Simple oil change and it's all gone wrong!
 

Just performed an oil change. (Adjusted the rocker valve clearances too. Most where a little loose. Also cleaned and re-gapped spark plugs. They all looked pretty healthy, just a light dusting of carbon.) Filled it up again with oil and took it for a spin....and it runs really poorly! What have I done wrong? The engine stutters and backfires. Rev it up to 3K and things improved and it sounded nice but that's not fun in London traffic. Between 1-2K the engine backfires and stutters, put it in gear and it shudders and jumps all over the place. Managed to hobble home and checked all plug leads correctly seated and connected. (My thinking is it's not firing on all cylinders now but I'm open to alternative suggestions!) . On the plus point it's now got new oil and no leaks but whys it running so poorly? Any ideas? Cars a '77 C3.0 and WAS running sweetly till I fiddled with it. Cheers Mark

It certainly sounds like crossed plug wires. You did say they were all "correctly seated and connected" but does that mean the firing order is correct? Literally, trace each lead from the distributor to the plug to double check each lead and see if the firing order matches.

I'll double check again tomorrow. I'm going to take the plugs out again too just to make sure they're correct. I was also wondering..what would be the consequencies of tightening up the rocker valve gaps too much/too little. I mean it's a bit subjective as to how the feeler gauge slides across them. I tried to get them all to feel the same but who knows if that feelings correct. (And it's a right fiddle to get to 3 and 6 inlets with the engine in...as I'm sure you all know!)

I'd definately take ossiblue's advice and only check for crossed plug wires first and see if she runs right. This is the only way to know for sure what the prob was. One troubleshooting task at a time.
Then you can fiddle with the plugs etc after that.
Good luck.

If it clears up higher revs, I would not suspect crossed wires. I would suspect bad wires or a fouled/incorrectly gapped plug. Go buy a pair of insulated spark plug boot pliars and gently pull the boots one by one while running (you'll probably get zapped at least once) then put back on and go to the next one. I had a very similar experience, the motion of pulling the wires the 1st time for a tune up, must have damaged the brittle core of the spark plug wire.

Wires sound suspect. Are the cracked or stiff? Also make sure the distributor cap is on correctly. Check for other wires damaged you may have bumped into.
Side note: make sure that the plugs are clean before putting them in. You don't want grit getting down in there. Also, be very careful not to cross-thread a plug. If you have problems getting a plug out stop and read up on cross-threaded plugs.

I bet it's either crossed wires (never remove more than one at a time), bad plug wires or dirty/incorrectly gapped plugs. Check all of your wires and vacuum hoses and be sure nothing got knocked loose. Good luck!

Might want to rev it in the dark and look for arching?

Open the trunk in the dark like carnutzzz said. I had a plug wire wiggle loose and easily spotted it when the car was running in the dark. The spark was arcing accross the connection and it's really easy to spot in the dark. You'll see a light show under your trunk if your wires are bad.

Gary

I recently did valve adjust and left one wire off. It ran OK but was not fully there but still ran reasonably well. Valve tolerance is fairly wide and I can't see that being an issue; too loose and you would hear them.

Trace wires from distributor to plug, mine had diagram of each. Also I had my dist cap off when doing valves to verify TDC for each valve. Possibly something got in there or got removed (like spring) if it was opened. I pulled the rotor also looking for a TDC mark, make sure it's on properly if removed.

Is this an SC? The first time I worked on an SC...I forgot that the distributor was turning counterclock rotation. I ended up with a very unhappy engine, backfiring missing and very poor performance. With the trottle opened up, higher revs, the car seemed to run a little better.

I agree that a troubleshooting system of one thing at a time makes really good sense, especially taking notes while doing it so you can remember where you were if interupted.

sounds like a crossed wire, or maybe a loose distributor cap.

Good luck!SmileWavy

Well...I traced the spark plug wires and they where all correct. Pulled off the Cap and re-seated. It did 'click' into place which I don't remember it doing last time. Un-screwed each plug (one at a time) and they where all 0.7mm. A couple had loose little screw thingys (the bit where the lead clicks onto) so I tightned these up and put a small amount of conducting grease on the ends. (NO not the threads!!) Visially checked the leads, a couple of rub mark where the outer earth stranding had frayed a little, which I taped up. Leads themselves looked sound. Plugged it all back up and started up. WOW...ran like a trooper! Nice familiar 'sewing machine' sound and not a single back fire. Revs nice and smooth throughout the range. Suddenly my mood was lifted and I love this stupid lump of metal again! May thanks for all your tip and suggestions. All I can think was that it was a loose connector/s somewhere in the chain. Amazing that something like that can make such a big difference in performance. Right..I'm off for a spin...