1974 Camaro Z28 fouling plugs

[hursst]

Hello,

   I have a 1974 Camaro Z28 with the stock High Performance 350, running points/condenser.  A few weeks ago, my car started running very rough, on 7, then 6 cylinders, or so it felt like.  I made it home, checked the plugs and they were all badly carbon fouled, two of which were so bad there's no way they were firing.

 

   I replaced all the plugs, replaced the points/consdenser with an original Delco Remy "Uniset" combined points/condenser, but I could not get the car to even start with this set.  I put the old points/condenser back on (the points look like new), reset the dwell and timing and it seems to run okay, but it has a little engine missing and bogs very slightly if I floor it and open up the secondaries.  I re-checked the plugs after about 25 miles and they are starting to foul again.  Strangely, one side of the plug insulators look brand new, while the other side is dirty with carbon on all the plugs (See photos (all the same plug)).

 

   I'm running a generic set of points/condenser with about 3,000 miles on them.  I also run an NOS Delco-Remy coil that has the same mileage on it, both replaced about 8 years ago.  It was tuned by an old-school Camaro specialist mechanic back then (who has since retired).  The car ran great for 8 years and 3,000+ miles, until it didn't.  I'm guessing my problem is either the car is running too rich, or I'm getting a weak spark, or maybe both.  The spark plugs are one level "hotter" than the originals.

 

   Any advice on what to do next?  A friend says replace the condenser and coil before trying to adjust the carburetor, but wanted to see what the experts think before I moved on, as I am a rank amateur mechanic, especially with electricals.  Thanks, as usual, for any advice.

 

-Chris

[West Peterson]

Check your grounds. Clean them up. Perhaps you're not getting enough voltage.

[hursst]

Thanks West, I'll go over all the grounds on the ignition system and see what I find.

[TerryB]

I would say you are running on the rich side.  The plugs seem dry so I don’t think it’s any kind of oil fouling.  Do you get blackish smoke out the tailpipes?  The one sided build up I believe is the side that is getting the blast of gas/air mix on the intake valve opening.  I am NOT a professional mechanic, just commenting on what I have seen in my personal experiences.

[hursst]

No black smoke.  Engine only has about 5,000 miles on it since a rebuild about 12 years ago.  I'm thinking it's just running rich as well, but trying to get as much advice first.  Thanks.

[TerryB]

Check the engine oil for the smell of gas.  If gas is getting into the oil it thins it out to the point it gets past the rings and burns but poorly.  Had this happen on a motorcycle I owned when the carb floats were sticking and the oil became super thin.

[EmTee]

Is that one plug in all of the pictures or four different plugs?  If the latter, it's odd that the insulator is half black and half white.  Stranger yet is that the black side is oriented the same way on each one.  Did you purposely index the electrodes on those plugs when they were installed?  Are the pictures of the plugs you replaced, or the replacements?

[rocketraider]

This is puzzling. I've never seen coloring like that on a plug insulator esp in 25 miles.

 

Did the original plugs have sooty or fluffy deposits when removed?

 

Possibly some chemical in the gasoline formulation? though most of that type deposit I've seen is reddish. With that in mind how old is the fuel?

[hursst]

Original plugs had huge chunks of carbon on them.  Fuel was not particularly old, maybe 3 months old at most before I ran it out.  It is driven about every 2 weeks.

[EmTee]

2 hours ago, hursst said:

Original plugs had huge chunks of carbon on them.

OK, it sounds to me like the carburetor is dumping fuel into the intake manifold.  I'm guessing the black side of the insulator was pointed toward the intake valve.  The white side likely faced the piston and was directly exposed to the combustion gasses.  Possibly a leaky power valve?  Is there any evidence of flooding on the outside of the carburetor or stains on the intake manifold?

[hursst]

No stains on the intake.  There is a light oil residue on the sides of the carb, as there is some dirt trapped in it.  I think I may have to try to find a shop to get this properly diagnosed and repaired.  This is beyond my ability.  It could be any number of things in the ignition and any number of things in the carb at this point.  I don't want to keep throwing parts at it that may have nothing to do with it.  I bought a new coil and that made it run slightly worse, amazingly.  I'll have to put the old coil back in.

[Rusty_OToole]

Millions of Chevs run rich usually because the choke pulloff failed. I would check that first, and also rebuild the carb if it has not been done for a while.

[hursst]

A lot of comments about the choke pull off or extra fuel coming in, which seems like a plausible cause for my problem and something I think I can test by just applying about 25lbs of vacuum to the inlet port, if any of the you tube videos are correct.  Thanks again, I can try that.  Carb was rebuilt in maybe 2014 or so.

Edited August 28, 2023 by hursst (see edit history)

[rocketraider]

Does this engine have a QuadraJet or Holley? I'm thinking 1974 emissions standards would almost mandate the Q-Jet but I'm not 100% sure what Chevrolet HP engines got that year.

 

Just for grins, add some fuel system cleaner to the gasoline. Berryman B12 or similar. There may be some goo in the carb float bowl keeping a Q-Jet power piston and jet from seating properly and leaking by enough to cause this.

