340 diagnosis misfire cause

['Cuda_Only]

Greetings....newer member here....first time posting.....
Purchased my dream card about a year ago.....'71 Cuda 340-4sp....numbers matching.....car has been driven less then 200 miles since I've owned it. I've had the carb (Thermoquad) rebuilt, replaced the electronic ignition/distributor and been trying to fine tune the carb. Had been noticing a slight misfire over the past several months and last week I decided to check the timing....slightly advanced and since then car idles rough when warm.....I tried setting re-adjusting timing but nothing seems to help and now I'm concerned there's a bigger issue with the motor. Any suggestions on where to start with diagnosis the issue? I don't know how many miles on the motor. I have not down a compression tesst or a leak down test yet. It seems to me the issue is more spark/ignition related, but hard to say for sure. Could this also be a vacuum leak issue?

I know there are a lot of variables and I'm probably not providing sufficient information, but any advice would help.

Also, if any members are based in Nor Cal, would appreciate any advice on a reputable shot for Mopars (I'm in Sacramento, CA).

 

[tonysrt]

Sometimes you have to change a part with a known good one to eliminate these kind of problems. I had the coil give me fits till I changed it and then all fine thereafter. Pull a plug wire and make sure your getting a good blue spark. Try pulling 1 plug wire at a time to see if misfire in a particular cylinder. Can even pull plugs and check the condition of being too rich or too lean. Unfortunately this is a trial and error process. No one said it would be easy, but solving the problem is so very rewarding. Good luck and let us know what you find.

 

[71droptop]

Could be a timing issue. Bring balancer to T.D.C. make sure rotor is on #1.
Distributor drive could be off 1 tooth of gear drive. Pretty sure slot in drive should be facing #1 cylinder.
You could use large flat tip screw driver to bump dist. drive backwards until in proper location.
Would email Nicks garage in Canada for the exact positioning of gear slot/ or any other mopar guru.
Good luck

 

[Steve340]

Have a look at the spark plugs and spark plug leads first.

 

[340challconvert]

Cuda_Only

to FEBO from New Jersey
As you and others have said; there are many variables when it comes to an engine miss. Is the miss continuous, or intermittent? (Important distinctions) You could go through the actions of shorting each plug wire to look for a specific cylinder, make sure that the there are no issues with the air gap/reluctor, or a worn distributor shaft. (Even a new distributor could be at fault) Pulling the plugs; examine any differences in color, misfiring plugs will look fouled showing the guilty cylinder. Vehicle electronics; any component could cause mis-firing. A worse case scenario; a sticky valve or worn rocker/pushrod all could cause a problem. I would try to eliminate the obvious potential sources then; Do you know someone with a shop and diagnostic scope? It could make it quicker and easier to determine the problem. Just my thoughts. Good luck

 

[DTCMMLF]

If your miss is not a constant miss, wait till dark. Open hood. Crank it and see if maybe fire is jumping to the manifold, AKA, bad insulation on a plug wire boot. Also look where the coil is and the distro cap, again in the dark.
Hillbilly troubleshooting!
RBHawk in 'Bama...
PS. I have 3 T-quads. All three idle rough after rebuilt. Think the meterings rods have to set JUST SO. And I have the Carter carb book for reference...

 

[Rons340]

If your miss is not a constant miss, wait till dark. Open hood. Crank it and see if maybe fire is jumping to the manifold, AKA, bad insulation on a plug wire boot. Also look where the coil is and the distro cap, again in the dark.
Hillbilly troubleshooting!
RBHawk in 'Bama...
PS. I have 3 T-quads. All three idle rough after rebuilt. Think the meterings rods have to set JUST SO. And I have the Carter carb book for reference...

This technique works great, I have used it many times! A little added trick is to take a spray bottle and mist the secondary side with water. If there is a spark leak anywhere this will enhance the test to find even the smallest issue.

 

[Rons340]

In regards to the original concern by 'Cuda_only, I would check for a vacuum leak first. Disconnect and plug any vacuum port, one at a time, to eliminate issues with any of the vacuum operated systems. Secondly, at idle, carefully use carb cleaner to spray around the carb and intake area. If you hit a vac leak it will suck the fluid into the leak and cause the engine idle to momentarily speed up. This will pinpoint the area of the leak. This will be a simple test to start with. Sometimes the simplest is the hardest to find.
In regards to a major mechanical problem with the engine, if it simply a slight, intermittent miss, the one thing that come to mind first is a worn valve guide. I would conclude that by process of elimination of the other areas since this rarely shows up in a compression test.
I hope this information is helpful.

 

[NoCar340]

The basic vacuum-leak and spark-scatter tests outlined above (especially with the spray bottle on the wires) are where I'd start as well.

The next thing I would do is pop the cap and look for any carbon tracking or degradation of the terminals. Look for the latter on the end of the rotor as well.

Since you're right there, try wobbling the distributor shaft horizontally in all directions. Rather than use an exact measurement, my rule of thumb is that if I can see the wobble, the distributor bushings are shot. I doubt this is the issue since you said you replaced it already, but even new stuff is defective or out-of-spec now and again. If you replaced it with an A-1 Cardone unit, chances are they didn't even change the bushings.

While you've got the cap off, turn the engine clockwise until the mark on the damper aligns with the furthest timing mark on the right (looking at the cover). Rotate the engine counter-clockwise, watching the rotor while you do. As soon as the rotor tip moves, stop and look at the timing scale. If you have more than 6-8° of crank movement before the rotor moves, replace the timing chain.

Grab your throttle shaft and try to move it sideways, i.e. fore and aft rather than rotate it. Try up and down just for kicks, too. Our shop recently worked on a '71 Cuda 340 auto, and the primary throttle-shaft bores were hammered. He either needs a replacement throttle body or to have the throttle-shaft bores bushed. That was the last remaining source of random misfire on that car: It's sucking air past the throttle shaft. It ran a whole bunch better after the timing chain and distributor bushings were replaced. Both were pretty bad (almost 20° of chain slop, .018" distributor runout if memory serves), but we're sure the last tiny annoying bit of misfire is the throttle-shaft vacuum leak. Along with the misfire, cranking times seem to vary a little and the idle will settle to a different RPM now and then.