CIS misfire

[GBStu]

Please can you help.......

Since rebuilding the engine last winter I've been experiencing a misfire when I apply "very light" throttle regardless of rpm. When I open the throttle the misfire goes away....

Stuff I've done so far:

1) Re-set cold pressure (warm pressure is about 47 psi)
2) Removed air box and resealed the two halves
3) New injector and injector sleeve o rings
4) Checked (many times) for vacuum leaks...none!!!
5) Adjusted mixture screw and ignition timing numerous occasions
6) Run a number a tanks of gas through with Techron
7) Checked injector spray pattern, all seems fine, except I cannot test the opening pressure.

For reference, I have 9.3:1 pistons, 964 cam and backdated headers with mb911 muffler on a 79sc engine.

Any thoughts would be gratefully received.....this misfire is sending me nuts!!!!!

Thanks,

Stu

[ianc]

Sounds like you've hitting the CIS pretty hard. Any chance it's ignition related? How are the plugs and wires? Timing spot on? Advance unit working properly?

ianc

[Mike Andrew]

WHEN YOU FIGURE IT OUT, PLEASE POST CAUSE. MANY OF US HAVE THE SAME PROBLEM.

[Dan Mc Intyre]

Hi Stu,

I have a similar situation with my car. Predominately in 2nd gear between about 2900 to 3300 at a "cruise" I get a burbling misfire. The car doesn't buck hard, but has this burble thing. If I let off ever so slightly it goes away and if I accelerate ever so slightly it goes away as well. I asked my mechanic about it and he says look to the fuel distributor.

I have no idea if that is the right answer or not. For the price of one of those things, I'll live with it for a while. I want to redo this engine eventually with an induction change so there is no real point to stick money in this thing now.

Good luck and please post what you eventually do to correct that issue.

Dan

[Paulporsche]

Dan, and possibly Stu,
Your cause may be a slightly too rich mixture. My engine was built and then installed by 2 extremely experienced pros, but I had some light throttle bucking. I cured it by nudging the mixture screw just a little bit leaner. This worked for years until my WUR finally went out of spec and had to be readjusted. I don't know if there was a relationship there because this was years ago and the whole CIS/control pressure thing was new to me. So I would check those pressures (Dan), check to make sure your sensor plate and plunger move freely, and check all ignition items.

[Blanco]

Originally posted by Mike Andrew
WHEN YOU FIGURE IT OUT, PLEASE POST CAUSE. MANY OF US HAVE THE SAME PROBLEM.

[Paulporsche]

Check Argeo's current post on coils and plug wires re his electrical problems.

[Dan Mc Intyre]

Hmmm, thanks Paul.

[GBStu]

Thanks for all your ideas......

Last night I richened up the mixture to a level that oscillated the idle rpm. Driving to work this morning the car was noticeable better.

Question.....how can I richen the mixture enough and NOT have oscillating idle? Perhaps I should try and lower warm pressure i.e. modify the pin or spring length in the warm up regulator?

Thanks, Stu

[Paulporsche]

Stu

Is your cold cp in spec? When do you get the misfire? Warm, cold, warming up?

Have you checked yourr wires, plugs, cap, rotor and distributor?

Are you sure your sensor plate and plunger are set correctly and moving freely?

[GBStu]

Paul,

The misfire occurs is greater when the car is warm and in hot weather I have checked the cold pressure and adjusted it within spec.

I have new magnecor wires, plugs and cap (distributor original)

The sensor plate seems to move freely....in fact I adjusted it anyway just to see if it made a difference (none).

Thanks......

[Paulporsche]

Since you have checked all the usuals...how about the coil and CD box? Do you have a Bosch CD? Have you checked/cleaned all connections?

Do you have a new fuel filter in there?

Are you sure the Magnecore wires are good and seating properly?

[Rondinone]

Here's a question: How many of you with this problem have replaced your fuel lines between the distributor and the injectors using aftermarket tubing?

I replaced mine last year using a method on this board and tubing (exactly the same part #) from McMaster-Carr. The new tubing is NOT sufficiently stiff, and at light throttle opening creates a pulsation in the fuel injector. That is, the injectors should open and spray continuously, but mine would click on and off proportionally to the throttle opening (and inversely proportionally to the length of the injector line!). At low throttle opening (cruising) the car had a classic lean miss. At higher throttle the pulsations become continuous and the car ran well.

I fixed the problem by reinstalling some old lines of a 77.

The original Porsche tubing on my car was 6X2mm. McMaster only sells 6X1mm. As far as I can tell 6X2mm line is not available anywhere.

[Purkinjefibre]

Hi GB, i am interested to know how you are setting the mixture. As far as i understand, this can't really be done acurately without an exhaust gas analyser. i.e. you need to set the CO2,
thoughts
Brett

[GBStu]

Rondinone.....you may be on to something there.

I replaced my injector lines with Nylon McMaster-Carr tubing, furthermore, on hot days (when the Nylon is less stiff) the car has a greater tendency to misfire.

I know what I'll be replacing tonight......

Thanks Rondinine

Stu

[Groesbeck Hurricane]

Another thought, what kind of plugs did you use (copper or platinum and/or anything about special plugs) and what gap did you adjust to? My "missing/bucking" went away when I replaced the permajunk, new wyres, scrapped the platinum plugs, and new cap/rotor with some cleaning of the visible connections in the distributor. I did several things at once as all needed to go so I do not know what the actual cause of my problems was.

[GBStu]

Brett,

I was setting the mixture by the "lift sensor plate" method.
See this thread:
CIS Idle Speed and Mixture Setting Without an Analyzer

David,

Just installed an MSD 6AL after my Permatune went south....If I recall I'm using NGL BP8ES plugs

Thanks for all your help

Stu

[Groesbeck Hurricane]

The NGK site recommends BPR5ES for the '79SC, but I don't know the differences. If I read this correctly, you have used Copper plugs, which is the preferred method. Did you replace the Bosch blue coil or are you still runing the same one in your car? Does this coil bench test as good?

I know I'm giving more rat holes, but I don't know any other suggestions than those listed above.

[Mysterytrain]

Regardless of what NGK/ Bosch recommends for a 78, you might want to re-think the plug. Porsche specified the hotter plug [W8DC] for the 78/79 sc with a lower 8.5:1 compression ratio and a cooler [W5DC ] plug for the 80/83 sc with a 9.3:1 compression ratio. I don't think this is your problem but its something else to look at. I realized that my 78 didn't fit the old spec when it evolved into 3.2 ss [9.5:1] and would be happier with cooler plug. Do you have the correct ignition distributor installed? Remember our cars have the vacuum advance when we come off idle and then the mechanical beyond that. I'm sure an 80/83 distributor would work but there would be a vacuum retard added to the picture. I recall my car running pretty crappy years ago when I forgot to reconnect the vacuum line for the distributor.

[pa911]

If the idle is oscillating than it's to rich.