Misterious behaviuor after rebuilding ...

[PaoloB]

I've already taken advance of your advices one year ago just before starting my 2.7 (type 911/81) engine overhaul.

The project has taken a lot of time (I'm just an hobbyist) and I've been supported by a professional mechanic and by the literature I bought from Pelicanparts.

I completely disassembled the engine and renewed almost every worn part (from rings to rockets, valve guides, timeserts for studs, etc. etc.) in order go get an all in specification engine. Well, to be short, a short after firing it up cyl. n. 1 started misfiring. We thought that it was a simple ingition/carburation problem and proceeded with the tune-up. After 300km we checked the carburation that resulted perfect for the other five cyl. but not for no. 1: black and just a little oily.
We started the investigation.

Results:
1) Ignition: perfect (completely renewed from the coil to the spark plugs and completely tested)

2) Distribution timing and compression OK (9.2:1 with a spec. of 8.5:1), no smoke, any trace of oil in intake or exhaust ducts (mechanically the engine is OK).

3) Injector (cleaned and tested before reassembling) excellent spray pattern, no air leaks found.

We tested CIS pressures that resulted in the tolerance, tried enriching mixture: sometimes works, but when the throttle is 3/4 open and the engine revs over 4500 rpm then the no.1 starts misfiring (also at idle). Changing the spark plugs seems to work but not as it should as it was a mere ignition problem. Unplugging no. 1 it at idle has a weaker effect than the other five (seem as even when it fires the combustion in weak).

The temperature seems not to be influent (sometimes the engine start with six from cold, sometimes only five or the misfiring appears only when hot).

The last hypothesis left is a strange CIS distributor behavior: in some condition (to be more clarified) line no. 1 may give less gasoline volume, that would result in a lean mixture for the cyl. no.1 (the other five cyl. are pretty OK) . Of course we are very dubious about this and I want to make a volume test, but we're running out of ideas.

Any suggestion ? We're forgetting something ? It's getting misterious !

Thank in advance for your help !

Paolo

[Tim L]

Pull that injector and check it again. I damaged one of my injectors when installing the intake. I had a miss in one cylinder and the plug was fouling. I pulled that injector and sure-enough I knocked the tip off so it wasn't spraying.

Tim

[PaoloB]

When the misfiring occurred we pulled out the injector and observed a quite good spray from that but still popping after reinserting it.
Assuming that is likely to be a carburation problem, I must understand if is the injector that is not sprying the right quantity of fuel or i the CIS distributor that has the port no.1 clogged. I've moved the line 1 to cyl. 2 and line 2 to cyl. 1 the result is no.1 still popping and even a more rugh idle.
I'll try inverting just the injectors .....and see.
Wayne any suggestion ?

Paolo

[tshih]

Was the distributor (ignition not fuel, new or rebuilt? I once installed a Pertronix ignitor for the 1977 911 2.7 CIS and it ran weirdly (popping and misfired at certain rpms) when warmed. Turned out to be a badly rusted advance weights inside the distributor. A new Bosch distributor ($400 from Florida parts place) cured the problem.

[PaoloB]

The ignition distributor is the original one which we chcked before reassembling. It anticipates correctly at every rpm (tested from idle to 5000 rpm.).
During the weekend I'll do some more tests.....
Any suggestion is very welcome.

Paolo

[Wayne 962]

Check compression and leak down, to assure that it's not a mechanical problem...

-Wayne

[stealthn]

Possible valve issue sticking open causing the misfire?