Help with vacuum gauge diagnosis???

The 216 in our '52 chevy shows a vacuum reading that bounces rapidly between about 9"-16" at idle. When I accelerate and decelerate the the gauge does drop and rise like you would expect but the needle doesnt smooth out, even while dropping and rising the needle bounces rapidly. All my resources I've checked dont really describe this. The closest idea I have come up with is blown head gasket (havent done a compression check yet) or perhaps poor valve adjustment (never worked on this engine before but am guessing it has mech. lifters??) Any ideas?? thanks-

 

do a compression check and get back to us with the results

 

Valves sound like they are out of adjustment or burnt.

 

So I hooked up the compression tester and tested the first three cylinders - nothing
Put the gauge up so I could see it while I was cranking - each compression stroke the needle jumped up to over 100 but then dropped down to zero again
Is there something wrong with my gauge (like it is not holding the pressure)? I just had a student use the same gauge on a 3.6 Buick and it held like 160 pounds??
Is my procedure off? I am cranking the engine for 4 good compression strokes, have the ignition disabled, am tightening the adaptor into the spark plug hole with a wrench
I am running these tests dry, would a wet test tell anything at this point?? thanks-

update - wet test shows no change...

 

At least 1 valve is not fully closing.

 

bad spring, keeper, retainer???

sounds like I should probably pull the valve cover and have a look??

 

Before you yank the valve covers, get the suspect holes up to TDC and throw some compressed air to 'em (leakdown test). Def. sounds like you're losing the wind somewhere. Leaving the covers on will help isolate bad rings(crankcase blowby)

 

Your compression gauge should have a schrader valve in the end of the hose to hold the pressure in until you release it manually, theyre easy to take out, and in some cases just get stuck open and dont hold the pressure. wet or dry wont make much of a difference if your valve isnt holding. if your getting 100psi out of one compression stroke without having subsequent compression strokes to build it up, thats not too shabby, check and or replace the valve and give it a try again. Vacuum diagnosis does sound like an intake valve isnt closing all of the way, whether it be from adjustment, debris, valve spring or whatever reason, sounds like your on the right track to finding out. Compression test should tell you pretty quickly which cylinder it is.

 

i think this is what ur looking for,hope it helps to understand ur readingshttp://www.secondchancegarage.com/public/186.cfm

 

leaky valve guides

 

let me try that url again for u so sorry
http://www.secondchancegarage.com/public/186.cfm

check out senario 6,its close to your readings u expressed

 

bad picture I know but...



the #6 intake pushrod is absolutely loose, doesnt feel like its resting on anything on the bottom and is like 1/2 away from the rocker on the top side...

could it have broken in half, "fallen" off the top of the lifter, ????

I dont have a manual for this engine so I dont know what the valvetrain arrangment is supposed to look like or adjustment procedures or anything like that - just know that what we have here is not right...thanks-

 

I'd call that excessive for sure. