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Registered User
Join Date: Oct 2005
Location: Asker, Norway
Posts: 7
Rough running 2,7 RS spec engine – help

I am new to this community, and what a vast resource of experience and knowledge! Hopefully – some of you may be able to give me a few ideas regarding my challenge. Please be tolerant to a few misspelled words as English is not my mother tongue, living north in Europe – Norway to be more exact. So to my problem or challenge, I have recently acquired a ’70 911 with a 2,7 RS spec engine. The overall condition of the car is very good, except that the engine had and still has the following symptoms:

When I bought the car, the engine started up and ran passable when cold but as it gradually warmed up the running got rougher, especially below 2000 – 2500 rpm and it would not idle at all when warm. “Every thing” has been checked, changed or overhauled like fuel tank cleaned, fuel filter in tank new, fuel hoses partly renewed, fuel pump checked, fuel filter in engine compartment new, fuel injection pump overhauled and calibrated (correct FI pump for 2,7 RS), fuel injectors new, air filter new, hot air hoses from the heat exchanger to FI pump is new, the pump control rod from the cross bar to the pump has been adjusted, valves adjusted, distributor overhauled and calibrated, timing checked etc… The engine was running only marginally better after all this, and I was about to give up. As a final attempt I decided to measure the ignition wires’ resistance and then to my BIG surprise realized that the wires for cylinder no 1 and 6 was mixed up at the distributor cap (by the previous owner..)! I have to admit that this was not a likely place to look for rough running in my head – you just don’t screw up the ignition wires I thought, but yes - some obviously can. But after correcting this, the engine has pretty much the same symptoms as before except that it now runs much better when cold. So my worry is that the mix up of the ignition wires may have caused some damage to the engine – but what do you think – any ideas? Or is there any other possible, overlooked cause?

Thankful for any suggestions.

Old 10-11-2005, 09:21 AM
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Registered User
Join Date: Nov 2001
Location: Central Kentucky
Posts: 729
Wow! mixed up ignition wires!
You have certainly been thorough in looking at all the possibilities but I would still think of these:
1) actual fuel delivery..undo the injectors and ensure that each cylinder is actually getting the fuel.
2) are you certain the fuel pump is not failing to maintain the pressures neccesary for a MFI system?
Richard Lane
1978 911SC
1973 "Clean" RS clone
Old 10-11-2005, 09:43 AM
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PRO Motorsports
Tyson Schmidt's Avatar
Join Date: Nov 2001
Location: Burbank, CA
Posts: 4,472
Sounds like you need to richen up the part load setting. Do you have the MFI adjustiment tools?

To richen the idle, you need the long 8mm hex head tool that reaches through the fan and through the little access hole on the back side of the shroud. You compress the spring and turn it until it engages, then turn it clockwise to richen, and counter-clockwise to lean. The motor must be stopped to do this, obviously. Turn it one click at a time until it will idle properly. You really should have it on a CO analyzer to get it right.

Unloaded, I like to get 8% CO at 2500-3000 rpm, and 5%-6% at idle.
'69 911E coupe' 2.7E MFI (retired 911-Spec racer)
'72 911T Targa (Formerly "Scruffy")
2004 GT3
Old 10-11-2005, 11:53 AM
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Registered User
Join Date: Oct 2005
Location: Asker, Norway
Posts: 7
Richard, I've retested the fuel delivery, all injectors and pump are all according to spec, "unfortunately". Regarding Tyson's suggestion, I will perform a gas analyze next week. But isn't a CO of 5-6% at idle a rather rich mixture - as far as I remember the original factory recommendation was 3,5%? But what about possible engine damage due to the mix up of the ignition wires - what do you think?

Old 10-13-2005, 12:46 PM
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