69 911T won't rev past 4500ish

[bobspace]

I thought my car was finally running well :-(

After replacing my ignition points with a pertronix ignitor, and tracking down the RF interfering components and replacing them with non interfering versions, it seemed like my car was FINALLY running well.

But it's developed a new problem: When accelerating under load, the engine won't rev past around 4500. At that point it starts popping and sputtering and won't rev any higher. The issue was sporadic for a few days, but now it's consistent.

This only happens under load. When the car is in neutral it revs fine.

Is this another ignition timing issue? Something else?

[oldie44]

Check the output of your fuel pump. I had the same problem on a 1970 911T that had been parked for a long time. Problem was that the pump was not able to keep the carb bowls full under load. New pump solved the problem.

[stownsen914]

if you’ve just replaced a few components, the condition is probably caused by one of them, or by the work you did to install them. Was one of the things you replaced the plug wires? Check that they’re seated wel, and that all the electrical connections you touched are attached well. If that all checks out, try reinstalling the old parts one by one, with test drives in between, until it runs well again, and you’ll have your culprit.

Scott

[bobspace]

Originally Posted by stownsen914

if you’ve just replaced a few components, the condition is probably caused by one of them, or by the work you did to install them. Was one of the things you replaced the plug wires? Check that they’re seated wel, and that all the electrical connections you touched are attached well. If that all checks out, try reinstalling the old parts one you one, with test drives in between, until it runs well again, and you’ll have your culprit.

Scott

Silly question: the last thing i replaced was the gas and oil filler caps (with porsche oem replacements). Those couldn’t cause a thing like this could they?

[Trackrash]

Sounds like a classic case of retarded timing. Put a light on it and see if your ignition timing is still advancing correctly.

Could be the fuel supply issue, but that would be under power, not necessarily RPM related.

[roblav]

Partially clogged main jets?

Possible, but more probably due to your recent fixes in ignition. Go back to points and condenser?

[bobspace]

Thanks everyone for the feedback!

Another note: It's very cold and wet where I am, currently. I live in the bay area and in crossing the bridge, I was literally driving though clouds when this was happening.

My mechanic suspects water in the fuel or carburetor icing.

Has anybody had this kind of performance due to moisture?

[Daves911L]

Pertronix are garbage. They work really well, until they don't. Some work fine for a long time, others fail virtually out of the box. You never know which you will get, or when or where they will quit. My guess it is something related to the pertronix. Put your points back in and see if that fixes it.

That said, I got some diesel-contaminated gas one time in a carbureted 911, and it caused similar symptoms. Started, idled, and ran reasonably well up to 4000 rpm or so, then just hit a brick wall of sputtering and mis-firing and refused to rev any higher.

[Trackrash]

Originally Posted by bobspace

Thanks everyone for the feedback!

Another note: It's very cold and wet where I am, currently. I live in the bay area and in crossing the bridge, I was literally driving though clouds when this was happening.

My mechanic suspects water in the fuel or carburetor icing.

Has anybody had this kind of performance due to moisture?

No. Moisture can cause misfiring, but not what I think you are describing. Carb icing in the Bay area, I think not.

Once upon a time when driving in really damp conditions my car would have a random missfire. Turned out to be a crack in my rotor.

If it were me, the first thing I would do is put a timing light on it and verify the ignition.