Surfer69

Another misfire mystery I cant solve. One day it seems fine the next I have one cylinder not firing but cant tell which one. Put timing light on all wires at idle and all eight had consistently same pulse. Fuel issue/starvation wouldnt cause one cylinder to misfire.

Was thinking if I drive around while its missing, could I pull over and use heat gun reader and check header tubes and see which one is colder? Doesnt miss when idling or at least I cant tell. Also checked cap, dwell, timing, pulled plugs and all is perfect. Still could be a bad plug not firing correctly I guess so maybe I will first try new set plugs.

All ideas appreciated.

Revi

What do you mean by miss-fire? Popping in the exhaust, etc..?

In the past I had somewhat of the same issue. The plug wire was backing off of the spark plug about a 1/4". Just enough that sometimes it fired, sometimes it didn't, but with just a casual look they all looked like they were on properly.

The next time you have the problem, try pushing on each plug wire at the spark plug. See if one has backed off a bit.

Antz81

Checking the temperature of the exhaust from each cylinder should tell you which is missing. Strange that you don't notice it at idle though.

Do check that the leads are all on the plugs properly as suggested, that's caught me out before too.

dochorsepower

What is the condition of your plug wires? Have you replaced them with good quality items?

jimvette999

prob an arcing plug wire, look under hood in dark garage with someone (non-goober) holding brake and applying light throttle while in gear....say 1200rpm. look for arc to manifold or other metallic surface.

lvmyvt76

crack inside the dist cap?? sometimes hard to spot the crack!

OMF

What gap do you use for the plugs? Under pressure in the cylinder, a plug could missfire if there is high resistance upstream from the plug. Sounds like you have checked most of these items though.
You definitely will get a miss fire in one cylinder under lean conditions caused by mixture distribution problems. This could be magnified by running too large sparkplug gaps.

Jebbysan

Dark garage as stated will tell you a lot.......
Also watch out for overcharging......if the voltage gets above 15 or so, it can mimic a misfire....
Use dielectric grease on both ends of each spark plug wire......

Jebby

cv67

Bad wire
Go get some cheapos from the auto parts and try them

7T1vette

If you have a 'points' ignition, you might replace the condenser [in the distributor]. And, if you have just recently changed it...then you should change it again, as the one you just installed is likely defective.

Other possibilities are: cracked distributor cap; bad coil; POOR GROUND CONNECTION TO THE DISTRIBUTOR (dizzy body to metal hold-down clamp to engine block) many folks paint that stuff and are unaware that they just lost distributor ground.

Antz81

Now that you menmention it I've had this before. The ground connection under my cap was loose. Behaved just like the OPs. Everything seemed fine at idle. But when driving it would randomly start missing, and then go away for a while (sometimes a few miles sometimes a few days). Wasn't until I was going to try a new ignition module that I found it.

Surfer69

OK just replaced plugs and found again number 2 black with medium tar gook on it. Drove around with new plugs and seems good like its gone now except my hesitation around 4500 is still there...but thats another thread.

Need to figure out why #2 is getting oil in cylinder. Maybe need to tighten head or something or rings are letting bypass get in.

7T1vette

Might be as simple as a bad plug wire or misassembled connector in the wire. You can check them with an ohmmeter. Remove wire from plug and other end from dist cap. Check resistance. If any of the wires have much higher resistance than others, replace them.

CanadaGrant

The ohmmeter check is a REALLY good idea. I spent the better part of a week chasing a miss but it was only after it was warmed up and not constant. Everything was new in the ignition system from the plugs to a new Pertronix unit with new cap, rotor, coil and plug wires. The plug wires (new) are those expensive ones with the braided shielding like it came from the factory with so that was the last thing I looked at. One of the connectors was improperly installed on the wire and it was arcing inside the boot until the boot and wire warmed up and expanded. Then it would start to miss intermittently. I finally caught it using a timing light on each wire but could have saved myself a bundle of time and aggravation if I had thought of using an ohmmeter to check resistance in the wire itself. Thanks for the tip!

lvmyvt76

done a compresson check, sounds like someone leaking thru a valve or rings maybe??

Surfer69

yeah I need to do a compression check on number 2 for sure.

7T1vette

{not if the #2 plug wire is bad...}

Surfer69

Final update:

Turned out to be debris in one of the main jets in the Qjet.

Now all is well. THX

cardo0

Cool! Thx for following this post up.

mikem350

If your rubber fuel lines are old, there may be more debris later, especially with todays gas.