64 Plymouth 273 Intermitent severe misfire.

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Wally-T

Wally-T

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Cochrane Alberta
Hello again folks. This is going to be quite a read so you may want to grab a coffee. I'll try to keep all symptoms and repairs in proper order to avoid confusion. (Wish me luck lol )
I changed plugs and wires last fall because of a rough idle.
I rebuilt the carb in April because of a bad hesitation and it worked beautifully.
I adjusted the valves because they were extremely noisy,a couple were just under .045 thou.
When I was adjusting the valves I noticed a lot of free play while rocking the crank back and forth so I pulled the distributor cap and found a scary amount of crank rotation before the rotor moved. Time for a timing chain and gears.
While I was waiting for the parts I was still driving the car (it's my daily driver in the summer) and one day when I was pulling on to the highway under heavy throttle and just before it shifted into third gear the engine took a nose dive, I thought it stalled but I pumped the throttle and it came back to life and I didn't have the problem again for about 20 miles.
About a week later it started bucking and chugging like it had water in the fuel or at least some kind of fuel problem so I thought I may as well take it to the shop and do the timing set and give it a proper tune up. While I was pulling it onto the hoist it started chugging again so I shut it off and coasted in so I could pull the top of the carb off and see if there was water in the bowl...nothing but pure gas. Shoot, I was hoping that was it.
So I installed the timing set along with intake gaskets, water pump, and points. The rotor was very loose on the shaft but the new one from Rock Auto was wrong so I waited for another. I installed the new rotor and a new cap and took it for a good 30 minute drive on the highway and it worked flawlessly. Got back to town and... CHUG CHUG CHUG. Got a new coil from NAPA... CHUG CHUG CHUG.
Now I'm thinking that maybe it's a voltage problem to the coil so I gave the coil battery voltage and it stopped Chugging!!! But when I removed the 12 volt source it kept running fine so I'm betting it will come back.
Well, that's the story so far. I guess it's not a full cup of coffee story but long enough.
So...what do you think? Thanks, Wally
 
Is the ignition all original? Coil, resistor? When you "gave the coil battery voltage" did you use a jumper wire from the battery to the coil?
 
Condensers will do some wacky things like that. Still have points? Also, the new condensers are junk. Get a hold of Halifaxhops for a old stock one.
 
Murray said:
Is the ignition all original? Coil, resistor? When you "gave the coil battery voltage" did you use a jumper wire from the battery to the coil?
Click to expand...
I forgot to mention that. Points, rotor, cap, coil and ballast resistor are new. I didn't have a condenser today but I'll put one on tomorrow. Yes, 12 volts from the battery.
 
toolmanmike said:
Condensers will do some wacky things like that. Still have points? Also, the new condensers are junk. Get a hold of Halifaxhops for a old stock one.
Click to expand...
It is points ignition and the condenser is the only thing I didn't change today. I'll try it tomorrow and re-post. Thanks.
 
What's your procedure for diagnosis other than throwin parts at it? Not bein a smartass, but until you actually DO some REAL diagnosis, you may well keep throwing parts at it. Have you done a compression test, for instance?
 
This one is strange, but am wondering about a overheating resistor failure since it goes away when you bypass it with a jumper.
 
RustyRatRod said:
What's your procedure for diagnosis other than throwin parts at it? Not bein a smartass, but until you actually DO some REAL diagnosis, you may well keep throwing parts at it. Have you done a compression test, for instance?
Click to expand...
A tech instructor always said, Never overlook the obvious. And start with the basics. I spent better than a decade at a dealer where the techs busted their tail diagnosing the exact cause of the problem which is all warranty would pay for while overlooking the basics like a proper tune up. Drove me crazy.
 
"Never overlook the obvious" is a nice way to put it. I was taught the "KISS" method. The "keep it simple stupid" method.
 
RustyRatRod said:
What's your procedure for diagnosis other than throwin parts at it? Not bein a smartass, but until you actually DO some REAL diagnosis, you may well keep throwing parts at it. Have you done a compression test, for instance?
Click to expand...
That's what I'm asking for. Some real diagnostic tips. Compression as far as I know won't cause an intermittent severe miss, or maybe you're just throwing that in there for extra pokes.
 
I think you might have a fuel starvation problem. Check for cracked fuel hoses. If they're good, check the sender and tank for debris.
 
Usually if a mechanical fuel pump fails or is weak, it's consistent, it doesn't sometimes work good, and other times not. Wouldn't hurt to test it though.
 
Do you have a tach/ dwell meter? Some of those have a points resistance scale. So check the points for condition. If you don't have one, Turn the key to "run" and check coil+ voltage. If it is full battery voltage the points are open. Bump the engine until the coil+ voltage drops. It should read about 6-10V or so. While you are at this, wiggle the bulkhead connector and see if the voltage drops badly.

