CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

1975 Chevy Nova 350 ( 8 Cyl )

The spark plugs foul out often. Only 2 are clean and 6 are just a mess. Mechanic says he adjusted carbo but it didn’t help. What can I do to stop spark plugs from fouling out in a month’s time. Please help.



Steve

Use hotter plugs. The piston rings may be worn resulting in oil consumption and fouled plugs.

Are the plugs fuel fouling or oil fouling?

If they’re oil fouling, you need to fix the source of the oil.

Can you tell us something about the engine and it’s mileage, history, and overall condition?
Any other symptoms, like burning oil?
Has any attempt been made to flush the system with an oil additive on the possibility that the oil rings are gummed up?
Has the compression been checked?

Does smoke come out the exhaust? Black or grey?

Thank all of you for responding. The plugs are black and crusted when removed, car isn’t burning oil but I’m going thru a lot of gas, mileage says79,000 but I know that can’t be right, it had 73,500 mi three years ago when purchased. I haven’t flushed or used any additives nor has the compression been checked. Once I put new plugs in, car runs perfect, however, If I don’t allow car to warm up with feet slightly on pedal for about a minute, it runs bad until I floor it a couple times and then it will run fine. overall cond is ok.

It sounds like carbon fouling from running too rich. Oil fouling leaves a black oily deposit. It sounds like this one’s dry.

While hotter plugs may help, before doing that I’d want to check the fuel pressure regulator. Excess fuel pressure could cause rich running.

You should not need to keep you foot on the pedal while the car warms up. Is it possible that the carb was misadjusted to compensate for a choke problem? If you don’t keep your foot on the pedal while it warms up, what’s it do?

You may want to find a good carb guy.

Thanks Mountainbike, If I remove my foot while warming it up, it shuts off. It will only remain running while foot is on pedal. Carbo was rebuilt April,08 due to sticking float then it was adjusted Jan,09 for running to rich. I will try the fuel press reg and have mech to readjust carbo again. Is the choke the reason why sometime I have to hold pedal half way down approx 10 secs and then give it a couple of strong revs to get car to stay running and run smooth at times?

If you aren’t getting a good spark, you may not be getting complete combustion. This would cause the spark plugs to foul out. The problem might be bad spark plug wires. One quick test is to start the car on a humid evening after the sun has gone down. Look for arcing around the plug wires. A more accurate way is to have a garage use a scope on the ignition system that will tell the mechanic how high the voltage is at each spark plug.

If the ignition isn’t the source of your problem, then I wonder if the float is set too high in the carburetor or the choke is sticking closed.

Do you have a good carb guy? That’s my first guess.

Honestly I don’t know. I had problems before and after carbo was rebuilt. It looks good though.

There are many potential problems that result in the same symptoms. I believe this car has an adjustable choke based on thermostatic spring control. Try setting that to specs first. I don’t remember a fuel pressure reg, as the carb fuel was controlled by needle and seat valve. The idle speed needle and jets needing cleaning is also another distinct possibility.

Thanks to all of you guys for your responses. I have to head off to work now and I will be back here tommorrow evening for more of you all suggestions.
Thanks

What kind of plugs are you using? I would use only ACDelco, and nothing hotter than an R45. I have seen some people use as hot as an R48, but I wouldn’t. For your apparent fuel problem, have you considered installing a manual choke? Sure, it’s very old school, even by 1975 standards, but it gives you maximum control over your warm-up, and if there’s any problem with it, you instantly know about it and can fix it quickly and easily. Many of my '70s and '80s cars I have owned I have retrofitted with manual chokes and have loved it. The kit sells at any auto parts store for less than $20 and installs in about an hour or less.

“the same mountainbike”

“While hotter plugs may help, before doing that I’d want to check the fuel
pressure regulator. Excess fuel pressure could cause rich running.”

Never knew a 75 Nova with a carb has a "fuel pressure regulator "

Try a new choke thermostat, and adjust your fast idle screw so it works. check the choke pull-off if there is one. It’s a little vacuum diaphragm. Make sure the vacuum advance is working too, not just holding vacuum. Back to the choke: There should be markings on the carb. If the choke thermostat is turned past medium, you can tell.

It definitely sounds like too rich a mixture. I could be as simple as an adjustment. But it could be more complicated.

Try to find a good carb guy to try and adjust it. If not it might be the carb itself and how it was rebuilt.

If adjusting doesn’t work you have a couple of options. Get a NEW aftermarket carb…or upgrade to a fuel-injection system (this is expensive). A new aftermarket carb is just a couple hundred bucks…Some are a direct replacement that will take you about 1/2 to replace.

Does this have an idle stop solenoid? In the early 70s these were added to prevent dieseling. With higher cylinder temps (higher T-stats + leaner operation) the pistins could draw fuel from the float bowl when you first turned the engine off and it would self-ignite. When the solenoid was deenergized (key turned “off”) an idle stop solenoid allowed the throttle to close completely and choke the engine, stopping the dieseling.

If it had am idle stop soelnoid and that was removed, the engine would stop dead when you took your foot off the pedal. Someone may have tried to compensate for the engine stopping by adjusting the carb richer.

I’m theorizing here, but on a vintage Nova anything is possible. As a matter of fact, now that I think of it, do you know if this is the original carb? If the carb is too big for the engine it may run too rich.

“The plugs are black and crusted when removed,”…A rich mixture leaves the plugs black, dry, will no visible crusty deposits. Crusty deposits are always oil fouling. This car has (should have) HEI ignition. which will fire almost any plug…But in the mean time, find an old-time mechanic who knows how to set up a Quadrajet carb. A simple, basic exhaust gas sample will verify his work. He should use a gas analyzer to PERFORM his work…

The OP should take a look at this, let us know which picture best matches his plugs: