1968 HELP! Car suddenly running bad.

[TexAg]

Okay, guys. It's been a while since I've posted, but I have a new problem. I've searched the threads, but nothing seems to fit. My car has just suddenly started running poorly.

Starts fine. Idles normally, but with an occasional hiccup. But, idle drops when I put it in gear, and it will eventually die if I keep my foot on the brake. In neutral or park, it will idle. Also, it stumbles on acceleration, but runs fine when driving.

I know there are a lot of threads about poorly running cars, but they all seem to refer to some recent change. I haven't changed anything on this car in months.

So, my two guesses are either fuel related, or spark related. Maybe bad gas? I'm going to change the fuel filter and maybe run a little carb cleaner through the system. What should I check for the ignition system? I'll check the timing this weekend, but I'm not sure why it would have changed, and I guess it wouldn't hurt to do a plug and distributor change, but again, that seems like a long-shot with the only symptoms being as described. Any other recommendations?

Car is a J code automatic.

Thanks in advance.

 

[somethingspecial]

I had the same thing on a J code Auto I had, then it finally died all together. With help from guys here, I changed the point condensor in the distributer and presto, the car ran fine. Might try that first, it's a cheap start. no pun intended. Does your car have Pertronix ignition? Mine failed, put points back in then the rough running started. Condensor failure. Just my experience.

 

[Midnight Special]

...Before you go to a lot of trouble, check all your vaccum lines, plugs & etc... especially if you have power brakes and if the car dies or idles down with brake pressure. My 'J' did the same thing after a long slumber... I found & replaced old lines & a few plugs, plus re-adjusted the idle mixture screws on the Holley. Problem solved!

 

[Mosesatm]

Another culprit may be dirt in the carburetor needle/seat, or a stuck needle.

 

[SD GTCS]

Check the coil also

 

[TexAg]

Great suggestions. Thanks, ya'll!

 

[rvrtrash]

All excellent suggestions. If it hadn't been for the comment about not changing anything in months, I would also suggest checking your vacuum line routing to the distributor. If the lines from the intake and carb are reversed on the ports in the thermostat housing cover, it will do this.

Steve

 

[robert campbell]

I have been at the lake all week. What carb are you running? Is it the stock Autolite 4300? Vacuum lines are always a good start. Does it have all the smog lines and the dual diaphragm distributor? Does you car have the Thermactor system? A vacuum gauge can be handy, but in your case a slowly dying engine at idle will display the vacuum leaving as it slows down. The real source of the issue may not be vacuum related.

A quick idle check is to start with your cold engine and slowly spin the idle mixture screws in until the seat down gently while counting the turns. Then remove them and prop the carb open with a long small screwdriver in the air horn. Spray Gumout or you favorite carb cleaner directly in the idle mixture screw holes. Put a towel under the screw holes to keep the cleaner off your intake if it is a pristine painted one. The cleaner is tough on paint!!

Dry it up and if you have small air compressor, blow out the screw holes also. The install the screws and turn them back out to the original setting. If they were at different turns out, split the difference and set them at the same turns out. Start the car and drive it until it is at full operational temp. Now you can experiment with the idle screws in 1/4 turn increments. Take them both out a quarter of a turn. If the engine starts to idle faster and better, then try out another 1/4 turn. Keep your hands out of the fan!! If the idle gets even faster you will need to back out the throttle stop screw on the left side to slow it down to normal. The optimum setting is when you turn it out and it makes no noticeable increase in idle RPM. If it does not then turn it back to where you just had it.

Also a bad diaphragm on the distributor can happen all at once. With the dual set up, the diaphragms actually pull against each other and the thermostat housing sensor works some magic (?) until the engine is warm.

If you are a not a concurs guy, eliminating some of this smog stuff can work wonders on how your car runs.

Rob

 

[TexAg]

Ok, more information..

Carb is an Edlebrock 4 barrel. I don't know which model. Checked vacuum and hoses. All looks good, and no problem with vacuum. Also checked PCV valve.

I pulled the idle screws and cleaned with carb cleaner as suggested. It appeared to make an improvement, until I took it for a drive.

The problem seems to get worse when the engine gets warm. Do I remember correctly that a coil/ignition problem will sometimes only present itself as the engine gets hot?

How do I check the coil, or do I just replace it? I hate to buy parts that I don't need. Also how would I check the distributor diaphragm?

Thanks again.

 

[gtcs1]

Ok, more information..

Carb is an Edlebrock 4 barrel. I don't know which model. Checked vacuum and hoses. All looks good, and no problem with vacuum. Also checked PCV valve.

I pulled the idle screws and cleaned with carb cleaner as suggested. It appeared to make an improvement, until I took it for a drive.

The problem seems to get worse when the engine gets warm. Do I remember correctly that a coil/ignition problem will sometimes only present itself as the engine gets hot?

How do I check the coil, or do I just replace it? I hate to buy parts that I don't need. Also how would I check the distributor diaphragm?

Thanks again.

Hi,

Someone was telling me to check the coil by using a timing light located in the car, run the cable thru the window inside the car, run your car and with a friend (to help checking ) check the timing light flashes when this happens if you see the light stopping, that could be a sign then of a bad coil.

