Drofseh

I changed my spark plugs and plug wires yesterday at my dad's house. Job went alright, car started right back up with no issues and I drove it home.

I went to start it again later that night and the tachometer was reading a little low at idle and when I gave it a little gas the tach needle dropped straight to zero. When I go back to idle the needle comes back up.

Any thoughts?

GISguy

Check distributor connection order. Check wires and make sure they're not touching each other. Check that everything is plugged in all the way. Check that the spark plugs have been tightened enough. Try the old wires again and see if you still have the issue. (if not, return/exchange the new wires)

Drofseh

Ahh joy, it is running rough today as well.

I will recheck connections and report back.

Ali SC3

iTrader: (10)

did you do the cap and the rotor also. sc400 has 2 distributors with 2 caps and 2 rotors, you should replace the caps and rotors or at the very least the rotors. they are probably falling apart. if they are in good shape then it could also be a coil has gone bad which is pretty common can happen at anytime.

Drofseh

I did not change the coils or rotors. They seem to be fine.

The reason for the plug and wire change was that i had one cylinder occasionally missing when the motor was cold. I figured that one of the wires was just barely getting a connection.

This turned out to be correct, when i took the old wire off plug #2 only half the stem came out. Got the other half out with needle nose pliers.

I figured that since i was going to do the wires i may as well do the plugs as well.

I've pulled all the new plugs and had a look at them. Numbers 3, 4, and 5 were sooty but the others were clean. #4 is on a different distributer from numbers 3 and 5.

I'm concerned i may have used too much dielectric grease on the stems of the wires when i plugged them in to the spark plugs.
If i did use too much would it be possible that the plug wire is shorting out to the sleeve, head, or block next to the spark plug?

Ali SC3

iTrader: (10)

yes it could also be a short, I haven't ever used that amount of grease most of the time I don't use any really.
If it was shorting out that could certainly cause issues but not sure if thats your exact problem.
on a sc400 I would still check the coils first, it could have happened after you corrected the wires.

Drofseh

So I did end up changing my coils and that seems to have solved all my problems.
I'm still really confused as to why that would stop my tach from working properly, but if it's fixed it's fixed.

Ali SC3

iTrader: (10)

if the coil does not fire properly then it does not trigger the IGF signal in the ignitor, and since the ignitor puts out the tach signal it does not put out the rpm signal properly