The Stovebolt Site Forums The Shop Area The HiPo Shop Exhaust valves scale is this normal?

I had to pull the headers for some other maintenance, noticed the coloring of the exhaust valves. The picture is #4 and #6 exhaust port but all the exhaust valves have similar appearance. The engine is a carbureted 350 with iron vortec heads, about 20,000 miles since I built it. I always run 91 octane pure gas when I can get it. I'm just curious if this is normal or if there is some problem lurking. I'm not that knowledgeable about this and appreciate comments from those who have the experience.


Mark

Your first sentence says it all- - - - -"headers". Look at the gasket seal (or lack of it) around the exhaust ports. You've got a pretty major air leak there, which is doing the same basic thing as an extremely lean mixture- - - -overheating the valves. I'm a little bit crippled from a bike wreck a few years ago, but I think I could still outrun somebody who wanted to give me a set of headers!
Jerry

Thanks for the input, I get it about headers. I will look at the gasket seal and make sure it is solid. I machined the header mounting surface before installation 20 years ago so I may have a chance for a tight seal. Is this appearance typical of a too lean mixture? All the valves look the same. The plugs I removed recently look like this photo and again all the plugs looked about the same. If both the plugs and the valves indicate too lean mixture I will rejet the carburetor. Thanks for the help

Mark

It's very difficult to judge mixture by spark plug color since the tree huggers took the lead additives out of gasoline. The old "tan to chocolate brown" color rule of thumb went away at least 30 years ago. About the only way to monitor mixture these days is with an exhaust gas analyzer or an oxygen sensor in the exhaust stream. Plugs will go from white-looking to sooty black (too rich) with virtually no color change from lean to rich along the way. The rule of thumb on headers was once to increase the mixture toward the rich side to compensate for better scavenging, but that was only at full throttle under racing conditions. Other than increasing the underhood temperature, burning spark plug wires, and creating exhaust leaks, I've never been able to measure much difference in actual performance, at least in the RPM ranges most street engines run.
Jerry

I know Oklahoma doesn't require vehicle inspections since around 2002, but if you can find a mechanic or service station that still has the exhaust gas analyzer they used to use for the inspections (some places were allowed to keep the equipment from what I understand), see if they'd put the sniffer up your tailpipe and tell you what they find. Running too rich is one of the things it used to identify.

There are various hand held air/fuel ratio meters on the market but a good quality one will cost around $2500.00. That's a bit much for a hotrodder's hobby shop. The big multi-function console testers with an oscilloscope, volt/amp tester, vacuum gauge, dwell meter, and 4-gas exhaust analyzer used to cost somewhere in the $25K range, and most of them were obsolete within less than 10 years. A shop had to do a lot of diagnostic work to justify the purchase of one.
Jerry

Thanks for the suggestions. If I don't find a resource for doing the a/f testing, I will probably just increase the jet size a notch or two and go on. After the next road trip I can check plugs for the black sooty look. At some point though I will install a bung in one head pipe for an O2 sensor. A volt meter would then provide a real time approximation of a/f ratio while driving and under load towing.

Mark

An oxygen sensor generates its own voltage, supposedly from 0-1 volt and it changes very rapidly. A "perfect" 14.7:1 air/fuel ratio which produces exhaust emissions of nothing but CO2 and water vapor is 0.5 volts. The term for that condition is "stoichiometric". You'll need some sort of stand-alone computer that is capable of sampling the O2 sensor output several times a second. The older GM computers had a clock circuit that worked at 10 Hertz (cycles per second) but I believe most of the newer systems have a much higher sample rate. At one time MOPAR was using a 90 hertz clock circuit. I believe there are some aftermarket kits that include a dash gauge with a digital O2 readout.
Jerry

Yes I saw one of those dash gage systems at a parking lot cruise-in last weekend. I don't know the accuracy of course but the reading changed rapidly as the owner reved the engine and idled it. I think that would be helpful. I know it has been discussed at length on this forum previously.

Mark

In a full-function (closed loop) fuel system that uses either an electronic controlled carburetor or fuel injection, the O2 sensor acts as a final check on the other fuel metering devices like throttle position, absolute pressure, coolant temp, fuel pressure, and bunch of other computer inputs that change constantly. The O2 sensor is constantly hunting for that elusive perfect .5 volts, and it's yelling at the computer- - - - -"RICH- - -LEAN- - - -LEAN- - - -RICH- - - -LEAN- - - -ETC." and it's doing it 10 times a second or more. No matter what the computer does to adjust the mixture or ignition timing, the O2 sensor yells "You're doing it wrong, DUMMY!"

O2 sensors are female!
Jerry

LOL! That's a description of a closed loop system that anyone can understand.
That's why if the O2 sensor goes bad, the computer just keeps doing it wrong, as the O2 sensor isn't nagging at it anymore.

One interesting thing that a lot of folks don't understand, is that a cylinder with a miss, such as a bad spark plug or plug wire, low compression, or some other reason that prevents the fuel in the cylinder from burning, will make the closed loop system go full rich, not lean. Even though there's an excess of unburned fuel in the exhaust stream, that's not what the O2 sensor is measuring. There's also excess oxygen, which wasn't consumed by the combustion process. The sensor interprets that as "lean". The O2 sensor sees too much oxygen, and yells "COMPUTER- - - -GIMME MORE FUEL!"- - - - -and the poor henpecked computer does what it's told!
Jerry

59 fleet I think your idea to richen-up a notch or two is a good move,looks OK on your plugs if it runs good better leave it alone.