SC miss-fire question

Bill Douglas · 11-26-2007, 10:42 AM

Hi guys I have a slight miss-fire in the SC. At low revs, say under 3,000rpm, it misses if I give it any throttle. If I back off the trottle it’s fine. When it’s completely cold there isn’t a miss, or the miss is barely there. Once the car is really warmed up, say I’ve been driving it fairly hard for an hour or two the miss can be up to 4,000rpm. Again if I nearly completely back off the trottle the miss goes away. I’m not often below 3k revs once it’s warmed up but I’m hoping to improve it’s drivability.

The engine is from an ‘82SC and is in my ‘79SC. The motor has had a professionally done full rebuild about 12,000 miles ago and runs beautifully, other than the small miss-fire. It has SSIs and no oxygen sensor or lambada (spelling) device. When it was in the original (1982) crashed spare parts car it didn’t have a lambada or oxygen sensor either.

I moved the ignition timing around but it didn’t change the miss. Any ideas?

Greek55 · 11-26-2007, 10:57 AM

A couple years ago I had the same problem. The PO had messed with the warm up regulator and it caused a miss under load. When cold I didn t feel the miss. Eventually, after changing plugs, wires, injectors we changed the WUR and have not had the missing problem since.

Superman · 11-26-2007, 11:02 AM

I suspect a momentary fuel/air mixture that is either slightly too lean or slightly too rich for good combustion. And so......if you still have the CIS system, then just go through the troubleshooting steps. One of the early steps is to make sure the air flow sensor plate is in the perfect location at rest. Just a guess, but that's what I suspect.

Steve@Rennsport · 11-26-2007, 11:17 AM

If its an '82 SC motor and operating without its oxysensor, you likely have a lean misfire condition that can be aleviated by adjusting the CO to around 2.0-2.5%.

Make sure that ALL your plug wires are fully seated, you are NOT using Bosch platinum plugs, and the cap & rotor are in excellent condition.

Bill Douglas · 11-26-2007, 01:18 PM

Hi Super and thanks Guys. I did a few tests on the WUR so I think thats ok. I hadn't thought of the mixture so I'll look into that. I know the sparkplug leads+distrubutor cap and rotor are ok as I've resistance tested them, but I may swap out the plugs as I've had a part failing NGK plug once before.

Superman · 11-26-2007, 01:54 PM

We've heard from the Man. Hi, Steve. Bill, I think Steve's saying the same thing as I. Those CIS systems are adjusted pretty darned lean from the factory, and the O2 sensor system just makes it leaner (under certain conditions). Keeping the O2 system unplugged is a good idea, from my perspective.

And yes, once that system is disabled, raising the fuel/air ratio helps. It could be that, at low engine speeds, when you crack the throttle open, you get vacuum (thin air) and the sensor plate is just not coming upward quite enough to get more fuel in before the engine stumbles on the increased air volume. Like Steve, I would strongly suggest first ensuring your ignition system is unimpeachable. That means new cap and rotor, new copper-core plugs (I like NGK) and test the ignition wires and connectors. That, and a rich-enough mixture, should do it. As far as I know. Which is not far.

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by Steve@Rennsport

Make sure that ALL your plug wires are fully seated, you are NOT using Bosch platinum plugs, and the cap & rotor are in excellent condition.

Steve,

Would you be kind and patience to us? Please elaborate a little bit more about it (spark plugs) and offer us some free education? Thanks.

Tony

Early_S_Man · 11-26-2007, 04:09 PM

Guys,

Make sure there isn't any fraying of the green coax cable where it enters the distributor, especially if it has turned brown anywhere along the length! If it has turned, replace immediately!!!

Several recent threads disuss the replacement ...

Even if it tests OK at 600 Ohms at the CDI, and 2.0 Volts AC during cranking at pins #7 & #31d ... it may be intermittant!

Bill Douglas · 11-26-2007, 06:16 PM

Thanks everyone for your help and interest. Warren the green wire looks fine, and the plugs+wires+cap are good, so I'll head down the route of richening the mixture. I guess I'll get the local Porsche place to do that and put it on the CO to get it to Steve's recomendation.