[dv657172]

Octane is 91 - Shell Premium

[steve25]

No where what’s needed for those cylinder pressures your coming up with!

[mgarblik]

To decarbonize, I make a mixture of 50% water and 50% a good chemical top engine cleaner. Some people just love Sea Foam. I guess that is OK in a 50-50 mixture. I much prefer Techron in a 50-50 mix. Stir your mixture together and get the engine up to operating temperature. Find a small, centrally located small vacuum fitting on the intake manifold or at the carburetor base. Hold the engine speed at 2000-2500 RPM and insert a 3/16"/5/32" small vacuum hose in the mixture and draw it through the engine. Do NOT stall the engine. You should see all kinds of crap and smoke out of the exhaust. After all the chemical mix has been run through the engine, I run about a cup of just water through next. After finished, engine should idle nice and smooth. Change oil and filter and go have some fun with your car. I would recommend some higher octane fuel if you can find it.

[1968GTO421]

Originally Posted by dv657172

Understood, the engine was cold when it produced the high compression figures of over 200, when the engine was hot the readings were all lower at 195 and below.

Compression ratio is 10:1 on the car, initial timing is 11 degrees and total is 32 degrees. I honestly am not happy with my vacuum advance as that appears to be able to create 19 degrees of advance. I haven't noticed any knock although admittedly the car hasnt been out much in quite awhile.

If the engine does have some carbon buildup what's the best way to clean it up without tearing it down?

Over 55 years ago I had '53 Chevy with a badly carboned up 235 six caused by the PO never having the carb rebuilt. I got a kit and rebuilt the old 1 bbl and put in new plugs. But it still had problems from all the carbon it. An old mechanic (started before WW II ) told me to put a small block under the throttle lever to run the engine about 2000 rpm and then turn the garden hose on a little and run it down the carb without stalling it. After a few tries I got the hose regulated enough to keep from stalling the engine. After a few minutes the steam cloud out the tailpipe had all this black stuff (carbon) in it. It took about 20 minutes but finally the cloud turned whitish again. Cleaned the sparkplugs again and went for a test drive. Ran so much better and could again run regular gas in it again.
FWIW

(A "pre-historic" version of what Mike recommended. Nowadays I would go with Mike's recommendation, its great advice.)

[Schurkey]

There's been all sorts of methods and procedures for removing carbon; ranging from "Italian Tune-Up" (Heavy throttle, high RPM) to dumping various chemicals down the throttle body (Solvents, water) or solvents into the gas tank, to connecting a "sand blaster" filled with RICE to a spark-plug hole.

Clearly, the cheapest, easiest, and least-distressing to the neighborhood is to use water at fast-idle as described above. Solvents work--but make so much tailpipe smoke the Fire Department may show up.

I drizzle a "urine stream" of water back and forth between the primary venturis at "beyond" fast idle--2000--3000 rpm. It's probably best to change oil afterwards, there'll be a lot of moisture in the crankcase. And don't be shy about using "too much" water. Use plenty!

[Sirrotica]

In the dealerships we used GM top engine cleaner, as per the directions, at home I just use plain water. Having used both, I really can't determine that one is better than the other. I know that with water, your wallet is happier.....

Run on, is a classic symtom of carbon accumulation after the ignition is shut off. No matter what though, the car will run much better, and idle smoother after you decarbonize it. I use a pop bottle, or a water bottle to pour it in, the small neck is useful to modulate the quantity you're pouring.

You will see some steam out the breathers if the engine is so equipped, entirely normal.

[dv657172]

Thanks for all the carbon cleaning info. I will definitely keep it in mind. Having checked the compression on the engine cold and all the cylinders were consistent I dont see this as a cylinder #7 issue.

Is it possible that by the time I get to checking #7 when the engine is hot that enough fuel has been pulled from the carb to try and cause the car to try and fire when cranked? The car has not had many miles since the engine was rebuilt although that was a while ago...

Also the octane I am using is actually 93, I previously stated 91 incorrectly.

Thanks for all the help and info
Really appreciate it