Misfiring all of a Sudden

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Mako21

Mako21

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1976 slant six 225/904 all stock.

About two weeks ago I had to take a trip on the highway at around 60 mph for 30 minutes. Once off the highway at the stop light she began running rough at idle. Never shut off on me but a distinct hiccup and I can distinctly hear it popping through the exhaust when idling. It happens mostly in Drive or Reverse, not so present in Park on Neutral.

It’s been like this for two weeks as I use my car almost daily and haven’t had a day off to look into it, because once it’s driving it’s just fine.

Tonight I went to check the timing and to my surprise I can’t check the timing. Number one cylinder is firing so erratic that I can’t get a steady strobe stream off the light. Number one cylinder is misfiring bad.

I did check the strobe output on all other cylinders and they were a steady flash stream.

New wires, cap, rotor and it still does it. So I changed the spark plug and it’s a little better but still popping and misfiring.

Should I re-lash the valves? I did them about two months ago and had no problems till my highway trip.

Any and all suggestions appreciated.
 
Pop in the exhaust is a lean condition normally. Maybe check the fuel filter and see if it has trash in it that's gotten into the carburetor.
 
Make sure you are not getting reluctor tip contact on #1 tip.
 
Try timing it on no6. Check the wire and plug or switch them with another cylinder.
 
You burned a exhaust valve with our new gas from Russia. Did you know their lowest octane over there is 92 and their mid grade is 95. And no ethanol added. Our government is screwing our engines up intentionally. Forcing the extermination of the combustion engines. All I can tell you all is run your engine on the rich side or dig its grave. Klotz leaded octane boost 2 bottles per tank.
 
To update, I put a new spark plug wire on it and no change.

I took out the original spark plug and it’s black. New plug in and she’s running fine. Purring real nice.

So now I need to figure why I’m running so rich on cylinder one, enough to be fouling a plug.
 
Here’s a picture of # 1 plug. The other five look great. It’s gapped at .035

image.jpg
 
Check the plug's resistance. Non-res should be close to zero, res plug should be 3000-7000 ohms.
 
Couldda been a bad plug. Hairline crack in it.....bad wire.....check the cap, too. Run a compression test. If #1 is low, then it's not burning all the fuel it should.
 
Happy Birthday RRR

Well, the spark plug is new and the wire set I bought brought eight so I put a new wire on there as well. New cap and rotor too.

Car ran good for about 10 miles and new it’s missing and fouling up again, plus black soot out exhaust.

I adjust the balances around 150 miles ago, maybe I should go back and readjust. Rather not mess with the carburetor.. what do you think? Could this be symptoms of incorrect valve lash on a single cylinder?
 
Pull the valve cover and inspect all the valve springs you may have a broke valve spring and if not relash the valves check the before clearance if you find one dramality off spec you may have found your problem. Spec for all slant sixes with mechanical factory cams is .010 for the intakes and .020 for the exhausts.
 
I’ll go ahead and check pre and post lash.

Don’t think it’s a broken valve spring (fingers crossed). I hooked up a vacuum gauge and still showing a healthy and steady 22 inches of vacuum at idle.
 
Mako21 said:
I’ll go ahead and check pre and post lash.

Don’t think it’s a broken valve spring (fingers crossed). I hooked up a vacuum gauge and still showing a healthy and steady 22 inches of vacuum at idle.
Click to expand...

Could be an exhaust valve then. An intake valve would cause a violent fluctuation.
 
Mako21 said:
I adjust the balances around 150 miles ago, maybe I should go back and readjust. Rather not mess with the carburetor.. what do you think? Could this be symptoms of incorrect valve lash on a single cylinder?
Click to expand...

I would bet this is the problem
 
Incorrect valve lash, as in incorrectly adjusted, does run well & then 'all of a sudden' become incorrect...

It is quite possible you have a valvetrain issue, bent prod, loose adjuster, wiped lobe/lifter, etc.
 
Mako21 said:
Happy Birthday RRR

Well, the spark plug is new and the wire set I bought brought eight so I put a new wire on there as well. New cap and rotor too.

Car ran good for about 10 miles and new it’s missing and fouling up again, plus black soot out exhaust.

I adjust the balances around 150 miles ago, maybe I should go back and readjust. Rather not mess with the carburetor.. what do you think? Could this be symptoms of incorrect valve lash on a single cylinder?
Click to expand...
New doesn't mean good.
 
To close this thread, several things I think led to my misfire.

1. I had recently disconnected the OSAC valve. I completely forgot to cap the carburetor line, so I had a small vacuum leak there for the past month or so, which may have contributed to the problem.

2. I went ahead and installed a new Echlin Napa brand PCV. Kept the old (original Chrysler stamped) one as a spare in the trunk.

3. New spark plug wire.

so far so good, thanks for the help guys.
 
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