 

And if it doesn't improve and you end up rebuilding the carb anyway, the B12 will give you a head start cleaning it.

[hursst]

Has a quadrajet.

[TerryB]

Are you using an electric fuel pump?  If yes, that could be a possible reason for a fuel issue.

[hursst]

No, original mechanical fuel pump.

[Larry Schramm]

Have you swapped out the coil?  Might be a marginal coil.  Do you have an oscilloscope to check the voltage output.  This could change as the engine gets hot.

[hursst]

Yes, I replaced the coil.  I ended up having the carb completely rebuilt.  Most likely, the cause was the float set too high, running a little rich, and the choke was most likely binding and not fully closing.  I have the car mostly dialed in now, but need to put a few more miles on it and check the plugs again to be sure.  Thanks for the advice.

[alsancle]

Just yesterday I was having a conversation with a buddy of mine about a rebuilt fuel pump.  The new spring looked identical to the original but was in fact 3 times stronger, causing the pump to go from 3.5 lbs to 10 lbs which was overwhelming the needle valve and causing the engine to run very rich.     It took a lot of work to figure that out.

[Milt Packard]

Here is what I think after dealing with ignition problems on my '58 235:

- it ran, not well but it ran with 'old plugs, points and condenser'

- it did not start with new plugs, points and condenser.

- I think you are looking at too many things.

- verify voltage from coil to points

[hursst]

Thanks Milt.  It's running fairly well now, but I still have to pull some plugs after my next long drive and see how they look.  I'll also do a voltage check to confirm.

[Str8-8-Dave]

If the car is equipped with a Rochester Quadrajet carburetor I would be looking at the float.  If that carburetor has a nitrophyl plastic float it may have absorbed enough fuel to prevent it from closing the float valve, at best, or sunk at worst.  I had a brand new 1968 Pontiac GTO with one of those carburetors and the nitrophyl floats failed twice, first time causing an engine compartment fire that did serious damage.   Second one failed on the freeway and when I slowed on the exit ramp the car began to run rough.  I nursed it to my destination by holding the gas pedal down far enough to keep it from stalling while holding the brake pedal to control my speed.  Nex very cold winter day I took the carburetor apart and removed the errant float, drove to the dealer and got another and installed it.  That fixed the rich condition instantly.  If you could find a brass float to replace the plastic float I would highly recommend replacing it.     

[hursst]

Just had the carb fully rebuilt by a Quadrajet specialist.

[pont35cpe]

Agree with Str8-8-Dave. Those plastic Quadrajet floats were bad about getting saturated, and not as buoyant as they should be. Being rebuilt, dosen`t mean the re-builder installed a new float.

[Bloo]

2 hours ago, Str8-8-Dave said:

If the car is equipped with a Rochester Quadrajet carburetor I would be looking at the float.  If that carburetor has a nitrophyl plastic float it may have absorbed enough fuel to prevent it from closing the float valve, at best, or sunk at worst.  I had a brand new 1968 Pontiac GTO with one of those carburetors and the nitrophyl floats failed twice, first time causing an engine compartment fire that did serious damage.   Second one failed on the freeway and when I slowed on the exit ramp the car began to run rough.  I nursed it to my destination by holding the gas pedal down far enough to keep it from stalling while holding the brake pedal to control my speed.  Nex very cold winter day I took the carburetor apart and removed the errant float, drove to the dealer and got another and installed it.  That fixed the rich condition instantly.  If you could find a brass float to replace the plastic float I would highly recommend replacing it.     

 

1 hour ago, pont35cpe said:

Agree with Str8-8-Dave. Those plastic Quadrajet floats were bad about getting saturated, and not as buoyant as they should be. Being rebuilt, dosen`t mean the re-builder installed a new float.

This point cannot be overstated. The service manual would have you weighing the float on a grams scale to see if it has sunk. Someone unfamiliar with these carburetors and floats might think that is OK because the service manual said so. It isn't OK. When the cars were in daily use, these floats usually lasted about 80K miles and then sunk. NEVER put a used nitrophyl float back in.

 

hursst did send it to a Quadrajet specialist, so it probably got a new float.

 

Edited December 31, 2023 by Bloo (see edit history)

[hursst]

Thank you for all the advice.  The float was checked by the specialist and he said it was set a little too high.  He did not mention it leaking.  I've had this carb rebuilt multiple times from the ground up, with just about every part replaced with the factory spec part from Rochester by this same specialist, including replacements of the floats with modern equivalents.  The specialist mentioned the float was set a little high and my choke mechanism was binding pretty badly at startup.  I suspect maybe the choke wasn't closing all the way when I was driving it as the primary culprit.  The choke binding is completely gone now.

 

The problems appear to be solved at this point, I've replaced or rebuilt just about everything possible.  I will be doing a 70 or so mile drive on Thursday to really test it out and see how the plugs look afterward.  I'll also check the voltage out so make sure it's up to spec.  I checked the new coil previously, and it appeared to be reasonable with its output.