wheatpj

I had my 71 Big Block in the garage searching for the cause of a miss that developed during some stop and go traffic on what was otherwise a perfect drive. I remember thinking earlier in the drive how well the car was driving and how good the engine sounded. The engine started missing on idle and pulling away from stops by ran fine after I got it to 3rd and 4th, even under moderate acceleration. I sort thought it might be too much heat; one of the header tube passes pretty close to the fuel line. I took the car home and didn’t tough a thing. Next day I run it around some back roads close to my house…no problems, runs fine. So went home and I let it cool down and pulled the plugs. All were sooty around the base but the electrodes were clean and insulators were white to light gray-brown. One plug was a little cruddy, but there was enough clean electrode that it shouldn’t be prone to misfire. I tested the compression. It has tested higher, but the engine was pretty cool. The compression readings were even cylinder to cylinder. Got everything back together except for the aircleaner, and started the engine for a listen. The engine was fast idling with the choke plate closed about a ¼. The light in the garage is so so, and after 4 or 5 minutes I notice the header tubes (Silver-Ceramic coated Hedman Hedders) are beginning to glow red. I knock the engine down to an idle, tubes stop glowing. I work the RPM up to 2000 (choke plate wide open) and hold it for about a minute or two and the tubes start glowing. Same thing with the aircleaner installed, but maybe not some fast. Am I too lean? The soot on the plugs would have led me to believe I was on the rich side. Is it normal for header tubes to glow like that?

gkull

glowing tubes on an unloaded motor is usually a sign of over rich A/F mix. The fuel is still burning in the pipes.

AzMotorhead

The black soot around the base of the plug is also a indication of running rich. Check your timing first. might have a bit too much retarded.. picking up and running better at higher rpm leads me to believe the mech adv is working as it should.

REELAV8R

This was my first thought as well. Timing too retarded.

jnb5101

is the vacuum advance connected to a manifold fitting?

wheatpj

I can jet down two sizes, but I worry about too lean. Won't too lean damage the engine.

wheatpj

Yes, vaccum advance connected. I was checking some of the vacuum connections, pulled one off, could hear a strong vacuum hiss and engine sped up. What does that indicate...not enough adavnce?

drwet

I would agree with checking the timing. Many years ago I helped a buddy fire up a freshly rebuilt small block for the first time. Of course we fired it up and immediately ran it up to about 2500 rpm for about 20 minutes. By the time we got this done it was starting to get dark. At the end of the twenty minutes the cast iron exhaust manifolds were glowing red. Turns out we had installed the distributor a little retarded (or maybe that was the guys who did it). Anyway, life lesson # 338 - if you run the engine with the timing retarded, the exhaust system gets hot.

Solid LT1

I'm going to put forth a couple of items here, if your running rich then your timing is too retarded causing fuel burn after exhaust valve opening. I think there is a chance you are too lean in fuel mixture and timing is retarded. You can buy a Infra Red non contact thermometer and read header pipe temperatures. You can also buy a Wide band air fuel meter for under $150 to give you a good idea of where your fuel mixture is running rather than guessing at it.

Years ago, I lit off my first Mopar 440 Six Pack build that had a big Engle solid lifter cam, ported heads and stock jetting in the carbs. We watched the headers glow red about 30-50 seconds after fire-up, I was breaking in that big flat tappet cam and did'nt want to shut it down. The headers glowed away destroying a sprayed aluminum coating while the cam/lifters got happy with each other. Eventually I went up 8 jet sizes in the primary carb and had quite a collection of re-drilled jets plates for the secondary carbs. That old lump of Mopar iron eventually ran 11.85@123MPH about what our stock C6 Z06 is capable of running today. I spent countless hours and dollars on that old lump of iron, now Joe Blow can buy that performance for no money down.......

scottyp99

When you pulled the vacuum hose off, you allowed unmetered air to enter the intake manifold, basically, you created a temporary vacuum leak. The fact that the rpm raised corroborates the rich air/fuel mixture theory. The mixture is rich, so if you give it a little extra air, it leans it out, coming closer to ideal, and rpm rises. Obviously, this is on the idle circuit, and has nothing to do with jetting. Have you ever set the idle mixture screws using a vacuum gauge?

Scott

7t9l82

if you have a holley carb or similar , before playing with the jetting you can open the transfer slot in the secondaries a little to lean out the idle. that goes with taking the vacuum hose off and it running better. but i don't have much detail on what you have.

427Hotrod

Yeah,,,but that 6 Pak will always have more cool points than anything bought today!

And all that time taught you how to tune and make a combo work. Many folks never quite get that and can't figure it out unless the OEM did it for them.

And 123 MPH is high 10's if it hooks...you must have been putting on quite a "slip and slide" show!

JIM

Solid LT1

Jim your one of the smart guys around here you have paid your dues too, I think your Vette is AWESOME! Waiting to see an episode of Street Outlaws where the fast Texas cars head North and give some driving lessons to those OK City boys (my favorite vehicle...Farm Truck!)

We bought the used 9"x29"drag slicks off of NHRA class racers running Stock Eliminator because we couldn't afford anything else, or perhaps didn't know any better in my youth I couldn't afford the good slick tubes, I bought truck tubes from NAPA.