Anyone on that?

gtcs1

 

[robert campbell]

Electric or heat stove or manual choke? It is not the coil. Do you have electronic ignition or points? Unplug the vacuum lines and suck on them and then place your tongue over the hose end. If it bleeds off immediately the diaphragm is bad. Do you have dual diaphragms?

Rob

 

[Mosesatm]

Clean the hose first!:wink:

 

[franklinair]

IIRC, Edelbrock 500CFM, electric choke. Pertronix ign, single distributor vacuum. Don't remember if I put a Pertronix flame thrower coil on it. Sounds like a vacuum leak to me, or the Pertronix failed.

Neil

 

[Mosesatm]

You may also want to check the fuel filter.

 

[TexAg]

Thanks, Neil! I knew you would come through. I agree that it does sound like vacuum, but it doesn't make sense that it gets worse as the motor get hot. I'll just replace all of the vacuum hoses. That's cheap. I'll also test the distributor diaphragm to see if it's holding. I'm planning on a fuel filter also, but again, it seems strange that it would only be a problem at idle, and not under load.

Anybody have a suggestion for testing to see if the Petronix failed?

 

[Mosesatm]

You're right. I had a senior moment and switched your issues.

The exact same thing happened with my J-code with a 4300 and am pretty sure the problem was dirt that found its way to the carb. I think the dirt was so small it went through the filters. The car ran great at full throttle but it wouldn't idle at all. I had to heel/toe the throttle and brake to keep it running at lights. My guess is that the dirt went past the needle/seats and plugged up the slot for the idle circuit, but I'm not sure because I removed the carb and replaced it with another. I then sent it to Rob for a rebuild.

 

[franklinair]

1.) Plug the vacuum source to the distributor. See if it corrects the problem. If not,
2.) Plug the vacuum source to the PB booster. See if it corrects the problem. If not,
3.) With the engine running, warmed up, spray WD40 around the edges of the intake manifold. If the intake gasket is leaking the engine will smooth out momentarily when the lubricant seals the leaking spot of the gasket. If not,
4.) Plug (block off) ALL vacuum outlets at the fitting at the rear of the intake manifold. This will eliminate all systems using vacuum: PB booster, distributor, AC. And don't forget the vacuum line to the transmission.
These steps pretty well isolate any vacuum leaks.
How old is the gas? If you're running ethanol in the gas, it could be a factor. (I don't use gas with ethanol additive.) Some gas stations sell non ethanol gas.
Hope this helps. Let me know, yea or nay.

Neil

 

[robert campbell]

My experience with Pertonix failure is complete failure and not a missing. I have had 4 cars in this winter with Petronix installed. I always wondered bout the adjustment of the light chopper. I called Pertronix and they indicated that the top of the light chopper should be even with the top of the electronic module. Most installation I have seen has the chopper to low. Pertonix said to put an o-ring under the chopper to space it up.

I did this and it helps them work better. Also on a couple installations I have noticed that the light chopper spaces the rotor up to high and the rotor is making minor contact with the distributor cap around the electrodes. There was a manufacturing burr on the bottom of the rotor and quick trip to the vice with a file allowed me to get it lower and stop the contact. I was surprised how much I removed on one application to get the rotor down and away from the cap.

My vote is still dirt and possibly a high float level. Many of the Edelbrocks I have worked on came out of the box with to high a float level. This would show up at hot idle. The float level should be set to a 1/2 inch drill bit under the toe of the float opposite the needle valve end with the gasket installed.

I can step you through the procedure to remove the top of the carb while on the car. You can then see how much dirt or water is in the float bowls. The ethanol gas really works on aluminum and craps them up in a short time.

Rob

 

[TexAg]

Thanks, all! I ran through the other vacuum leak possibilities yesterday, but didn't think to check the intake manifold gaskets. That actually makes sense to me as the leak may get worse as the gasket heats up. Right??

Anyway, I'll try that this week. If that doesn't do it, I'll take you up on the offer to walk me through the carb. I try never to mess with them, as I tend to make it worse. :grin:

The gas isn't too old, but that doesn't mean it isn't bad. As for ethanol, I try not to use it but its hard to not to around here.

Oh well, I was getting bored with just driving it around and enjoying it. It's about time to bang knuckles and get frustrated again..

As usual, thank you all for your contributions. That's what makes this a great family. I'll let you know what I find.

 

[BroadwayBlue]

You're right. I had a senior moment and switched your issues.

The exact same thing happened with my J-code with a 4300 and am pretty sure the problem was dirt that found its way to the carb. I think the dirt was so small it went through the filters. The car ran great at full throttle but it wouldn't idle at all. I had to heel/toe the throttle and brake to keep it running at lights. My guess is that the dirt went past the needle/seats and plugged up the slot for the idle circuit, but I'm not sure because I removed the carb and replaced it with another. I then sent it to Rob for a rebuild.

Arlie, I have the same exact problem with my C-code with a Holley carb. Sometimes mine tends to improve as the car warms up, sometimes not. While cold I have to do the same thing at stop lights, etc (heel/toe brakes and gas to keep it running).

Replaced the carb because the original one the prior owner put in seemed to be dumping too much gas. Holley agreed it was too big.

The replacement didn't fix the idle problem though. Still have it.

It's on the list to diagnose at some point. For now I double foot it.