Next measure coil NEG voltage. It should be less than 1 volt, better if less than 1/2 volt. This gives you some idea of points resistance. The higher this reading, the worse the points condition is

Do this as quickly as you can, you don't want to leave the key on very long with engine stopped. If you are in the habit of doing so, you may have damaged the points

Condensers are an "iffy" thing and modern Chineseoationized replacements are not particularly reliable

Next check spark. "Rig" a gap/ modified plug/ spark gap/ tester with a SOLID coil wire where you can see it. Crank the engine USING THE KEY. Do not jumper the starter relay for this test. The spark should be snappy and blue, and jump at least 3/8" and typically 1/2"

Check distributor for shaft play and stuck advance weights.

IF YOU DO NOT HAVE ONE wander over to MyMopar and download a free service manual.

If the timing seems reasonable, I'd start checking fuel. Get a pressure/ vacuum gauge, all real parts stores have them and they are not too much. You can check fuel pump pressure AND volume (read the service manual, chapter 9 I think) anyway fuel section.

Pull the plugs and post photo(s) of them
 
Wally-T said:
That's what I'm asking for. Some real diagnostic tips. Compression as far as I know won't cause an intermittent severe miss, or maybe you're just throwing that in there for extra pokes.
Click to expand...

I'm not poking anything. What if there's a broken ring? It can seal sometimes and sometimes not. Same with a partially burned valve or a worn guide....Hay, do what you want. My car runs great.
 
I've always heard that a condensor is always suspect. Change it and the points and set them perfectly of course. Next I always drop my fuel tank and clean it out. Yeah, it totally sucks but it's a good feeling to cross that off your list. You could be sucking up some tank debris every 10 minutes or so and clogging the pickup tube, which means zero fuel pressure.

I'd also test an extra carb.....some old original carbs on your engine may have been rebuilt but are "done" and junk.

If things persist after this, I'd probably run a voltmeter on your front seat on the "+" side of the coil just to affirm that your primary voltage is consistent.
 
full 12V to coil only shortens the life of the points, the condensor (capacitor) does the same, it extends point life. If that what works, your not gonna kill it by running it that way. change the ballast resistor again. Or check the coil voltage when its starting to chug. Then test across the HOT resistor with a multimeter and see what it reads for fun. It should read a pretty high resistance (low OHM). If you give the coil full 12V and it runs nice, you got no motor issues.
 
Last edited:
67Dart273 said:
Do you have a tach/ dwell meter? Some of those have a points resistance scale. So check the points for condition. If you don't have one, Turn the key to "run" and check coil+ voltage. If it is full battery voltage the points are open. Bump the engine until the coil+ voltage drops. It should read about 6-10V or so. While you are at this, wiggle the bulkhead connector and see if the voltage drops badly.

Next measure coil NEG voltage. It should be less than 1 volt, better if less than 1/2 volt. This gives you some idea of points resistance. The higher this reading, the worse the points condition is

Do this as quickly as you can, you don't want to leave the key on very long with engine stopped. If you are in the habit of doing so, you may have damaged the points

Condensers are an "iffy" thing and modern Chineseoationized replacements are not particularly reliable

Next check spark. "Rig" a gap/ modified plug/ spark gap/ tester with a SOLID coil wire where you can see it. Crank the engine USING THE KEY. Do not jumper the starter relay for this test. The spark should be snappy and blue, and jump at least 3/8" and typically 1/2"

Check distributor for shaft play and stuck advance weights.

IF YOU DO NOT HAVE ONE wander over to MyMopar and download a free service manual.

If the timing seems reasonable, I'd start checking fuel. Get a pressure/ vacuum gauge, all real parts stores have them and they are not too much. You can check fuel pump pressure AND volume (read the service manual, chapter 9 I think) anyway fuel section.

Pull the plugs and post photo(s) of them
Click to expand...
Awesome info, thanks.I forgot to mention, again, that it has a new fuel pump and gas tank. I suppose the pump could be faulty though. Also it can happen 30 seconds after cold start or not till 20 minutes after highway driving so temp doesn't matter. I do have a service manual but I'm not well schooled in diagrams. I'm sitting here with pages I printed out this morning from the manual and just looking at them makes me a bit tense haha. But thanks again guys, I have a ton of really helpful info to go through now. I'll keep you posted.
 
Also don't overlook the possibility of a sticking valve/ other valve train problems
 
toolmanmike said:
Condensers will do some wacky things like that. Still have points? Also, the new condensers are junk. Get a hold of Halifaxhops for a old stock one.
Click to expand...
Hey man, guess what??? It was the condenser!!! Thanks. I've put a couple hundred miles on it and it hasn't skipped a beat. I even ran the tank down to 1/8th. It's just a real nice cruiser now. Wahoo!!!
 
Wally-T said:
Hey man, guess what??? It was the condenser!!! Thanks. I've put a couple hundred miles on it and it hasn't skipped a beat. I even ran the tank down to 1/8th. It's just a real nice cruiser now. Wahoo!!!
Click to expand...
Good to hear it was something simple.
 
that is great news , lucky it was something cheap and easy to replace . that is the one thing I like about the old points distributors , with a few spare parts and a screwdriver you are never stuck for long , and the replacement parts are dirt cheap . not like the new cars , though they are reliable when they don't work it gets expensive in parts . like my nissan 800.00 in parts it is back on the road , no simple fix
 
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