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FIXED. Possible misfire and Tach swings widely at ~2700 RPM


wheelieking
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UPDATE: After scraping the apparent corrosion from the rotor tip and the inside of the distributor cap, I think most of the problem is gone.  I think the new cap/rotor/wires and coil I order will go a long way toward eliminating it completely.  Thanks!!

 

I just picked up a red '76 from a friend, so I'm trying to work out some initial bugs.  A used engine was swapped in by Midnight Motorsports here in Seattle a few years ago, and then it sat for relatively long periods because she was nervous to drive it.  I went for an extended drive last weekend and have put two tanks of gas through it and had no issues, however yesterday and today I've had an interesting thing crop up.  Once every 10 minutes or so, at a mild acceleration (most of the time going up a hill), it seems to misfire at seemingly the same RPM (~2700RPM).  On an onramp it happened at 4000 though. I was thinking it might be a vacuum advance problem, so I ordered a vacuum gauge and plan to test it tomorrow.  The misfire isn't just noise though, the whole car hesitates severely but briefly, and there isn't really a "bang" but its more like the sounds of a cylinder without the spark plug in it...  The other weird thing is that the tach needle swings around wildly at the same moment.  

 

I know this could end up being a number of issues, but would appreciate any insight here.  Thanks!

 

 

Edited by wheelieking
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Thanks!  I started pulling plug and coil wires and to this day the way the plug wires attach to the plugs baffles me but they seem ok.  I'm going to order a new cap, rotor, wires and coil though just to have things new and fresh.  Also, there is an electronic points unit in the distributor, but it's not clear that its a Pertronix brand unit.  Do those things fail?  I'll try to get a better picture tomorrow during the day though. 

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Hold your horses on the other stuff.  These things do fail.  I.E used to sell a new dizzy until the electronic points started to fail  It kinda looks like an off-brand set.  I use Pertronix, I think that there is another brand like Flame... something.  By your description, it sounds like the electronic points are your problem. There was someone else just a short time ago with the same symptoms,  but it was an I.E dizzy.

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And don't forget the spark plugs.  Even new ones have been known to be bad right out of the box.  A misfire under load can be a bad plug...

 

Also see if you can duplicate the situation at night--or at least run the engine in the dark, and observe the spark plug wires to see if you have a spark jumping from a wire to ground.  Old plug wires and "leak" high voltage electricity and weaken--or totally kill--the spark at the plug, as the path to ground via a defective wire is much easier than via the plug gap under pressure inside the cylinder. 

 

mike

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You guys are awesome, I'll focus on the ignition side of things.  Inside the distributor cap, the contacts have a little greenish corrosion (copper?), but the ends of the plug wires all look super clean.  As a side note, I drove it this morning and had a few instances of the misfire/backfire on the first real acceleration, nothing while driving steady.  A one point when it happened, it sounded like a short punchy "sneeze", and when I looked in the mirror I think I saw a very faint puff of brown/dark exhaust.  

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