Diagnosing misfire

[Neglected68]

I am trying to figure out if I have a misfire with my car. I have the Summit 600 cfm carb with a Weiand Stealth manifold and dual exhaust (no crossover pipe), but otherwise a stock 289. I hear what seems like a regular miss from the exhaust pipe on the passenger side, though the engine seems to be pulling well. When I was trying to isolate the possibly misfiring cylinder, I noticed that when I pull the spark plug wires from the distributor cap one at a time (following the firing order), only every other one causes the engine rpm to drop. Is this normal? :shrug I have put in new spark plugs and the wires seem to be in good shape. I haven't replaced the points, but the gap is correct. Could this be a timing issue? The timing is set at 6-7 degrees, but the vacuum advance is almost touching the top radiator hose. Could I have put the distributor in off by one tooth?

Thanks!

 

[AzPete]

Put a timing light on each plug wire to see if they are firing correctly. Ohm out the wires to verify they are good.

Then turn to the fuel. Missing at idle, you can use the mixture screws to see if it is that.

If still there, add some timing to it and see if it goes away.

Sometimes, when running no crossover pipe, the car will sound like it has a slight miss due to firing order and back pressure.

 

[6t6red]

Sometimes, when running no crossover pipe, the car will sound like it has a slight miss due to firing order and back pressure.

+1 I don't have a crossover pipe and that is how my car sounds. Without the crossover you don't get any scavenging to smooth out the exhaust.

 

[Mach1Rider]

If your vac adv is almost touching the rad hose and your at 6-7* it prob was installed a tooth off.
As far as the exh tone miss, the drivers side has the 7,8 firing then the #1 fires on the pass side.
So you get 2 cyl pulses on the left then one on the right.

 

[Neglected68]

Thanks for all the suggestions.

I did hook up the timing light to each spark plug wire and they all seemed to fire steadily, but I did not check the resistance through the wires. I did get shocked a couple of times, but I don't make a reliable voltmeter.

Bad gas could be the culprit - the tank is near empty now, and I have only burned about 8 gallons or so since I got the car running. I have a slight leak around the gas tank sender, so I will be draining the tank to fix that soon.

I have developed a small oil leak at the back of the intake manifold, so I will try to get the distributor on the right tooth when I reinstall the manifold.

The miss sounds sort of like an exhaust leak and is most noticeable at the tailpipe. Could one of my turbo mufflers started to rot away on the inside? They are more than 20 years old, but this is sunny California (never mind the fact that it is raining right now.)

 

[Anonymous]

Should be more like this so you have plenty of adjustment.

 

[Neglected68]

That picture is very helpful, Mark. Thanks. It is always the simple things that I miss. Well, the complex ones too.

 

[BobV]

I am trying to figure out if I have a misfire with my car. I have the Summit 600 cfm carb with a Weiand Stealth manifold and dual exhaust (no crossover pipe), but otherwise a stock 289. I hear what seems like a regular miss from the exhaust pipe on the passenger side, though the engine seems to be pulling well. When I was trying to isolate the possibly misfiring cylinder, I noticed that when I pull the spark plug wires from the distributor cap one at a time (following the firing order), only every other one causes the engine rpm to drop. Is this normal?

Sounds to me like your idle circuit on one side of the carb is "off"...
With a dual plane intake (like your Stealth), each side of the intake feeds every other cylinder, which matches up with what you're experiencing when pulling plug wires. What happens when you adjust the idle mixture screws? I'll BET, that one of the screws is either out of adjustment, or has no effect on the idle characteristics (clogged circuit).
The engine "pulls well" when driving, because the idle circuits have minimal effect at off-idle speeds.