backfiring and popping at 4000 rpm 911S 1970

[Tomy]

Hi,


Just have problems with backfiring and popping. When have 1000-3000 rpm is let say OK, after 3500 rpm start backfiring and popping.
At all till let say 3500 rpm is going smooth after 3500-4000rpm start to runing not smooth with little jumping I checked distributor point and it was to small gap - I correct it and now backfiring and popping more!!
Should I check timing after gap correction? before it was correct(timing).
My 911S is converted to Weber and have Bosch CD. Fuel pressure under load (engine working) is 3 psi. Without load (only pump switch on) is 2.5 psi.
Engine is in pefect shape as well as carbs. Where should I look for problem?
What to do and in what sequence?

[Tomy]

any idea?

[Mo_Gearhead]

Anytime you adjust the point gap (dwell time will change) and thus you should check/reset the timing.

Does it backfire under load (acceleration) or on deceleration ...or both?

You state the engine is in "perfect shape" ...does that mean you KNOW for certain the spark plugs and/or wires are NOT causing the problem?

[Tomy]

mostly under deceleration but running not smoothly under load

[Tomy]

I checked timing again ---it was retarded so I set up it correctly and have no backfiring and popping!
But after 5000 rpm have no power! even cant come above 6000 rpm what is going wrong? I change spark plug today and will see tomorrow results but I think it something other .... any idea why I loose power and cant come up 6000 rpm?

[Tomy]

New Spark plugs installed. I fire up and start drive little popping but was cold.After some miles when temp. comes to right level it start jumping and hunting popping and loose power as well. I was on the way to my son school so I thought it could be trouble .But came back home take timing lamp some tools and going to work with idea to checked once more ignition, after maybe 5 min car start to work perfect! I made maybe 10 mils and dtop the car before work office. Strange I don't know what it was. Maybe should I get out wires and put new once?
I have new spark plugs, new rotor, distributor gap adjusted, timing set up.
I will drive today some and see what happen.

[911pcars]

Check fuel pump pressure and volume. What is the fuel pressure at 5000rpm? Volume should be about a quart/min. or so w/engine OFF. Eliminate all spark when you perform this test.

Engine usually pops when mixture is too lean. That could happen with a borderline fuel pump. Resist the urge to replace parts w/o diagnosing.

Sherwood

[Grady Clay]

Tommy,

We need to do a ‘basic tune up’ and some serious diagnostics.

First, exactly where are you located? What is the weather?
What is the history of the engine?
When was it last running properly?
What did the old sparkplugs look like? Were they all the same?

Where is the timing free-revving at 6000 rpm?
What is the points gap/dwell?
What type new sparkplugs did you install? What gap?
How new are the high voltage ignition components (cap, rotor, wires, plug connectors)?

Get some extra fuel hose and temporarily extend the fuel pressure test gauge so it can be read in the cockpit while driving. Be very careful that there are ABSOLUTELY no fuel leaks. What is the fuel pressure under load?

What is the color of the exhaust pipe?
Any smoke? If so, under what conditions?

Best,
Grady

PS: Did you get the MFI with the car?
G.

[Zeke]

I'll bet it's time for new plug wires. I had the exact same problem and this fixed it.

[Tomy]

Sorry for one day delay.
First will answer to Grady:

I am in Europe and we have spring here but I drive only when is sunny.
/except yesterday but this I will write later.

engine is rebuild last year and in perfect shape at all

It was running perfect till this spring

all spark plugs was the same with black contamination (no wet at all but black carbonated) -was Bosch Iridium

now I put new NGK plugs with gap at 0.6 mm

timing correct at 6000 rpm at 35 degree

Points set at 0.3 mm

I install new rotor and plugs but still have old wire and cap and plug connector

Exhaust pipe is black but car is no smoking at all.

So I think that it could be a wires. See Like I write before I loose power but come home take some tools and start going work it was running let say perfect.
Yesterday was perfect sunshine till 2 o'clock after that start raining a lot. I never drive in rain with my Porsche but I like come back home so I start. After 15 maybe 20 minutes I loose power again witj popping again Just mile before home car stopped. I pull and shake wires and like to fire up - so it fire and working better It looks that it is wires but maybe not only wires ?

[Zeke]

Try running the engine at night so you can see if the spark jumps out of the wires.

