My '67 L79 is running pretty well, but it burns gas at a ridiculous rate...probably around 6-8 mpg. I recently pulled all the plugs and they all looked great, except for #8. Picture is below. I'm guessing potentially a stuck or burnt valve here, or potentially oil leaking into the combustion chamber. Doesn't seem to be the exhaust valve, as the rag test at the exhaust pipes does not point to any suction. Either way, this plug is black, crusty and not what one should see, especially considering there's less than 1000 miles on this plug. I just had shoulder surgery, which is a long recovery, so I won't be pulling heads or wrenching anytime soon. I'd like to find an honest, trustworthy shop that could diagnose and do the repairs. The 4 speed also likely needs a rebuild, as I have to double-clutch to shift into 4th to avoid grinding. I live about 2 hours west of Denver, CO, but am open to anywhere within a reasonable distance from here. I would likely ship the car if it's further than Denver, though. I've had bad experiences in the past with another car, so I'm really looking for a rock solid shop that has an impeccable track record of satisfaction. Thank you for your suggestions!

#8 plug. Black as coal and very crusty...not even 1K miles

 

What type of carb

 

Your fuel consumption rate, if accurate, suggests a good first step would be to get the air/fuel ratio correctly dialed in. If it were mine, I'd do that and then take another look at that #8 plug before letting anyone tear into the engine.

 

It's an original Holley 3810, rebuilt and fully restored by Eric at Vintage Musclecar Parts a couple of years ago.

 

If it was a mixture issue, wouldn't I be seeing it on more than one plug though? I've also done the idle mixture setting a number of times per the Holley spec, using the vacuum gauge on manifold vacuum.

 

then something is very wrong as you should be getting 12-15 mpg, unless you can't keep your foot out of it and your in the back two barrels all the time. while running does it smell rich and burns your eyes

 

Yep, it definitely smells rich. No burning eyes, but my clothes do tend to smell of gas a bit, as does the garage for a couple of days afterwards. (and I'm pretty gentle on the secondaries)

 

then you got some tuning to do before you go anywhere. start with the timing and dwell and go to the carb air fuel mixture screws. might have the wrong jets too.

 

Blown power valve? Do you recall any backfires?

 

My way of thinking about is that even if cylinder #8 was totally dead, such that you were running on 7 cylinders and if the mixture in those 7 was spot on, you'd be getting more than 6 - 8 MPG. So, regardless of whatever is going on with cyl #8, I don't think your carburetor is jetted correctly.

And one more point, the idle mixture screw(s) affect the idle mixture. Once you are running at hiway speeds, the jetting determines the air/fuel ratio. The mixture screws are out of the picture.

 

Call Vortecpro in Colorado bet he knows of competent people.

Once you tune the best you can do a leakdown test to better measure your engines healthy. no sense rebuilding if it doesnt need it. These didnt get great mpg new but should beat 6.

 

I don't now if Lars is still dong work but he can sort it out. Try emailing him. He is in CO.
V8FastCars@msn.com

Tom

 

Here is a basic question.....exactly how did you determine the milage? I'm not talking about the math. Dennis

 

I second contacting Lars

 

I'd definitely go Holley EFI! (sorry, just kidding....sometimes its fun to poke the bear)

 

I think Jim has a point - #8 is dead. The plug electrode shows no sign of firing from what I can see in the picture. First, determine if you have a bad plug or plug wire or distributor cap, rotor, etc. Check the porcelain to see if you have a crack. I have had many bad new plugs over the last 50+ years
Also, if you can, do a quick leakdown test, but if you can't, listen in the carburetor as you or someone cranks if over with the coil wire pulled, might not be able to tell if the valve seat good or not unless it is real bad. Listen for unusual lifter / rocker arm noise indicating excessive clearance or weak or broken valve spring. You don't have to pull the valve cover, just stick a hose in your ear and run it along the top of the valve cover. If you have no popping in the exhaust or carburetor, your probably have no mechanical issues.
I am hoping you have a bad sparkplug. If you move a known good one to #8 and clean up the one from #8 and put it in another cylinder the good plug was removed from and run the engine you can see if you get the same buildup and or performance (you have to have a miss with that plug) in a few miles.
You can use your timing light to monitor the plug / wire while the engine is running by putting it on the #8 wire and watching it flash at idle. Some plugs will fire until the compression increases with rpm, so that doesn't always give you a good indication, but may.

 

As I said, blown power valve symptoms…. Don’t want to get into the carb? Then try another Holley 3810, fresh plugs, you might be surprised

ps: regarding cylinder # eight, yeah some additional/ unique issue there, maybe sucking some extra oil from one or more of the known sources, combined with a super rich mixture, yeah, it’ll load up more than the others.

 

1000%

 

This is probably way off track, but are you running stock valve covers, or aftermarket-- and how is your pvc plumbed/

 

Everything is stock; pvc runs from the oil fill to the side of the carb.