CIS - Running Rough at Low RPMs

[jhladousa]

I've been fighting a problem all summer and needs some input on next steps. My car has a tendency to buck slightly at low RPMs, especially from take off in 1st, but is noticeable in any gear when running RPMs lower than 3K. It disappears at anything above 3K.

The car sat for many years prior to my acquiring it last summer. I have had many things done to the car this summer in order to baseline it as well as attempt to address the running issues without success. It is not a daily driver but is driven 3-4 days a week (about 3500 miles since April).

- Major Tune Up (Spring) Including
- Oil change
- Transmission Flush
- Brake Fluid Flush
- Valve Adjustment
- Cap & Rotor
- Pertronix Points Conversion
- New Plugs
- Plug Wires
- New Injectors
- Air Filter
- Fuel Filter

- Replaced Fuel Accumulator

- Replaced Ignition Coil

- Adjusted mixture to recommended settings

- During clutch replacement
- addressed all vacuum leaks
- Replaced injector sleeves and 0-rings
- Replaced all intake manifold sleeves & gaskets
- miscellaneous engine seals

- Swapped out CDI box with a known good Bosch, no change

I'm not sure what to pursue next....any input would be greatly appreciated!

Thanks!

[LakeCleElum]

I just solved this same problem with my '73.5 this morning.

Even though you have new injectors, my guess is that something has worked it's way thur the system and is partially blocking the fuel from being atomized. You're probably still getting fuel thru, it's just a bad pattern and doesn't burn well below 3,000 RPM (my exact problem). I isolated the cylinder by seeing which plug was darker than the others. (Remember, it's getting fuel, it just doesn't burn at low speeds).

My bad injector was #5. I compared the spray pattern between #4 and #5 and immediately saw #5 was bad. Although I plan to have my injectors professionally cleaned this winter, I was able to clean this one myself and my problem has cleared up.

This occurred even though I have recently replaced the fuel filter. I plan to drain and clean my tank this winter. You may consider this also if the car has sit a lot....G'luck...

[kach22i]

You have a fairly complete list there, amybe that's why no one has ventured to guess.

I will say this; there are shops who have not done the timing they have contracted to do, or they have done it wrong.

Sometimes even a new cap can have a hairline crack in it, undectectable to the naked eye.

Some people have changed out thier WUR and AAR to no good effect - could be a dead end, something to research.

I guess you know about the spraying starting fluid around your airbox and on hose lines to search for leaks. You must have a firee extinguisher at your side, "search" this topic up to find out why.

What year is this?

Do you still have points?

How much play is in your distriubtor?

My car threatens to buck at low speed, low rpm.............nothing as bad as when the timing was off. She starts right up and stays running now, used to stall when cold before and buck like a horse with a stone in it's shoe.

I found that slightly increasing my spark plug gap from published numbers helped, I was getting little back fires before and running rich at idle.

Without fixing the root cause you are going to put out of wack other related systems to make up for it. Not the best way, start with one topic and work your way though.

Good luck.

[jhladousa]

Thanks for the input – I’ll check the things mentioned.

Cap is new, and the issue was present before replacing it. I could probably find another one and swap it out just to check.

The points have been replaced with the Pertronix, but only after replacing the original points with new ones first. There was no change, so the Pertronix conversion was done as well.

Car is a 1975 911s. Original 2.7 that was rebuilt approximately 13,000 miles ago.

I’ll check out the plugs, injectors, and research the WUR and AAR. Another thought was the fuel distributor – any thoughts?

[LakeCleElum]

I think the injectors will get dirty before the FD gets plugged up. Injectors are much easier to get to also.

[ruf-porsche]

Get a set of cis gauge and see what the pressure are? You can't trouble shoot the problem until you know the base line.

[LakeCleElum]

Originally posted by ruf-porsche
Get a set of cis gauge and see what the pressure are? You can't trouble shoot the problem until you know the base line.

Unless you're checking the spray pattern of the injectors.

[ruf-porsche]

Originally posted by LakeCleElum
Unless you're checking the spray pattern of the injectors.

He installed new injectors in the engine

[LakeCleElum]

Only be be blocked by debris in a fuel system that has been sitting......Been there.....Just today in fact...

[por73911]

I cleaned the screen in the fuel tank and it made a WORLD of difference in the way my 911 ran. It had been sitting for years also.

[jhladousa]

I was also thinking that the tank could use a good cleaning. The car was only driven a couple times per year so when I got it the gas was pretty old. I'll check that screen out and see - perhaps look again at the fuel filter and see how gunked up it is.

I am also planning on getting some baseline readings from the pressure gaugues next week.

[stlrj]

Come to think of it, I did solve the bucking problem on my 78 3.0 when I was going back and forth with my project EFI install and CIS. At the time I had upgraded my ignition system with an HEI transistorized ignition module in place of the CD ignition, HEI wires with my plugs gapped at 0.060" so when I put the CIS back on, it never bucked at low rpms and I never gave it much thought.

I always knew the Porsche ignition system was a problem but never realized how bad it would make CIS look.


Cheers,

Joe
74 911 w/86 3.2

[kach22i]

Originally posted by stlrj
upgraded my ignition system with an HEI transistorized ignition module in place of the CD ignition, HEI wires with my plugs gapped at 0.060" so when I put the CIS back on, it never bucked at low rpms and I never gave it much thought.

That's a huge gap, I want that gap!

Can you install any of the above parts with CIS?

[Gunter]

Originally posted by jhladousa
I was also thinking that the tank could use a good cleaning. The car was only driven a couple times per year so when I got it the gas was pretty old. I'll check that screen out and see - perhaps look again at the fuel filter and see how gunked up it is.

I am also planning on getting some baseline readings from the pressure gaugues next week.

Greetings:
The tank would be the first thing to clean/inspect if a car has been sitting for years.
Unfortunately, many cars are stored with the tank only partially full and/or without Stabil.
Then, when starting up, the pump gets all the crud first, resulting in damage to the pump.
After cleaning the tank, the lines to the accumulator/filter have to be blown out.
Check/clean the tank, bottom tank-filter, lines.
Take out the new accumulator, turn it upside down and look for debris.

[Moke81]

With engine running lift up slightly on the sensor plate lever inside air cleaner. If it smooths out it might be that your sensor plate is out of alignment or dirty. My 81 was running like that at idel and the sensor plate was sticking from being dirty and was not closing right.

[al lkosmal]

All great suggestions, but don't neglect to check the ignition connections. I just went through this routine. I had a slight popping at idle, a bit of bucking at low RPMs. nice and smooth up to 4KRPM's and then misses like crazy. I adjusted my WUR, checked the spray pattern of my injectors, got out the Gunsen Gastester and checked/set the A/F ratio. As I was checking out the spark plug wires, I accidentily knocked the coil + wire off the coil. When I went to put it back on it was very loose. It's a flag terminal so I was going to squeeze it closed for better contact, when I noticed that it was burned through the side of the terminal. Must have been loose enough to have a gap that was arcing. The negative was loose too, so i replaced both connections with ring lugs. Runs sweet, actually better than before with all the misc. tune up stuff I ended up doing, as well.