2.7 Ignition Woes

[jmsnyder]

Hello everyone.

I'm working on a friend's car. It's a 75 911 with the 2.7. The fuel injection has been removed and replaced with Weber carbs by the previous owner.

When my friend bought the car and brought it to me, the set up was a bit of a mess. The car didn't run very well and still had all the wiring for the fuel injection laying around in the engine bay.

I eliminated the unnecessary wiring and did a typical tune up. I adjusted the valves, changed the oil, new plugs, new distributor cap and rotor, and put the carbs back to the default setting, tuning them from there.

After all of that, the car ran better but still not right. Sometimes it would run damn near perfect and other times it would clearly be running on 5 cylinders. What has been confusing me is that the misfire seems to be in either cylinder 3 OR cylinder 4. When one was running, the other would not. I could tell by adjusting the mixture screw on each individual cylinder until RPM was affected. If the screw didn't do anything, then it lead me to believe that cylinder is not firing. At any more than 1/4 throttle, the engine ran smooth, and pulled strong.

Next I removed the points to inspect them (Forgot to check the dwell angle first...) The points looked just fine and I put them back in gaping them to .012 and confirmed with the dwell meter which showed 39 degrees that the dwell was in spec.

After the dwell adjustment, all 6 cylinders are firing! But there's still occasional popping out of the exhaust at idle. Sometimes not at all then sometimes constantly for 20 seconds or so. This is where I need some insight. The car runs great otherwise so this is the last thing I need to work out.

For added insight, I did a leak-down test showed 98-99% in all the cylinders. Also, the car is running the original distributor with no vacuum line attached. I have the initial timing at about 5 degrees BTDC. I know for peak performance the distributor should be re-curved but I don't see why it wouldn't idle perfectly with the current distributor. Also, I did clean the idle jets in the carbs which didn't seem to make a difference.

Thanks for the help!

[Lapkritis]

With ignition sorted I would move to mixture. Laser thermometer each exhaust port/pipe when the issue is present to narrow down which cylinder is offending. Check the plug, verify with color tune. Could be an air leak/bypass screw out of adjustment. I'd have the vacuum gauge that sits atop the stack on each cylinder also to make sure they're balanced.

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[jmsnyder]

Originally Posted by Lapkritis

With ignition sorted I would move to mixture. Laser thermometer each exhaust port/pipe when the issue is present to narrow down which cylinder is offending. Check the plug, verify with color tune. Could be an air leak/bypass screw out of adjustment. I'd have the vacuum gauge that sits atop the stack on each cylinder also to make sure they're balanced.

Sent from my SM-N960U using Tapatalk

Thanks for the input. I have the same vacuum gauge which is what I used to tune the carbs. They are all balanced. I pulled some plugs and they looked good but they're brand new so it my be too soon to tell. But again, it runs fine other than at idle so I feel as if even if the plugs had a story to tell at idle, it would be erased by the smooth running or cruising around.

[juanbenae]

are you using the OE ignition box? carbs generally are going to need a hotter box & coil such as an MSD and blaster coil to generate enough spark. a recurved distributor is almost a must to get the advance the carbs like. also a worn distributor shaft will play havoc on a consistent timing effort.

if you are not already familiar check out paul's (1QuickS here) performance oriented web site. great weber info there on a wide range of carb set up and theory.

[zedsn]

I too would recommend a MSD box, blaster coil and pertronix. My 2.7 RS spec ran well for several years with these extras. I had a spare distributor recurved but didn't notice much difference from the one that was on the car already.

[pampadori]

sounds to me like the carbs need to be balanced and idle stops adjusted (i know you said you already did that but maybe crack them all open a little more).
if one cyl is not open as much as the others at idle, then it won't fire the mixture.
If you think cyl 3 or 4 are the culprits, whichever of those two has the greatest vacuum at idle is the one I would start working with. make the vacuum at idle less for that cyl. You'll have to go back and adjust the other 5 afterwards to sync again.

