Slight stumble under acceleration

SpeedracerIndy · 08-30-2003, 02:58 PM

My 81 ROW has just developed a slight stumble under light acceleration. It only happens when the car is warm, and only under light acceleration off of idle. If it starts to stumble, and I floor it, it is fine. I have not noticed any other problems. The car runs fine otherwise. What could this be?

stlrj · 08-30-2003, 04:00 PM

How old are your ignition cables?

Joe
86 Carrera

SpeedracerIndy · 08-30-2003, 07:15 PM

Ignition cables and plugs are about one year old. They appear to be in good shape. I was thinking it was either a vacuum leak or a problem with the advance in the distributor. Any ideas?

stlrj · 08-30-2003, 08:16 PM

You most likely replaced your cables with a stock Beru set which means you will be chasing down spark plug misses regularly as I have been doing for years in my 86 Carrera until I finally figured out my ignition cables were the problem, not my plugs. Meanwhile, you might as well put in a fresh set of plugs until next time.

Cheers,

Joe
86 Carrera

stlrj · 08-31-2003, 11:17 AM

Quote:

They appear to be in good shape

Impossible to tell of ignition cables are OK unless you had them megged individually but I doubt a set of Berus would be able to contain 12kv of spark without a short.

Since most of us do not have access to a megger then the next best thing I would suggest is spraying your ignition cables with water with your engine at idle. If it sputters or will not stay running, your cables are no good.

Cheers,

Joe
86 Carrera

RoninLB · 08-31-2003, 11:21 AM

I think it was Warren who posted on the connector "shake" ohm test. and that the problem with the Beru connector may only happen when the heat of the engine affects the connector.

stlrj · 03:57 PM

Ohm testing is only good for continutity but does not tell you if the insulation is capable of conducting high voltage without shorting out. An Ohm meter reads resistance on a 3 volt circuit and when you compare that to 15,000 volts on your ignition cables then you might understand how meaningless and Ohm reading is.

That's why I say to spray some water on your ignition cables and if they cannot tolerate any amount of moisture, you know you have a problem.

Cheers,

Joe
86 Carrera

Mike Kast · Mike Kast's Garage

needless to say... my misses were fuel related. but my 79 has had 230,000 miles of gas run through it. My first thought was plugs ,wires cap and rotor too.

SpeedracerIndy · 08-31-2003, 07:10 PM

Haven't driven the car because of all the rain around here, but... The wires are Magnacore and were replaced by the PO around a year ago. The original wires for an 81 would be of the shielded variety and I really don't have any interest in going back to them. I have heard that a way to check them is to run the engine in the dark and look for sparks or glowing around them. Also could spray with water while in the dark. Anyone tried this? The plugs are the same age, and the rotor and cap are less than a month old. I was thinking that it was related to the vaccum advance. First I will check plugs and wires though. Any other advice is welcome.

RazorRacer · 08-31-2003, 07:18 PM

CDiercks,

I'm talking over my head here, but when I first got my SC it had a stumble under acceleration when the engine was warm. The PO had tried to block the vacuum advance with a broken pencil tip. When the vacuum line warmed up, the vacuum would seap past the pencil tip.

I ended up completely blocking the vacuum advance and setting the timing to 13 degrees BTDC and it has performed perfectly since.

Your 81SC may have a vacuum advance and a vacuum retard on the distributor.

The first thing I would try would be to remove the vacuum advance and cap it off with something. Then I would drive the car and see if the stumbling stops. If so, then I would advance the timing to about 13 degrees BTDC.

stlrj · 09-01-2003, 05:15 PM

If your plugs are a year old, change them anyway before you do anything else.

Cheers,

Joe

SpeedracerIndy · 09-09-2003, 06:20 PM

Well, plugs are new and still same problem. I am almost positive that it is my pop-off valve popping. Whenever it stumbles, I hear it popping. It usually only happens when it has been running for a while. Also, it never did this until after I installed the popoff valve about a month ago. The car also has a cold start issue. The pop-off valve pops like crazy if I give it gas when I first start the car. So... Any ideas what is causing this problem? I have heard that a lean condition can cause popping of the valve when the engine is warm.

Doug Zielke · 09-09-2003, 06:24 PM

Have the mixture (CO) checked with a gas tester. These motors seem to like a setting around 2.5-3.5% CO

SpeedracerIndy · 09-09-2003, 06:28 PM

Thanks, good to hear I'm on the right track. Now, where do I get a CO tester? Is this something that is obtainable for the garage mechanic, or do I have to go to a shop to get it tested?

Doug Zielke · 09-09-2003, 06:30 PM

You local Porsche mechanic will have a proper tester. He should also be familiar with the CIS mixture adjustment.

ruf-porsche · ruf-porsche's Garage

You could have a vacuum leak around the popoff valve. You could try putting more epoxy around the valve if it is leaking, but wait until the epoxy has set before starting the car or else the vacuum will suck that epoxy down the airbox.

stlrj · 09-10-2003, 01:27 PM

Whenever it stumbles, I hear it popping

An ignition miss would cause it to stumble and pop your valve. I still would not rule out the ignition.

The popping of your pop off valve is only a symptom of something upstream causing it to pop.

Make sense?

Joe

SpeedracerIndy · 09-10-2003, 02:18 PM

Makes sence. I am going to start with the pop-off valve since it is the cheepest rout. If still no luck, I'll start disecting the ignition system. Thanks for the help.