7L CIS.

I was having an intermittent problem with engine cut outs. I would be in a steady state on the freeway, and the engine would suddenly go to zero (on the tach), backfire, and return to normal running. Total time, about 3 tenths of a second from normal to zero and normal again.

I did some research here, and came up with: points or CD box.

This weekend it went from doing it once in a blue moon to about every ten minutes, more often when it hadn't fully warmed up yet. Additionally, the car was idling very low... would stall at stop signs, unless you feathered the gas. So I decided to take the time to start "tuning".

Realizing I couldn't get a CD box on a Saturday, I started with points, cap and rotor. Unfortunately, I didn't plan ahead, so the rotor was a NAPA "this should fit", and not the correct part: There is no integral rev limiter, but it does fit on the distributor. Strict instructions to the girlfriend: DO NOT redline the engine. Correct part is being ordered and will be replaced as soon as it gets here.

That said, I installed the above. Futzed with the points quite a bit (it helps when you read the right spec the first time) gapped the points (old ones were slightly pitted) to .012, checked it with a dwell meter to 38%, right on the specs. Old rotor was a little pitted, and had some rust, so I used the "incorrect" one (will replace it as soon as Wayne gets the right one to me), new distributor cap (old one pitted).

Idle went up, things looked good. "While I was in there", I checked the timing. It seemed to be off... so I adjusted it. 5degrees ATDC, right? Vacuum hose connected. Checked the Haynes book. Book says 5ATDC with hose connected at 900 RPM, check. 30 degrees BTDC at 6000 RPM with hose disconnected, I didn't have a good way to check that so I took a guess that if the idle was on and the dwell was right and the 900 RPM timing was right, I'd be in the box for that. When I started adjusting, the tac marks on the pulley were no where to be seen... adjusted it to the 'leftmost' of the double tac marks. At this point, I'm thinking I'm cool... And the idle sounded fine.

Got in and drove... big problems: No advance, backfiring, generally slow... something's wrong. At this point, I didn't have many options, so I adjusted the timing back to where it was before I "fixed" it.

Now the car was running great. Good idle, no more backfires, no more sudden losses and the renewals of power. Things are golden, right?

Wait! I'm cruising down the San Diego Freeway at about 70. I should be pulling about 3,000 RPM, so I look down and... the Tach is bouncing all over the place! Not just floating on downshifts, like the research pages said (check the voltage), but while cruising at a steady state, the thing was always bouncing on average between 3500 and 5500 RPM, with wild, random swings from 1500 to 7000 RPM. It's not the engine that's doing this, it's the tach. And it wasn't doing this before I "re-adjusted" the timing...


I'm looking for ideas. CD unit? (The one there has been replaced with a permatune at some unknown time in the past). Voltage Regulator? Something else?????

[Jim Sims]

You need to find the TDC timing mark (Z1) and retime plus the points may have slipped. Time and then check/reset dwell. Hopefully, you have a pop off valve in the air box or the backfires can crack the airbox. Cheers, Jim

[bigchillcar]

Don,
Speaking as a fellow '74 owner...

I'd say forget the cd box...these don't fail intermittently - they're all or nothing at all. That's the good news.

It sounds like your coil could be involved. Check your connections, especially the one leading to the distributor. This can cause the wild tach fluctuations you've described.

You're getting there...either the coil or a points/cap/rotor issue.

Ryan

[fred cook]

I had THE EXACT SAME PROBLEM with my '80 911SC. It turned out to be a bad fuse holder for the fuel pump relay fuse in the front fuse box! Apparently the fuse contacts would overheat and weaken, contact would become minimal, and the fuel pump would shut of momentarily. I "rigged" a new fuse holder in place (temporary) to make sure that was "it" and sure enough, the problem went away. I finally rebuilt the entire fusebox, replacing the German style fuses with the new flat fuse holders. In the process of re-doing the fusebox, I discovered that most of the wires connected to the old fuse holders were badly oxidized (causing high resistance and heat) as much as 3/8" back under the insulation. I cut the wires back and used good quality crimp on connectors (some also soldered) to plug into the new fuse holders. There is really a lot more to this, but do a search on "fusebox" or "fuseholders" and you should find a lot to read.

Good Luck with your repairs,

Fred Cook

[Kevin Stewart]

You might check the cd box connector many of us have had problems with it being corroded, somebody on the bbs had one that was damaged (broken corroded wire) a couple inches from the plug, Kevin

i dont believe the early cars have alarms but it also was a big issue with sc cars shuting them down like yours