[OSC911]

If I remember correctly the initial timing should be st at 5 deg after top dead centre. Sounds weird but I am pretty sure it is the setting on a 2.7.

[jmsnyder]

Originally Posted by juanbenae

are you using the OE ignition box? carbs generally are going to need a hotter box & coil such as an MSD and blaster coil to generate enough spark. a recurved distributor is almost a must to get the advance the carbs like. also a worn distributor shaft will play havoc on a consistent timing effort.

It is the original ignition box and coil.

Originally Posted by pampadori

sounds to me like the carbs need to be balanced and idle stops adjusted (i know you said you already did that but maybe crack them all open a little more).
if one cyl is not open as much as the others at idle, then it won't fire the mixture.
If you think cyl 3 or 4 are the culprits, whichever of those two has the greatest vacuum at idle is the one I would start working with. make the vacuum at idle less for that cyl. You'll have to go back and adjust the other 5 afterwards to sync again.

I've tried all kinds of different configurations as far as the initial settings but always end up at nearly the same settings after tuning the carbs.

Originally Posted by OSC911

If I remember correctly the initial timing should be st at 5 deg after top dead centre. Sounds weird but I am pretty sure it is the setting on a 2.7.

You are correct, but that is with the vacuum line attached which retards the timing. Since there is no vacuum line I have it set at about 5 degrees before, otherwise the mechanical advance would only advance to 20 degrees or so at higher RPM. I have tried to see how it idles at 5 degrees after and it hardly runs.

Next I'm going to try a new set of points. I pulled the distributor today to get a closer look and noticed that part of the points arm that wears against the distributor shaft is worn to the point where it only touches the shaft at max lift. So although the points gap is correct at full lift, it would be not be opening as long as it should be. My dwell measurement being in spec would contradict that, but then again my dwell meter was handed down to me from my father who bought it in the 70s at Sears. If new points don't solve the problem, maybe the MSD setup will be the next step.

Here's a picture of what I'm talking about with the gap between the distributor shaft and points arm:

[gomezoneill]

Look at all that crap! a good start would be to R&R the dist. clean and check for runout. The block on the points looks worn.

[jmsnyder]

Originally Posted by gomezoneill

Look at all that crap! a good start would be to R&R the dist. clean and check for runout. The block on the points looks worn.

The close up picture makes it look much worse than it is, but it will go in clean for sure. The block is the part I was referring to. I wasn't sure what it was called.

[dicklague]

I have a 2.7 with MFI and after 5 years or so of running the points and stock CDI and coil I converted to Pertronix and a better aftermarket CDI. I used a Daytona-Sensors CDI which is a great unit, but even an MSD Streetfire at $160 is way better than the Bosch.

In my case MFI and yours carbs needs the best spark to make up for the variations in mixture through the range.

My Pertronix has been foolproof for 15 years.

[manbridge 74]

I’d start with putting things back to stock as the factory recommended and begin diagnosing from there. These cars don’t have a lot of running issues if kept in “the window.”

[jmsnyder]

Thanks everyone for the advice! Since I hate threads with no conclusion, here's what happened;

I took the distributor apart down to the advance mechanism and cleaned everything. Just for the time being, I picked up some fresh store-brand ignition points at Advance Auto. Once I got those installed and dialed into spec, I set the timing and started from scratch once again tuning the carbs. After getting everything dialed in, it runs damn near perfectly.

For anyone interested in the configuration that worked for me, here it is:
1975 Porsche 911 2.7
Weber 40IDA Carburetors
Idle air correction screws out about 1/8 turn. (each tweaked slightly to even out barrels)
Mixture screws out 2 turns.
Stock CDI, Distributor, and coil.
No vacuum line to distributor.
.012 inch points gap resulting in 34 degree dwell angle.
Initial timing 5 degrees BTDC at 950 RPM and 30 degrees BTDC at 6,000 RPM

I do have to keep the throttle open slightly on initial cold start for 20 seconds or so for it to idle on its own. Once it warms up, it idles smooth, pulls hard, and I'm happy with it.