1975 CIS issue

kplackmeyer · 06:33 PM

Hello to all.

I am new to the Forum, but not new to CIS. I recently picked up a 1975 911s from a former employer (mechanic and shop owner). I have known the car for over 20 years, and it has primarily been a "go fast" car, not an every day driver...but was driven regularly.

I am in the process of running the obligitory checks, but wanted to post my issue to see if anyone else has seen this one.

The car starts, runs and drives (at normal traffic driving) with no problems. However, above 4800 RPMs, it begins to fall on it's face (loses power, stumbles, won't accel, pops, etc).

Now here is the fun part...go hard around a left or right corner... BAM! head pressed to the back of the seat and away she goes, all the way up to redline. Straigten out...stumbles like a drunk on a Saturday nite.

Motor has about 10k on it, it's the 2.7 from 1975 but when redone all of the fixes were done (case helpers, racing studs, delete of thermal reactors, 11 blade fan, carrera oil cooler).

So far, System pressure and warm control pressure are ok, but I have the vacuum controlled pressure regulator, so I am picking up a Vacuum pump/gauge in the morning.

Need to check the fuel pump delivery rate at the return line...didn't have the appropriate hose to do it safely tonite. I checked it at the supply line, just for a base-line comparison, and it was good...but it's gotta be checked at the return.

Plunger and air flow sensor are both free...no binding. New fuel filter, holds pressure after shutoff, so accumulator seems ok.

Checked for vacuum leaks more times than I care to count. Can't find any.

My current thoughts are either a bum fuel pump not delivering enough volume, but under hard cornering it tightens up any play inside (I believe they are roller-style pumps?) or, the vacuum actuation of the Control Pressure regulator is bad at high load/high rpms...but I have NO good explination for the symptom going away in hard corners for that one.

Updates coming soon...but I wanted to throw it out there for comment.

Oh...and since this is my first post...I wanted to say thank you to everyone for all of the great info on this forum. It's amazing just how much great stuff there is.

kplackmeyer · 06-27-2007, 04:55 AM

Ahhh...forgot to add....(it was pretty late last nite!) The FIRST thing I did was pull the fuel tank sending unit to get a look in there. Really clean...so don't suspect any debris floating around, disloging on corners.

Porsche_monkey · 06-27-2007, 05:05 AM

You should still check the screen in the tank.

kplackmeyer · 06-27-2007, 05:29 AM

I will give the screen a shot. Tonite I will have the vacuum gauge, so I can properly check the control pressure...and I cannot recall how much gas is in the tank...but I'll pull the plug when it gets down. Can't hurt, right?

Porsche_monkey · 06-27-2007, 05:45 AM

Fuel pressure needs t be checked.

kplackmeyer · 06-27-2007, 05:58 AM

--System pressure is good. (4.7bar, spec is 4.5 to 5.2bar)
--Warm control pressure with vacuum line at atmosphere is good. (2.8 bar, spec is 2.7 to 3.1bar)
--Picking up vac pump today to check Warm control pressure with vacuum applied to control pressure regulator (should be 3.4 to 3.8bar with 520 to 546mbar of vacuum applied)

If the pressure with vac applied is off...that would make sense, except for the symptom going away in corners. I can't think of why the Control pressure regulator would be affected by cornering...

Porsche_monkey · 06-27-2007, 06:34 AM

I have hung the fuel pressure gauge in the back window to see it while I'm driving. Could be your losing your fuel pressure while driving.

Start there. Then we'll move on to timing etc.

kplackmeyer · 06-27-2007, 06:31 PM

Turns out...not a CIS problem at all.
I stopped at lunch to get the vacuum pump...and PBH's last post about timing, etc. was quite prophetic. Right next to the vac pump was a timing light. Haven't had one in a number of years, and knew I would soon. So I bought it.

Went thru all the pressures, delivery tests, etc. right after dinner. Everything checked out perfectly. Somewhat frustrated, I put all of the pieces back together, and went on to try out my new timing light with digital tach & advance.

BOY...was it off. But not just off...looks like my centrifugal weights are sticking, and perhaps some other problems in the distributor. Spec for the timing is TDC at 900 rpm with vac advance removed, and 10 deg with vac advance in place. If I set the advance at 0 deg with vac off (hose plugged) and then install the hose, it jumps to 26 deg! So...I instead set it at 10 deg with the vac set. It runs SOOOO much better, but still not 100%. Still a little missing at hi rpms, and a new problem that I read about the other day on the forum. When I decel, the RPMs stay up around 1700. If i let the clutch out a little with my foot on the break, until the rpm drops to 900 or so, it will stay there. And the timing drops back to where it's supposed to be.

I suppose it's time to pull the distributor and pull it apart and clean it out. Looks pretty dirty in there.

My distributor still has points, so my thought is that I will order the
"Ignitor II Electronic Ignition" a cap and rotor, the "Flame-Thrower II Coil". I already have a Perma Tune CD, but it looks pretty old. But I see a lot debate over points vs electronic ignition. So the debate on that continues.

I suppose the first step is just to pull it apart and see what's sticking....

Porsche_monkey · 06-28-2007, 03:42 AM

Oh yea.

I am the man!

Just a lucky guess.

azasadny · azasadny's Garage

Kerry,
Excellent! I have points and Bosch CD on my '74 and they work well together. i switched my CIS over to Webers last December and have very happy with the results. I have lots of spare CIS parts if you ever need any.

kplackmeyer · 06-30-2007, 04:51 PM

Pulled the ignition distributor last nite, and tore it all apart. All the pieces were there and complete (not like some of the stories I have read...broken bushings, disintegrated parts, etc). The bearings were free, no lateral play. But the weights were tight and full-o-crud. Cleaned everything thoroughly...lubed with 3-in-one upon re-assembly. Set the dwell, then timed it.

Runs like a scaled dog! When I first experienced the problem I thought it seemed like an advance problem, and I changed the dried out vac advance hose....it was quite porous. The vac advance was free and moving, so I moved on to fuel. Should have gone with my gut. But now I know the fuel sys is A-OK, and it runs like a champ....can't wipe the smile off my face.

Awesome when they are running 100%...

Porsche_monkey · 06-30-2007, 05:52 PM

Is your vacuum advance connected? I think most people use only mechanical advance on those cars. Try pulling it off and plugging it. It will likely run better.

kplackmeyer · 07-02-2007, 06:07 AM

Quote:

Originally posted by PBH
Is your vacuum advance connected? I think most people use only mechanical advance on those cars. Try pulling it off and plugging it. It will likely run better.

PBH,

It is connected. Per the manual, I timed it at 0 deg with it connected. When removed, it goes to 10 BTDC as it should. So if I disconnected it, would I time it to 10 deg for normal idle?

Any downsides to this? It runs really good right now...but it was running so sh!##y before that any improvement would seem amazing.

Porsche_monkey · 07-02-2007, 07:11 AM

I sold my 74 a while ago. I can't remember.

I would disconnect it and see how it drives. Then try 10 degrees back.

Maybe search the forum...

azasadny · azasadny's Garage

Kerry,
Check all of your vacuum lines as small vacuum leaks from older, cracked hoses can cause all kinds of problems. When I replaced all my vacuum and fuel lines and clamps a few years ago, I couldn't believe the improvement it made...