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Cold start, idle misfire
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Author:  wintermute [ Sun Jan 05, 2014 5:40 pm ]
Post subject:  Cold start, idle misfire
When I start my Valiant in the cold (30's ºF), it has a bit of an intermittent miss while idling. It does seem to clear up as the engine warms, though.

My plugs are gapped properly, haven't touched the distributor or carb. The previous owner installed an electric choke. I just adjusted the valves last night and that made a HUGE difference in valvetrain noise–I doubt it's ever been done.

This miss did exist before the valve adjustment. Any tips on things to check?
Author:  wjajr [ Sun Jan 05, 2014 7:42 pm ]
Post subject: 
Intermittent miss could be caused by a degraded points if so equipped, bad plug wires leaking voltage, fouled plugs, worn cam lobe causing bad adjustment of one valve, and other things.

Have you been able to determine which cylinder is missing, or is it a random miss floating around to all six cylinders?

Dose the miss manifest at higher rpm, and or does engine smooth out, or can't tell?


If you can locate cylinder that is missing, swap its plug wire with its twin in length and see if the miss moves to the other cylinder.


Edit:
I just reread your post, my short term memory is shot I guess.
Quote:
My plugs are gapped properly, haven't touched the distributor or carb.

Have a look at the rotor button, and distributor cap, look for carbon tracks inside & outside of cap where voltage has been bleeding off, and corroded contacts. If the plugs, cap, rotor, and points if equiped look old I would replace them.
Author:  oldskoolracer [ Mon Jan 06, 2014 2:55 pm ]
Post subject: 
Check for vacuum leaks when it is first started... Mine was doing the same until I replaced the intake/exhaust to head gasket. When it is down that cold the leak will be worse, as it heats and the manifolds expand they'll seal up a little better. I checked, and checked, and replaced a bunch of stuff before I figured it out. :shock: It only did it first thing in the morn, or after sitting for hours in the freezing cold, sounded like one or more cylinders just werent firing at all, and would gradually smooth out after it ran for a few minutes.
Author:  Aggressive Ted [ Mon Jan 06, 2014 7:58 pm ]
Post subject: 
If all the above checks out......
Run longer lash settings and it will sound much steadier.
Author:  wintermute [ Wed Jan 08, 2014 7:25 am ]
Post subject: 
Quote:
Check for vacuum leaks when it is first started... Mine was doing the same until I replaced the intake/exhaust to head gasket. When it is down that cold the leak will be worse, as it heats and the manifolds expand they'll seal up a little better. I checked, and checked, and replaced a bunch of stuff before I figured it out. :shock: It only did it first thing in the morn, or after sitting for hours in the freezing cold, sounded like one or more cylinders just werent firing at all, and would gradually smooth out after it ran for a few minutes.
This seems most likely, as the behavior is very similar to this sort of vacuum leak; Add throttle, it smooths out. Warm the engine up, it smooths out.

The funny thing is now that the valves have been adjusted, I can hear the leak in little spurts. I thought it was just air getting by the temperature controlled intake door. :oops:

I've only had the car since December 7th, so I really have no idea how it behaves in anything approaching warm weather :o
Author:  Aggressive Ted [ Wed Jan 08, 2014 9:01 am ]
Post subject: 
It may be carboned up pretty bad....and the glowing carbon can cause a slight misfire at idle. A good regimate of Gumout in each tank or Seafoam and a long trip or water/alcohol injection and a air-oil separator can help clean up the valves and head.
I do all three and can idle for hours in traffic at 600 rpm with out missing a beat. Same with warm ups in the morning.....very steady, no popping or mis-fires. It should clean up over time....
Author:  oldskoolracer [ Wed Jan 08, 2014 10:01 am ]
Post subject: 
If you do the gaskets as I mentioned go with the Remflex ones, you will no regret it and will NEVER look back to another brand of gaskets! After that, do as Ted mentioned and run a good fuel system cleaner through a couple tank fulls, and dont be afraid to hit the highway and wind the engine up a couple few times. Simply doing that has cleared up alot of running issues in previous cars I have owned.

Once done with that, air/oil seperator makes a huge difference with incoming contamination at idle. If not done already, convert to HEI, it drastically improves starting, idle, and throttle response. If equipped with an EGR, check it, make sure its clean and not stuck open. It will cause idle issues at all temps, but may be worse in the cold as well when the engine wants a hotter richer mixture to get warmed up. :D
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