What is causing my misfiring while driving ?

[Evan Frucht]

Hi i have a 1964 plymouth fury. Original 318 motor, the cam has a regrind on it from PO, it has dual exhaust and a factory 4 barrel.

It always has idled pretty rough. Even to the point of dying occasionally after one missed beat. I've turned up the Idle speed and that helped but it will still happened occasionally. The car always ran great so I just attributed the lopey rough idle to the unknown cam grind.

So in general the car would drive great and really smoothed out once I gave it some gas.

Recently I drove it all the way up to Big Bear Lake, 7000 elevation. It's a couple hour drive but I have taken this car on long drives before without any issue. It made it no problem. Anyways, on the way back home I noticed the car misfired a few times on the freeway. It just lost its power and you could feel the car slow down for a split second. Then it got normal again. Now it has been doing this on and off for the last couple weeks and it might be getting worse but sometimes it doesnt do it either.

Any ideas?

My first thought was maybe the points, condenser, or coil is failing ? So I bought new ones.

Does that sound like it could be the issue.

The coil is the original one that came with the car when I bought it... have no idea how old it is.

Points probabaly have a few thousand miles on them or so. There is a dark spot that kind of looks like a pit on the center of the points contact surface.

Any other ideas on what is causing this "misfire" or stumble.

I figured if it was a carb issue it would not have just started one second to next while driving... but maybe I'm wrong

 

[turbine68rt]

Pull some plugs and check their condition. When replacing points keep in mind the dwell/gap will affect timing. Reset after confirming correct dwell. Do the mixture screws provide any change in idle quality? Fuel and air filters good? Just some thoughts.

 

[Ron H]

First off, did the ’64 318 have a 4bbl factory option? Asking as my ’63 didn’t (I installed one). Of course there could be a slew of possible gremlins taking you to fuel and ignition issues getting into a common engine tuning process. You say the points have a burn spot, that is a quick concern needing fixing. Check the gap, and also plugs for carbon deposits or incorrect gap, and the wires. Is the dist cap in good shape? Can check the carb fuel/air ratio adjustment (I’ve done this using a vacuum gauge and RPM test) and check the engine timing. There could be carbon build up on a valve or cylinder that can be investigated by using a vac gauge and compression test. I had some build up on one valve I got rid of running mystery oil for a few months. In any case run through a thorough engine tuning process to assure it’s tuned properly. One issue I encountered when installing my 4bbl is the rear throttle plates were contacting the manifold causing poor running (when I’d open it up) and fixed it by adding a spacer.

 

[R413]

That’s usually a hollow spot on the middle of the points.

Have you checked your valve adjustment? That’s first thing on any tune problem. If they are wearing in they can get tight and not close and it will run crummy, gets worse when hot.

Do you know if this a solid or hydraulic lifter camshaft? Still need to check the adjustment with either type because you have adjustable rocker arms.

 

[Ron H]

Yep, as turbine68rt posted, simple things include filters and some carbs also have a screen filter in them. An engine 'cleaning' could help using seafoam or as I've done running Marvel...in any case make sure engine is tuned.

 

[Evan Frucht]

Thanks for all the replies. I got this car a couple years ago and have been driving it around a decent amount the past year. A year ago it got a good tune up and the carburetor was completely cleaned/gone thru and rebuilt. But it's been a while since all that...

It's just a weird problem this misfiring that keeps getting sort of worse but also doesn't happen for 20 minutes at a time and then will return for 5 minutes. Given I dont have a long experience driving these cars I just wanted to see if the condition threw up any specific red flags. I know these types of issues can be hard to describe and diagnose

I have not really got into the engine much and have not adjusted the valves since I bought it becuase the engine always ran fairly well with everything else. I probably should do that.

Since I have then already I will replace the points, cap, rotor, condenser and coil today and see what that does.

The engine is pretty much original, but sure on what lifters. I know the cam is a stock core but re-ground, dont know its grind specs. The 4 barrel is factory cast iron Mopar for a poly 318 but not original to the car, I dont think. I have a carter AFB 650cfm.

A lot of stuff is only a year old like the air filter, the plug wires, spark plugs... I have also sorted out a lot of electrical issues so far but it still has a few.

The fuel filter is a bit dirty but I feel like I've seen worse...

Anyways I'll adress these issues hopefully it is just basic maintence that it needs.

Will report back

Thanks

 

[Big Bad Dad]

Fuel filter full of crap caused me a similar problem a couple years ago. Ethanol gas causes fuel tank rust and problems.

