A little help needed with high speed mis-fire

[1QuickS]

Pretty sure it is electrical but a little hand holding would be good.

Engine is:

• 2.3 with S-cams
• twin plug with Bosch coils
• twin Permatunes
• upper and lower banks independently switchable
• points in good condition with 0.016" gap
• distributor advances freely
• plug wires ten years old but low mileage and car is garage queen

Symptoms:

• revs fine to 7200 in first at WOT
• second gear begins stutter at 6800 RPM and tach drops out
• third the stutter occurs at 6200 RPM and tach drops out

Battery shows 12.5 volts when parked and when engine runs voltage drops to 12.25.

I haven't checked timing lately, plugs are less than 500 miles and reasonably good for mixture.

I have a spare, unused Permatune to swap out. All three of these are 12 years old but again, low mileage, garage queen.

Is it possible that both coils can be fired by one CD? Perhaps I could serially try the three CDs firing both coils to isolate a CD issue.

Do my symptoms suggest that as more power is demanded at lower RPM in higher gears that the power of the spark is weak??? I would think my twin ignition system would rule this issue out, perhaps one CD would be weak but the other would cover that problem.

Tach is tied to only one CD so perhaps that CD is faulty and delivers an erratic tach response at the time the stutter starts.

This has been in place for awhile but I haven't been driving car due to ongoing efforts that kept car off the roads. I don't recall if there is an event that triggered this issue.

High speed stutter would indicate lean but I doubt this and then there is the tach response that indicates electrical issue.

Any suggestions as to probable suspicious item to check first? Perhaps timing check at 6K first?

Thanks!

[Flieger]

I would change the wires first. 10 years in a garage is enough to deteriorate elastomers. Battery voltage sounds a tad low too.

[1QuickS]

I was eye-ing new wires this weekend. Thinking about making my own set like I did before.

Battery is low, I just put an overnight charge to it since it wasn't holding a good charge...this is unusual. Battery is just a year or so old (DieHard) and wires, connections are really clean, tight.

Also at night with lights on and revs low the alternator light is dim.

[Harvey Weidman]

Paul, did you do your single plug at a time method?
Can you wire the tach to the other CD?
Sure sounds like a CD....wait till the smart guys respond...
Although the dim battery light is a warning of maybe a weak cell?
Or low alt output....mine measures about 14 when revved....
H

[1QuickS]

I've been reading about high speed issues on Pelican and most seem to go to voltage regulator/alternator issues or poor connection integrity. The low voltage at the battery has me concerned since it seems it should be in the 13.5 volt range when engine is running. I didn't check voltage while revving...you might hear me when I do it since you are only 30 miles away and my open exhaust can probably be heard by you when I hit 6K.

[edmayo]

Paul, if the voltage is dropping out due to charging problem that would sure seem to me to cause a problem at the CD box (boxes) Once above idle your charging voltage should be in the 13.5 range minimum, as Harvey said. I would sure correct that before going any further.

[1QuickS]

Thank you Ed. I'll check voltage first.

[kent olsen]

Paul, Check all the grounds also. Electricity won't flow unless you have good grounds and they are the easiest to check.

[kent olsen]

Paul it does sound electrical but just another experience I've had. Years ago I had a similar problem with a small block chevy in my 65 corvette, 1st gear good to redline then each progressive gear would misfire at a lower rpm. It turned out to be the fuel pump. I replaced the mechanical pump with a Holley and that's all it needed.

[dho]

Paul, I had the same symptoms you and Kent describe on a 5.0 Mustang. I was convinced it was fuel related and went through the entire fuel system. The issue was actually a dying coil - it couldn't supply the spark needed for the demand at higher rpms. Replaced the coil and the problem was solved. Do you have any way to confirm the issue is not fuel related? Or, conversely, to measure the spark under load?

dho