Ok, first let me start off by saying I have changed several sets of plugs and wires before on other cars without a problem. Today we went down to Auto Zone and spent $123 bucks on all new brake parts, various filters, spark plugs, wires, dist. cap, rotor, ect. I get home tonight and put the new plugs, wires, dist. cap and rotor in, fire it up and it's got a terrible misfire and the engine is shakeing violently. First thought was I got the firing order off. Checked firing order at least 10 times, tried the old cap and wires, tried everything I can think of and I still have this misfire. Car starts up easily. Is it possible that one of the new spark plugs are bad? Their new Motorcraft plugs. I finally gave up and called it a night at 11:30, I'm dead tired, been working the car all day.
Yes, they are gapped at .035. The guy at Auto Zone told me to gap at that #. (they were pretty close out of the box) Forgot to mention, engine is a 74' 250 I6.
Jamie; might be the carb needs to be adjusted, or the points need to be adjusted. Terry did a rough gap on them. Gap them to .017, on one of the distributer cam lobes, and it should run a lot better.
Yes, the plugs can be bad. I was helping a friend put a 390 in to a 66 F100. We got it all together and running only to have a miss. After looking at everything several times we turned off the lights in the shop and found the problem. 2 of the Autolite plugs were bad. You could see ablue fuz around between the porcelen (white part) and the metal. Changed plugs and it was fine. clint
It's very possible for a plug to be bad out of the box. It's just a question of figuring out which one is bad. I just had a plug fracture without much help changing a set on the Bronco recently. Another thing is did you use dilectric grease in the plug boots like many do? If you used too much and it got spread on the wires, it can create a short condition as the spark travels along the grease. I've seen this happen also.
Well, I'll try adjusting the carb and points today. I'll also pull it all back apart today and recheck everything else. I appreciate it.
i had the same thing happen before and it was the plug.....just like they said before me..the porcelain can crack especially when you are tightening it and you wont notice it at all....
I have tried EVERYTHING short of going and getting another set of plugs. Tried the new plug wires, the old plug wires, the new and old distributor cap and rotor, adjusted the points, cleaned the points, adjusted the points again, tried 2 carburetors, tried opening the gap up some on the plugs, tried closeing the gap some on the plugs, triple checked the firing order... ...and it's still misfiring. Only thing that made any diffrence was closing the gap up some on the plugs, that made it idle a little smoother. I'm going to go get another set of plugs tonight. If that dosen't work then I might give up on this engine, and drop a 200 I'm getting this weekend in it. It ran pretty smooth and overall, ran decent until I changed the plugs...
Run the car for a while and pull the plugs one at a time. If the plug is wet when you pull it this is your subject plug/cylinder. Another option is to remove the plug wires one at a time while the engine is running. If you see no noticeable change in the way the engine is running this is your subject plug /cylinder. Be carefull and don't get bit while you doing this! (If you do get bit, it will only hurt for a minute or two also be glad it doesn't have an MSD ignition on it)
Jamie, trace out very vacuum line and replace the hoses. Next, check for play in the distributor. Did you replace the coil? Are you still using that carb off the 300? That engine sat for over 5 years. Take the valve cover off, turn the ignition off, use a screwdriver on the solenoid to the bump the starter around, and watch the valve springs. See if all go and up down the same and if any are sticking. While the cover is off, clean out what crud you can. Put paper towels in the oil return holes of the head to prevent chunks from going back to the pan or possibly getting stuck in the system. If all the valves are opening and closing properly, button the valve cover back up. An engine that has sat that long is going to need massaging.
I got TWO bad Motorcraft plugs for my '87 F150 about a month or so ago. They were bad out of the box! I had to go buy more - problem solved! So..it can happen.
Yea, their Motorcraft plugs. I did notice cylinder #4 and #6 were a little wet when I pulled them. Something else worth noting is I can reverse the plug wires on 4 and 6 and the engine still runs the same. I can also disconnect cylinder #1 and it runs the same. My dad said something about checking the numbers on the plugs, the heat range might be off, but the number on the new plugs match the number on the old plugs. I'm stumped. Terry, I rebuilt the original carb yesterday and have tried it and the 300 carb both and it runs identical with both. I ran the car for close to an hour yesterday after I rebuilt the original carb, and it was smooth as could be. Changed the plugs and this started. Ohh, If anyones intrested in knowing, a throttle cable off a 78 Econoline with a 300 I6 fits perfectly on a 74 maverick with a 250 I6.
Jamie; pull the valve cover, and look at the rockers while the car is running. See if they look like they are moving in approximately the same manner. If one or more is moving very little, it might be a lifter or push rod, or something stupid like that. I think, also, the coil might be a problem. Don't know if Terry left the coil on it that we put on it while we were tinkering with it at his house. Good luck!!!!!
