B5QShip

Hi everyone, I apologize but this is a long one....a result of about 5 years of failure. My father and I rebuilt a '74 911 (2.7 w/ S cams) over the course of the past 5 years (I was away at college, and my dad never worked on it without me around). The problem right now is that the car has no power (have to burn clutch to pull car in and out of driveway), idles poorly and backfires often. We've verified high voltage going to all plugs (plugs look fine, and inductive timing shows spark timing is good), the distributor is set correctly (we believe), injectors are spraying well (verified 4 cylinders). When we pull one plug at a time while the car is running, only 2 pulled plugs appear to give an audible change in rpm/engine tone. We checked compression on one cylinder that didn't produce an audible change in engine tone when its plug was pulled and it was fine. We just drained the gas tank and put in new fuel.

This was actually the 2nd time we had torn the engine apart. The first time, the engine had the same symptoms, in addition to leaking a bit of oil and some audible valve tap The crank also bound when the car sat due to improper case through-bolt torquing. The clicking noise turned out to be valves in 2 cylinders slightly impacting pistons due to machining on the case for which we hadn't compensated with spacers beneath the cylinders. We remedied this & verified proper valve seating. Crankshaft binding was also corrected. The second time through we were even more careful with the sketchy cam timing, and at the time, were pretty confident we got it right. Everything looked peachy, even after sitting for so long.

Does anyone have an idea of what may be the problem? Should we get a wideband and check mixture, check compression on all cylinders, try replacing all plugs, reverify cam timing, etc??

My dad's a PhD & has rebuilt numerous cars, I'm an engineering PhD student with oil running through my veins, we have every 911 rebuilding manual known to man, have made every 2.7l reliability upgrade (with the exception of the 3.0 swap) ,but yet we cannot get this damn thing to run properly.


In humbled frustration

- Ian

Steve Weiner-Rennsport Systems

Hi Ian:

Needless to say, if you have compression, a good correctly timed spark, fuel to each & every cylinder, and correct valve timing, its got to run OK,....

My recommendation is to go back to the basics with diagnostics:

1) Check mixture with an accurate wideband A/F meter.

2) Perform compression & leakdown tests in all cylinders.

3) Make sure the ignition system is PERFECT and the timing is correct.

4) Perhaps recheck valve timing, based on the outcome of the above.

5) Check fuel pressure at the inlet of the fuel filter for pressure as well as delivery rates.

Good luck!!!

m21sniper

When i did the T-belt on my 928 i thought i'd put it in wrong because it seemed like i was about 50hp short.

A simple timing advance corrected the problem. Hopefully, it's just as simple a solution for you. Good luck.

B5QShip

How far off would valve timing have to be to produce these symptoms? I think the S cams are supposed to idle a bit rough, but the car can't move itself.

How can we ensure "the ignition system is PERFECT"? The plugs and wiring from the distributor cap is brand new.

A wideband is a few hundred, no? (just wondering )

How much is this car worth in its current state? If it ran well? Its got brand new brakes/wheel bearings, all necessary 2.7 reliability upgrades, new clutch, a great looking red body, etc. I think my dad is fed up and really wants to dump it.

Steve Weiner-Rennsport Systems

Not far.

2.7 "S" cams idle VERY smoothly. Early ('67-'73) "S" cams are lopey.

LOL,...The ignition must be perfect. Points, point wiring, all primary ignition wiring, plugs, cap, rotor, coil, must be in perfect shape.

A good wide band runs $ 350. Look at the Innovative LM-1. Cheap ones are simply toys and you need real accurate A/F data.