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Got a skip coming from the back three... Conclusion, maybe
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Author:  wjajr [ Sun Apr 01, 2012 1:19 pm ]
Post subject:  Got a skip coming from the back three... Conclusion, maybe
My engine has developed an intermittent skip or misfire from the last three cylinders. Engine has headers and duel exhaust to the rear bumper, and I can detect a difference between the pulse / sound of left and right pipe.

Knowing that the last three are giving a problem narrows the search, but I’m still having trouble detecting which cylinder is offending. After pulling and examining plug numbers 4, 5, and 6, there did not appear to be any significant coloring differences between them, all showed very good mixture & combustion. If I had to guess, perhaps #5’s terminus of its threads exposed to combustion may have been a wee bit darker.

Next I tried pulling plug wires off of each plug one at a time, the boot on #4 and 6 leak so that portion of the project was immediately abandoned. Number five allowed boot pull back until spark would no longer jump, and made some difference in exhaust sound. The coil is Blaster 2 with orange box, and blaster 2 ballast. Plug wires are 7mm high temperature, most of them do not tightly latch onto the Ngk plug, won’t easily pull off, but will slide in and out a millimeter or two.

There has not been a compression test performed on the engine in a few thousand miles, and lash was reset cold last month to 0.016 and 0.021 if I recall correctly. (I don’t have my notes on that adjustment handy). I don’t have the equipment to perform a leak down test.
Possible causes I should think are; a burnt or maladjusted valve, or bad plug wire. Plug wires were installed on car when purchase February ‘08, they look like new, and are grease free.

Tomorrow I will attempt to check lash hot with engine running. Loping low rpm makes this process difficult, and oil often gets flung all over. Now I’m just whining.

Any other ideas on how to detect which cylinder is giving problems is welcome.

Bill
Author:  Aggressive Ted [ Sun Apr 01, 2012 2:44 pm ]
Post subject: 
Bill,

Hopefully not an intake leak, but you might try spraying around them to see.
Author:  olafla [ Sun Apr 01, 2012 9:37 pm ]
Post subject: 
You may switch the rear 3 spark plug wires (and later the plugs) to the front 3, and vice versa, and see if the misfiring moves to the other ex pipe.
Author:  wjajr [ Mon Apr 02, 2012 1:39 pm ]
Post subject: 
Today I was able to run through a few tests, and check valve lash on old Skippy.

Five compression strokes of engine yielded 150 psi on five cylinders except number five came in at 145 psi.

Number five required six compression strokes until gage stopped moving, and number six needed seven compression strokes.

I reset the lash to 0.016â€￾ intake, and 0.021â€￾ exhaust. For some reason some of the lash setting from a few weeks ago moved a few thousands. I don’t know if this is a result of my inability to operate a feeler gage, or the damn things are self-adjusting. Some of the adjustment screws are not as hard to move as others, and are perhaps backing off...

As Olaf suggested, I swapped the back three plug wires one cylinder at a time with their front brothers, no different effect was produced. I did use insulated pliers, and removed a previously loosened boot on number 4 to 6 to see if that made a difference in the cadence. Number six showed the least difference in the skip, so I’m now suspecting number six as the problem area.

The wires leak from the conductor end of the boot when wire is removed from the plug, and located near a ground. Perhaps it is time to upgrade to a better wire set. I can’t locate notes on what high grade wire set Dan recommended to fit NGK plugs, and stand up to high voltage.

All six spark plugs, gaped at 0.035 inches, showed as very good with a light tan/gray grounding electrode, off white porcelain with a small tan spot on one side, and terminus of threads a flat black coating that won’t transfer to one finger when rubbed.

Previously these NGK UR5 plugs have been very clean and white, they have only been installed for 1983 miles, and just recently after rejecting down to 54, installing 4.5 power valve, and driving perhaps 120 miles this spring shown any color.

I am suspecting that for the fifteen miles I had 51 jets installed the car was a lean stumbling mess, number six may have been too lean.

Ted, no vacuum leaks found at intake gasket.

Any other tests I can do before pulling the head to examine the valves and seats?
Author:  SlantSixDan [ Mon Apr 02, 2012 1:57 pm ]
Post subject: 
Notes are here and here. Call 'em up and tell 'em about your spark plugs, etc.

Sounds like your engine's in fundamentally good shape, though I'm curious what the compression reading was on the first, second, and third compression (the engine was warmed up and the choke and throttle were fully open and all six plugs were removed during the compression test, right?).

