persistent 914-6 engine miss

[r_towle]

so, you have changed the valve clearances, Is the skip still present?

Is the skip consistent, or is it random?
If consistent, its going to be either valves, piston rings (stuck) ignition or carb
If its random...I would start with carbs (clean, reset, test) then redo the electronics

I did notice you have checked everything but you have not mentioned the carbs.
Old cars, bad fuel, time sitting....carbs get gunky and act strangely.

[RSOHC]

I am waiting to get the muffler heat shield back from powder coating before I install the muffler and start the engine.

Lauran

[RSOHC]

I have set the lash of all the intake and exhaust valve clearances at 0.10 mm (cold) using a Stomski Racing valve lash fixture and a digital dial indicator. The new leak down values (dry) are now as follow:

96 psi and 6% for #1,
100 psi and 6% for #2,
100 psi and 2% for #3,
100 psi and 0% for #4,
100 psi and 2% for #5,
100 psi and 2% for #6.
When the engine is running there is still a persistent miss.
I can hear pops from both the tail pipe and the carb on the driver side of the engine.
When I pull the spark plug wire off of the # 6 spark plug, the rpm of the engine increases, the miss goes away, and the engine easily revs to 6,000 rpm. When I pull the spark plug boot off of the #6 spark plug and hold it near one of the nuts holding on the valve cover, I can hear a clicking sound associated with the arc from the boot to the nut. When I reinstall the boot on #6 and do the test on #5, I hear the same clicking sound. I have changed the #6 spark plug wire and I have changed the #6 spark plug and the miss is still occurring.

The spark plugs are NGKBP7ES with the gap set at 0.025 inches. The spark plug wires are BERU 0300.890.309.

At this point it seems to me that the valve lash is too tight when set at 0.10 mm and both the intake valve and the exhaust valve are not closing all the way. Should I set the lash for #6 at some valve looser than 0.10 mm so that the valves can close all the way?

[930cabman]

Valve spring binding or broken, I am reaching here but trying to help

[RSOHC]

For #6 only, one of the two valve springs (inner and outer outer) on each valve (intake and/or exhaust) being broken is a possibility and also weak valve springs for the intake valve and exhaust valve are a possibility.

[RSOHC]

As additional information, the records for the car show that the engine was rebuilt 36 years ago and that 24,000 miles have been driven since the engine was rebuilt. Based on this information, is it time to look at replacing all of the valve springs in the engine?

[John Rogers]

Next time you start the engine do it at night with NO lights on. Look to see there there are any glowing places on the plug wires or jumping arcs as it sounds like a problem with ignition.
John

[RSOHC]

I have ordered a full set of valve springs, a full set of keepers, intake and exhaust valve spring removal tools from Pelican, and an engine/transmission removal tool for use with a floor jack from Pelican. I have also ordered a valve spring tester from Speedway and calibration spring from CompCams.

[RSOHC]

I have added rocker arm shaft seals and rocker arm adjuster screws to my shopping list.

Lauran

[930cabman]

Keep us posted with your progress

[RSOHC]

I have removed and tested the intake valve spring for cylinder #6.
For the seat pressure tested at 40.2 psi which is 1.6 psi below the specification range.
For the full pressure it tested at 163.9 psi which is 3.6 psi below the specification range.
Therefore, this spring needs to be replaced.
I am expecting new genuine Porsche springs on Friday, July 21.
I will replace the #6 intake spring and then test the #6 exhaust spring.
It appears that this may be the solution to my problem.

Lauran

[930cabman]

Any chance they are getting bound?

A pound or two wouldn't seem to make a big difference on a street machine, but maybe

[RSOHC]

For the seat pressure, the reading should be 44 psi with a range of 41.8 psi to 446.2 psi.
My reading of 40.2 psi is 3.8 psi below the target value of 44 psi.

for full pressure, the reading should be 176.5 psi with a range of 167.5 psi to 185.3 psi.
My reading of 163.9 psi is 12.6 psi below the target value of 176.5 psi.

Lauran

[RSOHC]

I am in need of some ideas on how to install the keepers when using the Pelican Parts spring compressor. The windows in the tool is too small for my large fingers. Also gravity is not working in my favor. I have tried a magnet, a small flat tip screw driver, wheel bearing grease and vaseline. I am waiting for some automatic transmission assembly lube to be delivered.

I have also tried compressing the spring and installing zip ties to hold the spring in the compressed state. The spring is not compressed enough to install the keepers. Any ideas are welcome.

Lauran

[RSOHC]

What would I do if I had coil bind?
Would I change the valve spring?

Lauran

[Dave at Pelican Parts]

Remember that we have a 911 Engine Rebuilding forum here on this website. You may get more focused help with this type of question there.

911 Engine Rebuilding Forum - Pelican Parts Forums

--DD

[RSOHC]

I am posting on "911 Engine Rebuilding Forum - Pelican Parts Forums"

[rennzeit]

I'm surprised you did not mention any checking of the carb mixtures yet. No response about the color reading of the plugs or comparison between cylinders. No attempt to adjust idle mixture or check of air balance cylinder to cylinder. The carbs are far more likely to be out of adjustment or dirty/clogged jets than a valve spring a little low causing some pops at idle. I know you have the engine out now and are stuck trying to replace a spring keeper, but before you spend more moneyon parts you have to check air balance and the mixtures!

[RSOHC]

Hi Neil,

All of the intake and exhaust springs have been replaced.
Leakdown tests were done on the engine cold.
The numbers are as follow:

Cylinder #1 ******* 0 % leakdown at 100 psi
Cylinder #2******** 18% leakdown at 100 psi
Cylinder #3******** 12% leakdown at 97 psi
Cylinder #4 ******* 0% leakdown at 100 psi
Cylinder #5 ******* 14% leakdown at 100 psi
Cylinder #6 ******* 3% leakdown at 100 psi

The engine is still running rough with all six spark plug wires connected and smooth with #6 spark plug wire disconnected.
I am thinking that there may be a problem with the camshaft lobes for cylinder #6 or the possibility of a bent rod for cylinder #6.
What are your recommendations for next steps?

Thanks

Lauran

[911problems]

My thoughts are you have a lot of time and money spent replacing springs that are NOT the cause of a misfire at idle. (the spring pressure to keep a valve in its seat at low rpms is way less than the spec)
it is many ,many times more likely to be a fuel/spark issue than an internal mechanical issue.
If your cranking cylinder. pressures are even and high (which they are) it is not a cam timing or cam wear issue, so don't go down that rabbit hole.
my first thought is some type of crossfiring due to uneven resistances in the ignition. This explains the weird symptom of the engine smoothing out when no.6 is disconnected. double check all your connections and cap condition.replace coil.try another CD box.
The next step in logical trouble shooting would be remove, thoroughly clean and check the Webers. Make sure all the jets and internal parts are the same. make sure throttle shaft bushes are good. do lots of reading and understand the function of each circuit in the carb, so you are educated to pick up any little problem.
Next I would weld in bungs on each exhaust side and hook up Sensors so you can monitor your A/F ratios as you retune the engine with your nice freshly rebuilt carbs.
I really, really doubt its an internal mechanical issue, and if you go much further into the engine you are going to be looking at a full rebuild due to project creep.
best of luck, and keep us updated !