The Stovebolt Site Forums The Shop Area Engine Shop One plug not firing

I am having trouble figuring out why the drivers side plug closest to the fire wall (71 chevy 350) isn't firing and causing a miss..I have replaced the plug twice to make sure I didn't get a new dud plug, the wires,checked the rotor and the metal points inside and outside, at the top of the distributor cap,both have clean metal...I haven't pulled the rotor yet to see if it's points ign underneath, I could see a condenser/capacitor but what's stumping me is just this one not firing..wouldn't seem like a points or rotor malfunction or small crack in the dist cap?...I notice a bit of smoke coming from that side of the dual exhaust and figured it may be due to the non firing plug and or a valve guide seal problem since this truck had sit quite a while in the prev owner's yard...could it be a valve malfunction maybe? but this motor starts really easy and idles fine and sounds tight for having that one plug prob.. btw I replaced the vacuum adv line and made sure there was good vac on it and no leakage... like to get this old dually running smooth...

thanks in advance for any ides or suggestions


Mike

First I'd run a compression test to see if it's within 10% of the other cylinders. Next put a phillips screw driver in the sparkplug end of the wire and see if you get a strong blue spark to ground with the engine cranking or running. If it has compression and spark this leaves only fuel but that's unlikely with the other seven working.

I will check for the arcing...I have pulled the offending plug and it comes out clean with a little gas residue on it...in stark contrast to the others ...

any simple ways to check that cyl's compression besides going out and buying a gauge I would use prolly once or twice?

I thought about a valve problem but the thing starts easy and idles well with no strange noises...could this be symptomatic of stuck valves or badly burned seats maybe?

thanks

Mike

Crank it up in the dark and make sure it isn't arcing somewhere along the plug wire. Look for any spark where it isn't suppose to be.

That coil will fire two plugs at a time, one on the compression stroke with fuel in the cylinder, and the other on the overlap position with nothing but exhaust gas in the head. It's called a "waste spark" system. Both plugs connected to that coil should be misfiring if there's an ignition-related problem. If only one cylinder is involved, you must have an open or a short in the wire going to that plug. That's assuming you've got fuel and compression to the cylinder that's missing. Swap both the plug and the wire to the bad cylinder, even if they're supposed to be "new".
Jerry

Ditto on the "in the dark" idea. . . I'll never forget a friend starting up a truck he'd just bought in the dark. Little sparks everywhere from old plug wires. Pretty, but not desirable.

How about swapping plug wires to see if the problem migrates?

How did you initially know it was that plug in the first place?

-Michael

You may have a burnt exhaust valve on that old of an engine. Compression test is in order. Also, pull the valve cover and see if the rockers on that cylinder are moving the same as the others. Could be if it sat for a while, that a valve stuck and has let a push rod fall out of place.

Had this on a chevelle once. Something to look for is in the distributor. The shaft that the points ride on was worn so bad that the points were not opening on that cylinder. Surprised the engine was runnning at all. Easy check with an ohm meter or feeler guage. Just something else to look at.
Tim

I replaced the plugs and the wires myself...when I took the old
plugs out the one nearest the firewall on the dr side was completely full of gunk and no doubt fouled bigtime...when the new plugs ran for a while I noticed it still had a miss so I pulled the one in that spot and the ones in front of it...they all came out looking blacker and showing signs of firing except the mentioned one which came out looking brand new with some gas residue...put a new plug in it's place just in case it was a dud "new" plug and same thing...still missing and coming out clean...
tonite when it gets dark I'll look for errant arching and see if I can do the screwdriver thing to see if there's a spark at that plug at all if I don't see any random errant arcing...as well as wire swapping

in regards to a compression check any suggestions on doing that without having to buy an expensive tool I may only use once?

thanks

Mike

You can pick up a compression gauge on e-bay for under $20. that will work. The local auto parts place probably has one for not much more. You can spend $120. for one but for what you need to know one of the cheaper ones would work. Not a bad tool to have.

First find out wether you have spark to that cyl. or not. If not check the point gap. like TRK says sometimes a dist shaft is worn or bent enough that if the points are gapped too close one cyl will be the first to missfire. If you havn't replaced the cap it can be a hairline carbon track it it that may be very hard to find or may not show up at all.

Thanks,

I'm going to go by the local flea mkt tomm and see if I can find a deal on a gauge if not will check local parts stores for price..I guess it would be good to take a spare plug with me so I can get the right threads? anybody know what the ballpark reading should be?

anyway did the arc test tonite after it got dark...couldn't see any errant sparks and did the screwdriver test for fire...the plug wire does in fact have spark...must be a valve or compression issue...bummer...

anybody have any idea what a full valve job should run $$ wise as opposed to having an engine re-build kit put in? seems like if there's a stuck or burned valve on an eng this old I can't see the others being in great shape...

TIA


Mike

i have a small block 400 that had a broken rocker.

could be the dist cap. That one contact may not be making contact. You should be able to pull that wire a little bit with it running and hear it arching. Worth a shot.

take a extra plug, put it on the end of #7 plug wire,ground plug and check for spark. if no spark you have bad/broken plug wire. you can also ohm out plug wire (check for resitance)

thanks guys...I did the spark test and it appears there is fire...I tried to get a compression gauge yesterday at the flea mkt but the only one I could find was pretty old and ate up looking..prolly wouldn't work...

I think the next thing I may try is pulling the valve cover and seeing if I can see the rockers moving as they should...if they are I would think maybe burnt valve(s)?

any other suggestions well appreciated...

thanks

Mike