 

[Evan Frucht]

well just spent an hour or so replacing...

points
condenser
cap
rotor

set the gap at .016ish
I bumped up my initial timing to about 20 or so
I had it a little lower but it seems to like this a bit better

cleaned fuel filter

overall, at first I thought I fixed it. the car seemed to run a little better after tuning the ignition but then the problem started again. It doesn't seem to be as bad, maybe just because the car is more tuned in general, but it is still hesitating, or misfiring, or stumbling. I really don't know what to call it.

it almost FEELS like a belt is slipping or something, I know that's not the case but its weird and I lose my power for a just a split second. You can feel the car kind of hesitate for a second. Giving it more or less gas doesnt seem to do much. if anything revving it up faster actually helps sometimes.

I guess I will check plugs tmrw.

It's frustrating because I am busy in the middle of rebuilding my VW engine, and am sort of depending on this car a little at the moment as my driver.

 

[bandit67]

In checking the basics, the rule is, if it skips at low rpms, replace the plug wires, if it skips at higher rpms, replace the plugs. But that seems to work on steady misfires, not random as yours sounds.....

 

[Evan Frucht]

mine is random but it also seems to pretty much happen higher up in the rpm range. it doesn't really stumble off idle... only once im going. first time I noticed it happened on the freeway. then after that just on and off while I'm driving around

plugs were installed new about a year ago. I will check them tmrw though

anything i should be looking for in specific here

 

[Ski 61701]

mine is random but it also seems to pretty much happen higher up in the rpm range. it doesn't really stumble off idle... only once im going. first time I noticed it happened on the freeway. then after that just on and off while I'm driving around

plugs were installed new about a year ago. I will check them tmrw though

anything i should be looking for in specific here

I would ohm check your wires. Even though they are a year old, they might be the cause of the intermittent miss.

 

[Evan Frucht]

I would ohm check your wires. Even though they are a year old, they might be the cause of the intermittent miss.

Damn I hope not, there are pricey wires. The red MSD ones.

Probably didn't even need them as I don't have an electronic ignition

 

[matthon]

I've heard that the coil should really be mounted vertical, even though it is typically mounted horizontal on these old Mopars, possibly to keep it cool.
It sounds like the car is fine until it warms up, based on what I gather.
I could be wrong, but I modified my mount to make it vertical and keep it from contacting the intake, probably just out of habit now.

I had a similar issue with a rebuilt carb, a tiny piece of lint circulating inside which finally got stuck and I was able to pull it out.

 

[Ski 61701]

Damn I hope not, there are pricey wires. The red MSD ones.

Probably didn't even need them as I don't have an electronic ignition

I seriously hope they are fine, but I got bit by pricey wires once brand new never run i got to dyno, fired right up but motor just did not sound right. Ohmd the coil wire on bbm and it was over 4000k on that short wire alone. Other wires were worse. luckily my buddy had another set of wires and saved the day.
Not sure if you did this, check your ballast resistor, it might be getting ready to go. see what you have n test it, both cold and once you've warmed up the car see what you get.

 

[kiwigtx]

I had a terrible case of backfires many years ago.....random and usually under load. Turned out that I had one bad spark plug - replaced it and the car went fine. Even a new plug can go bad...especially if they are Champions.

 

[Ski 61701]

I had a terrible case of backfires many years ago.....random and usually under load. Turned out that I had one bad spark plug - replaced it and the car went fine. Even a new plug can go bad...especially if they are Champions.

Another old school trick is to get a spray bottle and when its dark out and motor is running, lightly spray water starting at plug and work up to the cap. Look for spark arcing. If you have a tach hooked up, have someone watch it while you check each wire look for rpm drop. then check coil wire too. Also make sure you have a good charging system check battery when car is running make sure you have at least 13.5 volts preferably 14.7 running. low voltage will give you grief too,

 

[Darter6]

Any idea if the engine has been rebuilt or is it 56 years old ?
I know at my age I miss a few beats on occasion too.
So many thing to think about.Maybe it's time to do a leak down test

 

[Don Frelier]

Maybe the fuel pump is weak or debris in the sending unit pickup sock.
Sounds like fuel starvation to me.

 

[turbine68rt]

mine is random but it also seems to pretty much happen higher up in the rpm range. it doesn't really stumble off idle... only once im going. first time I noticed it happened on the freeway. then after that just on and off while I'm driving around

plugs were installed new about a year ago. I will check them tmrw though

anything i should be looking for in specific here

Black sooty buildup or wet oily coating. Aside from correcting what is actually causing it, if that's what you find, you could go up in heat range.

 

[Evan Frucht]

Any idea if the engine has been rebuilt or is it 56 years old ?
I know at my age I miss a few beats on occasion too.
So many thing to think about.Maybe it's time to do a leak down test

Ya it might be getting around to that time. The engine was rebuilt by the previous owner... supposedly.

It does have plenty of power and has always driven very well and smoothly other than the rough idle which I have always attributed to the unknown cam grind.