persistent 914-6 engine miss
 

I have a persistent engine miss.

I decided to pull and then reinstall the ignition wire off each spark plug one at a time to see how the rpm of the engine changed.

For cylinder 1 though 5 the rpm decreased from 666 rpm 630 rpm when the spark plug wire was removed and went back to 666 rpm when the wire was reinstalled.

However, for cylinder number 6 the rpm increased from 666 rpm to 766 rpm when the spark plug was removed and went back to 666 rpm when the wire was reinstalled.

The condition for the number 6 cylinder seems impossible.

Is this a clue on how to solve my engine miss problem?

Lauran

Rpm
 

Your engine is possessed

Have you done a compression test to verify the cylinder doesn't have a mechanical problem (i.e. bent valve or something)?

Several questions: How old are the plugs and plug wires?
- How is the inside of the distributor cap look, I.E. oil. burn marks, ETC?
- As already noted how is a compression test results?
- When running are all 6 plugs getting the same voltage?
- Are plugs tight and clean, I.E. no oil or carbon tracks?
- When running are there any unusual noises when using a stethoscope?
- What sort of intake system do you have I.E. Webers and how is air/fuel reading looking? More importantly do you have an A/F meter installed and does it work?
- What are temps at each exhaust tube about 2 inches from the head, are all 6 the same?

For measuring voltage at the plug, Snap-On makes a voltage reader that you hook into the plug wire then onto the plug. Expensive BUT it can show where a problem could be.
Good Luck, John

I have found in the snap-on catalog a blue point tool called an "in-line Spark Tester part number YA840". Is this the correct tool? Lauran

I would suggest contacting a local Snap-On person in their truck and check it out in person since I can't remember the part number or such. You could also contact a race support shop to see what they use. The ignition system I had in my vintage race car had a Bosch distributor, points eliminated, 9mm wires with solid wire core I bought from Summit Racing, MSD box and all this stuff made 16.5K voltage at the spark plug at idle and 26K at 3000 RPM.
John

The compression test results are 125 psi #1, 110 psi #2, 115 psi #3, 115 psi #5, 115 psi #6.
The inside of the cap looks good and is new.
The resistance test results of the plug wires are 1,407 ohms #1; 1,407 ohms #2; 1,405 ohms #3;1,329 ohms #4; 1,446 ohms #5; 1,431 ohms #6.
The resistance test result of the coil wire is 0.5 ohms.
The spark plugs are new and tight.
There are no unusual noises when the engine is running.
The engine has factory webers.
I have not yet done an A/F test.
I have not yet done a temperature test of the each exhaust tube.

Lauran

Since the Six engine is an early 911T engine (basically), you might also ask on the early 911 forum. There are probably more people there with experience on this engine than here.

--DD

Check the ignition points.
Not a usual issue with CD ignition but I just experienced missing and poor running this past
weekend. Virtually no gap.....despite correct lube on the rubbing block.

The points have been converted to a Pertronix I system.

You might have a bad spark plug in #6 hole. Try swaping #6 plug with another cylinder and then repeat the cylinder drop test you did earlier and see if the rpm icrease goes with the spark plug or stays with #6.

I replaced the spark plug and the miss is still there.

I changed the spark plug wire and the situation is the same. I have ordered a genuine cap and rotor.

In addition to inspecting the ignition system, you should probably double-check the other basics. What's the compression like; is it consistent with the other cylinders? Are the valve clearances in spec? How's the fuel mixture, is there anything odd with that one throat of the carburetor? Do the spark plugs have a consistent color?

--DD

How are your leakdown #'s? valve clearances?

Leak down on number #6 was so high that the pressure only went to 50 psi rather than 100 psi on the pressure gauge as per the instructions. This was done at TDC. The valve adjusters were not adjusted loose to make sure that the intake and exhaust valves were closed as requested in the instructions.

When I checked the intake valve clearances are all too tight. I have not checked the exhaust valves.

I have ordered a Stomski Racing valve lash fixture and a Stewmac Digital indicator.

firing order correct?
Seems odd to have an increase in RPMs when you remove one cylinder.
It may be fighting.

rich

The firing order is correct.

The Stomski Racing valve lash fixture and the Stewmac digital Indicator worked great. I needed to cut more threads on some of the lower studs on the head to get within the range of the small digital gauge. Most of the valve settings were too tight. The valves had not been set since the year 1999. The car had been driven about 1,100 miles since then. I am also changing the oil and will be using Porsche brand 20w/50.

On May 17, 2023 I did an engine leak down (dry) after intake and exhaust valve clearances were set. The values are 96 psi and 6% for #1, 100 psi and 6% for #2, 100 psi and 2% for #3, 100 psi and 0% for #4, 100psi and 2% for #5, and 100 psi and 2% for #6.