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Misfire with Jumpy Tach
This has me stumped..
They key is the jumping tach, seems like a primary ignition problem. Under load the car misses. Seems to be at its smoothest while at 3k, but once back under load it misses again. The tach jumps 1-2k either way. Wiggled all grounds at idle, no change. The cap has had pitted terminals for a while now, but not sue that would bounce the tach. If the tach just flucuated a little to match the drop in RPM i'd suspect fuel as well, but it jumps up as well as down. Thoughts?
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60+ views, must have others stumped as well.
I figured the culprit would be the coil, dist, or bad ground. The issue is Mallory was bought out by MSD, direct replacement parts are not available. |
Jumpy tach sounds like an alternator problem (I tried to view earlier and it never opened).
I personally would check the battery and the Alternator, sounds like a voltage issue. |
It sure sounds like you have an intermittant loss of spark problem. That could account for the tach needle fluctuations too. As you said, it could be the coil, distributor cap or your distributor pick-up. I had a a Mallory uni-light many years ago and can remember having to wipe off the distributor's pick-up light every now and then due to ozone deposits. It would cause it to miss if the lens got cloudy.
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It's strange that at idle the tach jumps all over the place, sometimes as high as 4-4.5k rpm, all while just idling. You can see/feel the misfire at idle as well. Very strange. I'm at a point where i'm either going to build a 347 stroker or do a 351 stroker so unfortunaltly i can't replace the distributer until i know what kind of build i'm doing. I need a quick bandaid. |
I changed out the coil with an MSD Blaster SS and went through every ground wire, still having the same issue.
As mentioned earlier, i'm replacing the motor with a 392 so i'd prefer not to throw $250 into a new distributor if i don't have to. But i have 2-3 more races on this motor before i'm ready for the swap. Would bad plug wires cause the tach to jump? Any additional thoughts? |
I would disconnect the tachometer, and observe engine behaviour.
A fault tach can cause all of this. Since it is connected to the coil negative, any internal short to ground is like a random ignition cut off. |
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I appreciate the suggestions! |
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Sounds like it must be the distributor. What else can make the tach jump that much? What wires have you got? I would run spiral wound plug wire, and resistor plugs. Gary |
Alternator is a possibility, it happened to me
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Does your Mallory distributor use an optical module? Mine does and I blew out three of them in rapid succession. There is a device that installs between the coil and the distributor that is said to "clamp" (Mallory's term) voltage spikes...once I installed one things started working again, but it was a lengthy investigation to solve my problem.
Hope you get it fixed!! Dughy :cool: |
So I ordered new 9mm Ford Motorsport wires, that wasn't it. I pulled the valve covers, everything looked good. All the plugs looked good, compression was 140-170 on all calendars. Still misfiring and popping as I slowly rev it.
I hate to replace the $553 Mallory distributor if that's not it. Anything else? |
Did you look to see if the center contact on the rotor is making good contact with the cap center electrode?
Also, somewhere there was a thread on distributor phasing.... Ground and power connectors/wires all in good condition. If you source the ignition key-on power from the ignition switch, perhaps try a jumper and temporarily power it from a good known source. Ignition switch could be on the fritz... Easy enough to do a resistance test on the wires... |
Did you check the mechanicals on the distributor? Is the drive gear slipping? Are the bearings shot? Is the distributor a tight fit?
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Thanks for the suggestions! |
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