'75 SC runs poorly when warmed up

[BiL911SC]

Here I go. My brother-in-law's 1975 911SC starts missing and the tach jumps once it is warmed up and on the road. It appears to have a new cap, rotor, points,wires, ignition box. I'm thinking electrical due to the tach bounce. Any ideas to save me trouble-shooting time?

[TibetanT]

Hello There:

First, I don't mean to be a jerk about this, but the Super Carrera was built between the years of 1978 and 1983. So, I am not sure what you mean by 1975 911SC??

Anyway, start with the basics, fuel, spark and compression. Make sure you are getting spark to the cylinders, fuel at the right time and that the compression within the cylinders is not going South.

My first thoughts, other than being confused about the year of the car, is that maybe the spark plug wires are not put back in the right place. Also, if this is a 911SC 1978-1983 motor, make sure that you check the firing order, especially since the rotation is counter-clockwise. The latter fact messed me up the first time I worked on one of these cars.

Lastly, I did notice this is your first post, so...WELCOME!

Hope this helps AND someone else with more knowledge chimes in to help as well.

Good luck and keep us posted on your progress.

[azasadny]

Welcome aboard! Be sure the rotor is seated firmly and correctly on the distributor shaft. This is very easy to mess up and can cause all sorts of issues. Start with the basics and double-check everything!

[Tim Hancock]

My '79 SC slightly hesistates/stumbles on hard acceleration when it is fully warmed up also lately. It runs fine prior to the engine getting fully warmed up. It probably could be a number of things. I have yet to take the time to check the timing and fuel pressures, but the timing and fuel pressures were good earlier this spring and it on occasion exhibited this same hesitation then but on a smaller scale.

I am wondering if the coil or cdi may be going bad and acting up when heat soaked?

[1982911SCTarga]

Tim, you can always check your CDI by swapping with a known good box (but you already know that). I don't think the failure mode of a coil or CDI box is usually a slow degradation but more of an either/or, on or off.

Are you sure the engine isn't sucking additional air somewhere when you're getting on it and it's warmed up? My shadetree 2 pfennigs worth of knowledge.

Brian

[Brent Lineker]

Check the points again,my ignition was left on for a few minutes while I was troubleshooting a headlight issue,the points got burned and caused the erratic tach and was missfiring.Replaced the points and all was good.Hope this helps..

[911s55]

[QUOTE=BiL911SC;3455192]It appears to have a new cap, rotor, points,wires, ignition box. QUOTE]

You need to be sure about these things, if CDI box has been changed, to what?

[Paulporsche]

You say that it appears that these items are new. Is your BIL available to verify this?

Quite often it is the points that cause this type of behavior. Have you checked the dwell angle?

A failing alternator/voltage regulator can also cause tach needle jumping, but I would check those points first.

[Tim Hancock]

Is the original poster's car indeed a '75 or a SC? I don't think any of the SCs have points.

Disregard, I re-read and he claimed it has new points, so it can't be a SC.

[Paulporsche]

Tim,

Do you have a Permatune CDI? there have been many posts about them acting up when hot.

[Tim Hancock]

Originally Posted by Paulporsche

Tim,

Do you have a Permatune CDI? there have been many posts about them acting up when hot.

No, it is stock. I was thinking it was a wur or injector issue initially, but I checked the injectors and the warm and cold control pressures last spring and they were both good yet it even then was starting to show the hot running issues. The fact that temperature is involved makes me wonder if the coil or ignition unit is somehow.

It obviously is not on the top of my to do list right now and is still daily drivable, but I was hoping maybe this guy's problem in the original post might be relevant to my problem.

[Paulporsche]

Tim,

Aside from the usuals (wires, fuel pressures, CDi, etc) check distributor to see you are getting correct advance.

[grudk]

Originally Posted by TibetanT

Hello There:

First, I don't mean to be a jerk about this, but the Super Carrera was built between the years of 1978 and 1983. So, I am not sure what you mean by 1975 911SC??

The 74/75 Carreras are stamped as '911SC' so this is one explanation

[BiL911SC]

I just wanted to thank everyone. What a great forum. I may have to buy a 911! Perhaps my memory is faulty, it is probably a 911S. I'll check when I go out to work on it this weekend. Why work on your brother-in-laws car? Did I mention he works for a beer distributor?

[TibetanT]

Grudk:

I didn't know the 74/75 Carreras were stamped with 911SC. Thanks for clueing me in on this fact. What's the factory's reason for doing this?

[grudk]

Not sure. When I bought mine and saw the stamp, I thought perhaps I had bought an SC. But all the 74/5 Carreras have this.

[BiL911SC]

I found a loose, corroded wire from the distributor terminal to the fan housing. Seems like an odd way to wire the distributor. I cleaned it up and tightened it down. The car ran better and longer before the problem cropped up again. He managed to drive it for about a half hour, and put CHEAP gas in it!!! (I'm going to strangle him.) He said the tach started bouncing again, actually pegging it, and his low beams don't work either. I'm going after that too. Perhaps these issues are related. He did say it ran better than ever after I found the loose coil wire though.....

[Dennis Kalma]

I have a '75, albeit with a heavily modified '79 engine in it.

In terms of the bouncing tachometer, you may want to check the connections at the battery and ground connections as well. In my case, I found that the red wires connected right at the battery terminal were not making good connections, fixed that and the problem went away.

To be sure, there is about 5 wires bolted to the terminal, you probably want to check for corrosion or even poor connection from fatique of the wire. NOt a bit deal to fix..you can replace the connectors if required.

I would also support the whole idea of new cap/rotor/points/condenser/spark plugs....regardless of what the PO said, it is safer to just start from a known set of components.

Considering you will own this car today (as you will shoot your brother in law for the gas thing) I would also go through the engine wiring and fuse boxes witha fine tooth comb, looking for loose or damaged wires. I would spin every fuse and make sure that they looked OK (not corroded or anything), and if you are anal, take each fuse out, clean all the contacts with a bit of 3M Scotchbrite pad and again improve your confidence.

I presume you have already done the basic stuff such as change fluids, checked tires, brake pads just as a safety thing. That car is a serious +140mph car, I would hate for you guys to become statistics due to simple things....

Dennis

[BiL911SC]

Thanks Dennis. I was indeed headed for the battery connections next. The cap & rotor look perfect although I haven't inspected the points closely yet.
I have seen very strange things occur as a result of poor electrical grounds. At least we live in the desert so rust and corrosion aren't extreme.

[914efi]

I have a '75 carrera, what do mean by 'stamped' 911SC?
Thanks