I bought mine form a guy named Len Schnider, he raced a 69 428CJ Mustang at one point in time he had the NHRA national record in C/Stock automatic. The Machine shop I used was Hollums Machine in Fremont, CA Larry was a pretty decent guy back then his shop is in the SA Design book How to Blueprint Engines by Car Craft Editor Rick Voegelin, Larry moved to Washington back in the 1990's his brother-in-law Ron Lindvig was a super smart engine guy too, has a shop in Medford, OR and he built the L-88 motor that Elliot Forbes Robison used to win the final A Production SCCA Championship in late 70's. The guy who did cylinder heads on that AP Vette is a friend and without a doubt the SMARTEST motor guy, I have ever had the pleasure to know, name is Bob Wirth, this guy is as smart if not smarter the Smokey Yunick, currently building an Austrailan 400 cu/in Pro Stock motor, the one he built for the guy last year was an inherited project with many mis-matched parts Bob would have never specified for building one of those bullets, the new motor ought to be something, I watched dyno runs on the mis matched motor and it pulled 10,800RPM......1100HP UNBELIEVEABLE sound. I like Corvettes but, my real passion is motors and I think you are the same, we also agree that one of the better Detriot platforms is the Corvette, it just took me a while (over 10 years of first playing with Mopars, then Camaros to realize how great the Corvette is.)

I think attendees at the 2013 SEMA Optima Ultimate Street Car Challenge are going to be schooled into what a great platform Zora Duntov gave us when Danny Popp and Brian Hobaugh show up with the family C3 and C2 Corvettes (can you say "dad can I borrow the keys to the Corvette?) All these guys dumping tons of $$ into Fbody cars are going to learn a live axel in the back just isn't where it is at no matter how much you dump into your 3 link/4link whatever link suspension.

LOVE your C2 Jim keep it going straight down the road

wheatpj

Okay, I checked the timing today. It was at 12 BTDC. I played around with it. It didn't like 16 BTDC, and it definitely didn’t the like the factory 8 BTDC setting. I am now set at a 14 degrees advanced .

I noticed I didn’t pick up much of a jump in ignition timing when I plugged the vacuum line back into advance. Vacuum from the line didn’t seem very strong. I was getting ready to hook up a vacuum gauge when I noticed that the vacuum pulloff on the Quadra-jet had separated. I guess I could have plugged that port and kept going, but I didn’t. I found another pulloff so I will be back at it tomorrow.

The idle mixture was adjusted with a vacuum gauge and also checked by tipping off the airflow. I ended up backing out the mixture screws though to combat backfiring through the exhaust on de-acceleration.

This leads me back to my original question, am I too lean? Do exhaust temperatures increase under a lean mixture? Would this cause the header tubes to glow or is an over rich mixture necessary for that? Can too lean damage an engine. I have re-jetted several times since the carburetor came back from the rebuilt. The jets that were put in it seem really small for a 454. Off idle and cruise was good, but the engine sounded like it was really lean. The plugs looked really hot (white all over). I think the jets in there now are 78s. Plugs now are sooty at the base, but the electrodes and insulators are white/light gray-brown. Exhaust tips are sooty too. I can go back down a couple of sizes, but I don’t want to do anything that would harm the engine. The headers are a recent addition and I would have thought there addition would cause the need to fatten up the mixture. Looking for advice.

Thanks

bluedawg

You should be 8 to 12 degrees btdc or before top dead center. Firing after tdc sends burning air fuel out the exhaust causing your glowing header pipes.

Solid LT1

What carb is on your motor Holley 750? 850? Need the List on choke tower. Thinking you have List 4779 would be 750 double pumper #78 primary jets should be in the ballpark for correct jetting, do you have any vacuum leaks? Have you eliminated the headlight vacuum feed? Power brake vacuum feed? Glowing headers are usually a sign of retarded timing, you said timing is 14BTDC that is plenty of initial advance to rule out being retarded as long as balancer marks are true ( is balancer stock? Could the outer ring have spun?) What are your idle vacuum readings? I would be checking vacuum hoses by pinching off with pliers also using carb cleaner spray and spray at carb base intake to cylinder heads too see if RPMs pick up, sure sign of vacuum leak. Try tightening intake bolts, carb nuts. Good luck, come back after trying some of these tests.

bluedawg

He said 14 atdc.

wheatpj

My bad, all the marks on the tab are before TDC. My timing is advanced 14 degrees.

sorry for the confusion...I will edit my post accordingly

Thanks

wheatpj

I have a Quadra-jet. The balancer is stock; it may have shifted a little. The engine doesn't run well at the factory setting of 8 degree advance. At that setting it runs kind of lazy. It runs better (idle speeds up and quality improve and more get up and go) with more advance. When I get to 16 degree advanced the engine acts like it's too much.

The ignition advance is off a port on the carb. I will replace the vacuum pull-off and check the readings.

The jets now in the Quadra-jet are #78s.

scottyp99

When you say you're at 14* advance, is that with the vacuum advance hooked up, by any chance? And, are you using ported vacuum or manifold vacuum for vacuum advance? If any of these terms are confusing, here is a good article explaining the basics of ignition timing:

http://forums.corvetteforum.com/c3-t...long-post.html

Scott