Also sounds like your plug wires are long past due, which makes me wonder about your cap and rotor. Could be an arc path, visible or invisible, internal or external.
Author:  Slanted Opinion [ Mon Apr 02, 2012 3:23 pm ]
Post subject: 
Just for fun, try running it at night, with the lights off, and letting your eyes adjust to the dark. Take a good look at the wires, cap, etc. sometimes, but not always, you can spot the telltale blue glow of a leaky wire, or boot, or cap.

- Mac
Author:  emsvitil [ Mon Apr 02, 2012 4:50 pm ]
Post subject: 
Is the pipe plug installed on the #6 runner or is it going to the brake booster (possible brake booster leak if it's connected)?
Author:  SlantSixDan [ Mon Apr 02, 2012 5:13 pm ]
Post subject: 
Quote:
Just for fun, try running it at night, with the lights off, and letting your eyes adjust to the dark.
…armed with a trigger sprayer full of water. Mist the water over the plug wires and distributor; see if you can cause lightning and misfire.
Author:  wjajr [ Tue Apr 03, 2012 4:50 am ]
Post subject: 
Dan:
Quote:
though I'm curious what the compression reading was on the first, second, and third compression
I didn’t read gage at each compression stroke. Perhaps I should get a remote starter button so I can read gage as engine roll over. Remember the cam is far from stock with a ton of overlap.
Quote:
the engine was warmed up and the choke and throttle were fully open and all six plugs were removed during the compression test
Yup

This morning it suddenly occurred to my atrophied brain housed safely within its thick scull to use the compression tester’s threaded hose to connect compressor to combustion chamber, and listen for air escaping once both valves have closed.

I’ll try filling combustion chamber with air later today once back home.
Quote:
Also sounds like your plug wires are long past due
Agreed.
Quote:
Could be an arc path, visible or invisible, internal or external.
I looked for tracks, saw none, but last winter while distributor was removed from car during engine compartment beautification project, I cleaned cap & wires to look like new.

The present cap has aluminum contacts, which I had removed the usual deposit build-up, and the little screw off cap on top. I have no idea how long wires and cap have been in service. That said, along each wire there is printed; 7mm, high temperature, 96. Perhaps these are historical parts from the last century…

Engine is equipped with peanut head. Dose Magnecor make up the full set with boots, or is it up to me to source boots from Lectric Limited, and assemble wires & boots? Any particular person I should talk to at Magnecor, or just the service desk folks?
Quote:
I would suspect your motor mounts have sagged severely if you're getting plug wires touching the inner fender
Would someone have a measurement between inner fender and say top of head to check for sag? Rubber section of mount has a crack parallel to and next to its bottom where attached to K member. I had never noticed this before.
Author:  wjajr [ Tue Apr 03, 2012 4:53 am ]
Post subject: 
Ed:
Quote:
Is the pipe plug installed on the #6 runner or is it going to the brake booster (possible brake booster leak if it's connected)?
No drilled bung in #6 runner.
Author:  SlantSixDan [ Tue Apr 03, 2012 8:43 am ]
Post subject: 
peanut-plug head is much easier to get plug boots for; Magnecor has lots of varieties in stock and no outsourcing will be necessary. Get rid of the low-quality distributor cap and put in a good one instead. I suspect once you go through and replace the maintenance parts in the secondary side of the ignition your miss might go away.
Author:  wjajr [ Tue Apr 03, 2012 10:52 am ]
Post subject: 
Dan:
Quote:
I suspect once you go through and replace the maintenance parts in the secondary side of the ignition your miss might go away.
That would be damn nice.
Author:  wjajr [ Tue Apr 03, 2012 12:28 pm ]
Post subject: 
I just completed pressurizing cylinders #4 through #6 with 90 psi air, and not one little hiss could be detected from carburetor or tail pipe; I’m a much happier guy now!!!


That leaves secondary electrical components being the cause of skip as Dan suspected, and that stuff is on its way.


I’ll update after new parts are installed.
Author:  wjajr [ Tue Apr 10, 2012 2:23 pm ]
Post subject:  Up Date
New Magnecor plug wires were delivered today joining new cap and rotor, and I installed them in the Dart. Still have a skip, but now I know it is from number six.

This leaves two possibilities causing this skip: a defective spark plug; or vacuum leak around number six intake manifold leg at head. I replaced the plug, result was no change.

After squirting copious amounts of water around number six intake runner, I still can’t tell if there is a vacuum leak at that junction. Short of removing, and replacing intake gasket is there any other possible causes I’m missing here?

Bill
Author:  SlantSixDan [ Tue Apr 10, 2012 2:43 pm ]
Post subject: 
Got any vacuum lines tapped in at the #6 intake runner?
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