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I have a ’69 396 BB engine in a 69 Camaro that I’m working. The engine was totally rebuilt 10 years ago and I installed new rod bearings, rings, mild performance cam, new timing gears, and ground the valves and seats. When the engine was fired up it ran like a fresh motor, not issues. It had a stock points ignition and a 4 barrel Qjet.
The engine then sat for 8 years due to unexpected issues in life. I rebuilt the carb to insure no stale gas/varnish issues. Upgraded to a new HEI distributor and new wires. I insured that the distributor was installed correctly ( #1 at TDC with rotor pointing to #1 tower). Fired it up and the vacuum gauge fluctuates rapidly between 11-14 inches. It backfires through mufflers. Adjusting the timing does not improve the symptoms. Ran a compression test, cylinders came in at 110 psi with minor variance. Any ideas on the root cause? Vacuum leak? Sticking valves? I want to do a cylinder leakage test next. Should I do that with a warm engine?
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The rapidly pulsating vacuum gauge leads me to lean toward a mechanical issue. I would remove the valve covers and make sure all the rocker arms are still opening the valves fully and they are all closing again..
I am thinking you may have a lifter problem such as the valve is gummed up or the plunger pumped up or wont leak down. This will hold the exhaust valve open where you can get a backfire in the exhaust from unburnt fuel. Remember, you turn these down past zero lash to center the plunger.
Check to see if all the push tubes are squirting, this will tell you if all the lifters are leaking off. It should squirt a little on every lift and that can affect your vacuum reading.
Assuming he has hydraulic lifters not solid lifters. If hydraulic and sticking or not pumping up due to varnish you could try this old trick. Run engine till warm not hot, add 1-pint of kerosene to the oil in the crankcase, run engine 5-minutes, DRAIN HOT OIL WELL, change filter, put in fresh oil, run engine until operating temp. and notice any change in vac readings. NOTE: This can be risky depending on internal condition of engine-you say it is fresh. I’ve done this about 25 times over my life WITH STORED LARGE V8 ENGINES ONLY and have an 80% success rate with no mechanical failures for 100%. The kerosene is the equivalent of putting a small amount of penetrating oil inside the entire engine. “There is no replacement for displacement!”:)
Disconnect the exhaust system…then try to run it…..
I have seen this before when cars have sat a while… the exhaust may be partially blocked… They can get plugged up from varmits, their nests and so on.
[quote=”sunset” post=64389]Assuming he has hydraulic lifters not solid lifters. If hydraulic and sticking or not pumping up due to varnish you could try this old trick. Run engine till warm not hot, add 1-pint of kerosene to the oil in the crankcase, run engine 5-minutes, DRAIN HOT OIL WELL, change filter, put in fresh oil, run engine until operating temp. and notice any change in vac readings. [/quote]
I believe this is an oval port engine with hydraulic lifters. Only a true muscle car had the solid tappet camshaft such as a Chevelle.I was going to suggest backing off the valves to zero lash to identify the valve causing the misfire and possibly adding some Marvels Mystery Oil to work on the stuck lifter. I typically use 4 oz per quart of oil but have used as much as 35% on occasion.
I have used fuel oil (Diesel) as a solvent with great success and its like using the solvent based engine flush. 20% makes a good cleaning flush but if you mix a little of the detergent flush in with the kerosene, it really cleans the deposits away. I do a simple flush once a year to get rid of deposits that affect the piston rings and cause undue drag on the engine, it usually runs very good afterwards.
I learned about after working on diesels that had fuel dilution problems, the internal parts were very clean and polished from the detergent action of the diesel fuel. A machinist also told me to dribble some diesel down the carburetor and let it idle for a while and that helps free up gummed up valves.
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