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What can cause an intermittent misfire at any RPM which

appears to increase the RPM...
What can cause an intermittent misfire at any RPM which appears to increase the RPM slightly? Have checked compression, leakdown, changed plugs, ohm'd wires, distributor cap, points and condenser are relatively new. Removed spark plug wires one by one without change in intermittency. Has good spark also. FYI: the engine is an Oldsmobile 455cid with 75k miles.
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Answered in 1 minute by:
1/7/2018
rpmdiag
rpmdiag, Tech and Instructor
Category: Classic Car
Satisfied Customers: 9,114
Experience: Since 1975
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Hi, I'm Ron and I will be helping you today.
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The ignition coil can be breaking down too. Also, keep in mind that an ohm test and a load test isn't the same. The wires can still have issues. A scope will show you the truth.

Please understand that I don't know your skill level or tools
you have to assist us. Also understand that I can't see, smell,
hear, or touch the vehicle either, so your patience is
appreciated. JA is a great Q/A site, but it does not replace a
repair shop. So please don’t shoot the messenger if I have to
deliver bad news:)
If you are satisfied with my answer, I would greatly
appreciate a 5 star rating now before you leave the site and I
will still assist you after you have rated me too if another
question pops up. I don’t get credit for helping you until
you rate me

Thanks, Ron

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Did you have any more questions for me at this time?

Thanks again, Ron

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Customer reply replied 7 months ago
Hello Ron,
I am not a newby, neither a pro. Just an old school shade tree mechanic and engine builder whom has not worked on an old school engine in many years. So long gone are the timing lights, dwell meters and o'scopes. I am however very familiar with Electrical characteristics of components as a retired Electrical Engineer.
As my description of the problem stated, the RPM glitch didn't go away when removing the plug wires. Also it is interesting the RPM jumps up 50 to 100 RPM and is variable.

Can still be wires ( they're not new or a coil. Coil is most likely.

Thanks again and please remember to rate me 5 stars now
and keep this link in case you need me again later on.
I don’t get credit for assisting you until you rate me

Ron

rpmdiag
rpmdiag, Tech and Instructor
Category: Classic Car
Satisfied Customers: 9,114
Experience: Since 1975
Verified
rpmdiag and 87 other Classic Car Specialists are ready to help you
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Customer reply replied 7 months ago
Right on! I'll replace the coil and wires. Hopefully, one is the cause.

Okay, keep me posted.

Thanks again, Ron

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