Backfire and jumpy tach

[LETITBE]

Hello,
i just bought a 75' 911s and drove it back home 1,200 miles. It ran perfect the first 1,000 or so miles and then i noticed the tach needle starting to jump around. Later on the motor would cut out. Now i just drove it around for a few miles and everything seemed to be fine but just as i got a few blocks from home the tach started jumping and motor cutting out again as well as a backfire. It is almost like i was turning off the key and then back on.

Hope one of you might offer some insight as to what the problem could be. I parked the car in garage and left it running for a while and it idles
perfect.

Thanks in advance for your help to a new member.

[crustychief]

post a pic of your car and check your alternator.

[LETITBE]

[Brent Lineker]

Points are either pitted or dirty would be my guess.The tach will read erratic when points are bad as well as run crappy.My .02 worth.

[crustychief]

Nice Targa! Welcome to the forum.

[Winter]

Killer! Another white '75 targa. Just like mine, only nicer! Except my tach works. ;-)

Good luck with it!

---

Tom '75 Targa ("stinky")

[scottb]

A jumpy tach is a classic sign of alternator failure, but since you drove another 200 miles after the problem started, I'm thinking that it's something else.

You should use a voltmeter to check voltage at the battery while the car's running. It should be in the 13 volt range, IIRC.

If the voltage shows normal at the alternator, then I think checking the points is a good idea.

[Paulporsche]

Check the points first as others have said.

To post a pic, use Go Advanced in Quick Reply. Click on Upload a Photo. browse your computer for the photo, select it and hit Upload. You can preview it if you want, or just click on Submit.

[LETITBE]

Paul,

i am going to check the points first. I have pulled the distributor cap off,
now how do i get the rotor off to get to the points?

Thanks for the help and any advice is appreciated. I am new to porsche
i just got the car and now i can't drive it. The car runs perfect until it warms up and the cuts out and backfires. I cant get but a few blocks away so i can't get to a mechanic unless i get it towed.

Thanks again,
letitbe

[ninelevenick]

Mine had similar issues, and it turned out to be the ignition module. Although, when the ignition cut out, I had to turn the key to get the motor running again.

[LETITBE]

Mine acts like i turned the key off but i haven't. It starts right up again on it's own.
I have run it in the garage with the lights on to see if they are diming or going of but everything stays bright and the battery is staying charged. I am searching for a mobile mechanic in okc to come look at it.

[john walker's workshop]

the points are probably barely opening, or burnt.

[Paulporsche]

The rotor just pulls straight up and off the shaft. There is no screw or locking device holding it on.

You can set the points w/ a feeler gauge, but I like to use a dwell meter, which I feel gives you a more accurate setting. There should be no burning, pitting or transferrence of metal from one side of the points to the other.

If it looks like they are old, you should also change the cap and rotor. Also check the condition of the ignition wires.

Let us know if that takes care of it. If not, as others have said, it might be your CDI box, wiring, connectors, etc.

Do you have a Permatune CDI? There are numerous reports of those going bad once they get hot.

[LETITBE]

Paul,
Thank you for your help as well as the rest of you that replied. I have decided to have the car picked up and taken to Bob Dumonts here in Okla. City. I am not experienced at all and I don't want to make things worse. It sounds like the problem could be minor but who knows. The car is a bone stock survivor with original paint and all and it deserves a professionals attention. The CDI BOX is the original BOSCH and it looks like the voltage regulator is original as well.
I will be sure to post what the problem was when it is fixed and hopefully those of you who have the ability can solve your problem.

Thanks everyone! This is a great site with great people.

[LETITBE]

Thanks guys, you are great to help a new guy.
I just picked up the car and it runs like new.
You were right, only the points.

The shop said they have a kit they install in the distributor that eliminates the points and said the car will run even better. Cost was $230.00 total.
Anyone done this and what is it called?

By they way, i'm new to the forum, what does it mean when someone replies "bump"?

Thanks again,
letitbe

[Paulporsche]

Glad to hear that's all it was.

Check the New Parts Catalog here. PP has Pertronix electronic ignition parts that replace the points for under $100.

Also, search this BB for threads on installations of this type of system.

[teenerted1]

too late now but you also want to look at the the green coaxial cable coming off the distributor. if this get frayed it can arch and produce the same simptions too.

[Walter_Middie]

LetItBe,

Good to hear you got it running right without too much hassle.

By they way, i'm new to the forum, what does it mean when someone replies "bump"?

When someone replies to a post, it is bumped to the top of the list of Threads. So someone might just add a "bump" to a thread to get it back to the top, and get the attention it deserves.

I see you bought the best car, the best year, and in the best color. You are obviously a man of great wisdom and taste. Welcome to Pelican.

[RoninLB]

buy some auto repair books , tools, and Pelican's "101 Projects for the 911".


make sure you have a digital camera to post pics detailing issues with your next project.