Thread: Please help me understand this....

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Hey you guys. So I put in new plugs and the same things are happening as above. But in looking around the net for why this could be happening I see that a leaking intake manifold or an unplugged vacuum hose/fitting seems to have all symptoms I am experiencing.
-hesitation when acceleration
-misfiring
-rough idle
-overheating
So I will do what Denny suggested and spray some starter fluid/cleaner around looking for rpm changes. Yes I have a working fire extinguisher just in case and yes it is in the garage!!!!! :-)) BTW a pic of the beast "we" are working on is hopefully attached.

Few things to try run it in total darkness and look for spark plug wires jumping spark--------
take valve covers off and check to see if pushrods are rotating as valves lift/close( Lobe wiped on cam-----lifters should rotate and also will spin pushrods) turn carb around and see if missing cylinders change---plug all vacme lines------

My first "go to" tune up tool when there's nothing obviously wrong with the simple things is a leak down tester. It's also a really goo way to keep track of what the engine has going on internally. JMO

Don't overlook a leak at the inside bottom of the intake at the interface of the cylinder head, where you may or may not find a leak with a sprayed flammable substance. Such a leak will allow oily vapors from the oil pan to be sucked into the intake port, disrupting air/fuel mixture balance and providing a vacuum leak. It will also foul the spark plugs with oil. If you find nothing pointing to a leak, pull the intake and examine the gaskets to insure that they are being pinched with equal force top and bottom, side to side. The manifold could have been mis-machined. If nothing is found there, examine the manifold for cracks. We used to use a Zyglo process back in the day, but there may be better ways today.

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Have a CO2 extinguisher handy. You don't want to clean up the mess a dry chem will make, don't ask how I know.

Vacuum leaks from the underside of the intake manifold can be found by performing a simple vacuum gauge test.

1. Remove any breathers and plug or cover their holes in the valve covers with tape.
2. Remove the PCV and plug or cover its hole in the valve cover with tape.
3. Remove the oil dip stick and connect a vacuum gauge to the dip stick housing.
4. Start the motor and read the vacuum gauge.

If the vacuum gauge shows vacuum, you have a leak under the intake manifold.

Hi you guys, here is an update on the SBC that won't fire on cyl. 8. Again compression is 160, did leak down and got air out of the valve cover portal when pressurizing #8. Other cylinders leak air through that hole also to some degree but none through exhaust or carb. Oil is clean (shavings etc.)and the rockers appear to be working well so I don't think I have a flat cam. This is crazy... My neighbor, a pretty good home style mechanic even says WTF!?

Have you tried just changing the plug with a new one. I have bought plugs, that just did not fire, brand new.
The other thing, you can pull #8 plug, and bump the starter, until you know #8 is coming up on compression. At that point, you turn the engine by hand so the piston is at top dead center. Then, you want to make sure the distributor rotor is pointing to #8 plug. Also, don't forget to check the distributor reluctor, or cam to make sure they are in good shape. And is the distributor bushing in good shape with no side to side play.

I think Denny has nailed it, you probably have a damaged reluctance tooth

More good ideas. Here's more:

-all new plugs
-moved plug wires around, still problem with #8
-HEI dist was new 5 years ago with at most 2 hours of use (but yes could still be bad)
-as noted before, seem to be getting good spark to plug
-dist is tight, no play but the reluctance tooth...? Dont know what that is: help!
-yes the rotor does not seem to point directly at the cap #8 point.... but if its off there wouldn't it be off at all the cap points... but its firing on the other cylinders.
M

Change #8 plug with say #6 plug. Does #8 still not fire ??? Or does #6 now not fire ??? If it's the same problem, be sure to check that distributor cap real good for a crack. And, if you try the plug out and check spark, that will not be the same under compression. Just change the plug first, and see what happens...

How (exactly) did you determine the rotor is misaligned only on #8?

That is extremely odd.