[Grady Clay]

Tommy,

I recommend you replace the cap, wires and plug connectors. You can buy the wires & plug connectors from our host as an assembly. Normal Beru or equivalent will be just fine.

Carefully inspect the rotor for any sign of arcing or damage. Check the rev-limit spring. If you suspect the rotor, there is an inexpensive VW rotor (no rev-limiter) that you can use to test with. Please don’t be tempted to leave the test rotor in. The engine the rev-limiter will save is yours.

Your engine should use NGK BP8ES. Once running correctly, BP9ES might be appropriate for sustained high speed driving in hot weather.
Check that the points are still at 0.3 mm.
Check that no distributor cam grease got on the points.
I would be a little conservative on the ignition timing. Perhaps 32º - 33º might be a starting point. What octane fuel do you have available? Remember, this engine has 9.8:1 compression ratio.

The rain provided a good clue. Hopefully you have a de-humidified garage so the 911 can thoroughly dry.

Best,
Grady

[Tomy]

First thanks a lot Grady and Milt too.
My spark plugs are NGK BPR7EIX I have also new BPR9EVX in my garage.
Here we have too choose 3 kind of octane petrol :94,98 and 100 (shell V-power)
I use to use 98 octane petrol.

[Tomy]

I like to say f...k.

I just replace wires and again the same I loose power...no popping but I have problem to going higher rpm and fill that something is wrong like -30% power under load. What is going on?

[Richard Burns]

I had a similar problem which turned out to be a fault in my distributor. The part that holds the rotor was not moving freely on the drive shaft as it had not been lubricated. After rebiulding the distributor the problem disapeared.If you decide to do this look out for the circlip that holds the two together it is under the rotor and can be a pain to get out and is easily lost.
Originally it felt as though I was hitting the rev limiter at about 5200 then 4500 and was getting progressively worse.

[pozee]

Tomy,

Do yourself a favor and look inside your gas tank.

What do you see?

Any residue that does not belong? How does your fuel filter look?

[t6dpilot]

I am struggling through a very similiar problem and am having no success. My 2.2S motor consistently cuts out at 5800 and has done it since I bought it about a year and a half ago. I have subsequently gone completely through the ignition system replacing plugs, wires, rotor, cap, points. I reset the dwell and timing to correct specs. Mechanical advance on the distributor is working fine. I verified that the problem is not with the CDI box or coil by installing known working units - problem is still there. I also checked the fuel delivery volume and it is about a liter in 40 seconds (if I remember correctly). Fuel pressure checked and reset to 3.3 psi. No debris in fuel tank or carburator bowls. Verified main jet, emulsion tube, idle jet, and venturi sizes - all correct for 40IDA carbs on a 2.2L motor. I STILL HAVE THIS PROBLEM!

Recently did a leakdown and all cylinders are great 3%. The next step is to swap my 71T Zeniths for the Webers to see if the problem still recurrs. If so, then maybe cam timing? Beyond that I am baffled too. By the way, the motor is a pretty fresh rebuild by a reputable shop in SoCal and has maybe 10k-12k miles on it. I will be very interested to see what you find.

[mobius911]

Hang in there, you'll get it fixed.

You should definitely follow the previous recommendations. I've also had the following affect high rev performance:

1. Distributor advance not correct due to aging and wear (you can use a timing light with an advance dial to check this through the rev range, and build an advance curve)
2. Original Bosch CDI failing to deliver spark or cutting out intermittently at higher temps and revs (swap in a known good one). The symptons were popping and backfiring and cutting out.
3. Rev-limited rotor cutting out too early (replaced with new). Same symptoms as #2.

I just had my distributor rebuilt, and even though I thought it was ok before, there is a definite improvement.

Good luck!

[mhackney]

I have a 70S coupe also and it took a couple of years to sort it. Similar problems as yours. I actually think as I fixed on e problem, a new problem would be exposed somewhere else. I did end up getting rid of the Pertronix after market CDI and replaced it with a proper Bosch CDI and coil. That was the source of the biggest issue. However, I was still having a lot of popping and backfiring a few months later. Long story short, see attached photos!





That is a dead and dehydrated mouse plugging the tube that heats the MFI thermostat. Removed the mouse, no more backfires!

[boxster03]

maybe microswitch not adj. correctly.