Hot running issues

[Bikesmith01]

Hey, Everybody!
I'm having trouble tracking down an issue and I'm hoping you guys can give me a hand.

My wife and I bought a 1975 911s a couple months ago. It was running great when we got it and it's still mostly good, but there's an issue developing.

The car runs a little rough for the first twenty seconds from cold, but I understand that's not unusual. After the first twenty seconds it runs beautifully while cold and while warm. After 30-40 minutes, though, it starts stumbling on initial acceleration. It idles fine (a little bit of RPM wandering, but nothing I'd be concerned about). Then, when accelerating from idle, the engine stumbles until it hits maybe 1800 RPM. Then it goes right back to normal through the rest of the rev range. If I let the engine cool for half an hour the problem is still there, but the next day it's gone until the engine is fully hot again.

The previous owner had the engine rebuilt by a Porsche dealer and the car has about 1600 miles since then, so I'd be surprised if there was anything fundamentally wrong in the heart of the engine.

My first thought was that the mixture was going too lean when fully hot, but that's not the case. It's putting out right around 3.0% CO. Since that's higher than spec I tried bringing it down closer to the normal 2%, but that actually made it worse. So I have it back at 3%.

I took a look at timing, too. It wasn't set very well. It was at 8 degrees ATDC at idle with a point gap of about 3-4 thousandths. I adjusted dwell (currently 36.4 degrees) and set timing to the specified 5 degrees ATDC. That made the engine run smoother while cold and warm, but didn't help the hot running stumble.

I haven't checked valve clearances yet. The car came with no oil cooler, so I have one on order (Still waiting, Pelican!). When that shows up I intend to install it and check the valves at the same time while the oil is drained. So, valve clearances could be an issue. But I doubt it considering the timing of the issue. I plan on replacing the spark plugs, too, while I have them out.

Another thing I suspect is the ignition transformer. I've seen coils cause problems with hot running. I checked resistance and it was within spec, 0.6 ohms on the primary coil and 740 ohms on the secondary. But I'm still not convinced it's good. At almost $400, though, it's an expensive item to swap out on the basis of "maybe it will help."

Also, the issue seems to pop up after about the same amount of drive time regardless of engine heat. There were times when it was hotter here in Colorado than the oil temp got up to 250ish (I know. That's why I'm putting in a cooler). Driving today, the temp never got above 210. Either way, the issue started after about 30-40 minutes. Makes me think it's something being heated by the engine (like the ignition transformer) rather than the heat in the engine itself.

So that's what I've got. Any thoughts?
Thanks in advance for your help!
-Jon

[PeteKz]

Quick thoughts:
1. As you noted this is typical of a coil going bad. Find someone to swap with, or buy a non-OE coil for a CDI ignition. Pertronix and MSD sell these for about $60.
2. Distributor flyweights may be sticking. Although engine was recently rebuilt, did they clean and lube the dizzy?
3. Valves: not likely in 1600 miles since rebuild, and not typical of hot running problem, because valve clearances increase as the engine warms up.

[Bikesmith01]

PeteKz,
Thanks for the reply! That's helpful!
1) I'm perfectly happy spending $60 to see if it helps, especially for a problem that really feels like a coil issue to me. But I'm completely unfamiliar with CDI, so I'm not sure how to find a coil that won't damage it. For instance, Pelican says this coil (https://www.pelicanparts.com/More_Info/PELPT40611.htm) "Fits your 1975 Porsche 911s," but the description doesn't say that it works with CDI. The description on the $400 coil does say "for capacitive discharge only"

Any tips for making sure I get the right thing, or links?

2) That's an excellent thought. I don't know how likely it is to be the culprit, but it's so easy I might as well check it out.

3) Agreed. I'm going to check the valves anyway, but I doubt I'll find anything wrong there.

Thanks again!
-Jon

[Bikesmith01]

PeteKz,
Thanks for the reply! That's helpful!
1) I'm perfectly happy spending $60 to see if it helps, especially for a problem that really feels like a coil issue to me. But I'm completely unfamiliar with CDI, so I'm not sure how to find a coil that won't damage it. For instance, Pelican says this coil (https://www.pelicanparts.com/More_Info/PELPT40611.htm) "Fits your 1975 Porsche 911s," but the description doesn't say that it works with CDI. The description on the $400 coil does say "for capacitive discharge only"

Any tips for making sure I get the right thing, or links?

2) That's an excellent thought. I don't know how likely it is to be the culprit, but it's so easy I might as well check it out.

3) Agreed. I'm going to check the valves anyway, but I doubt I'll find anything wrong there.

[PeteKz]

I forget the part number and it’s not coming up in my forum search. Check with John Walker. Hae has a specific recommendation.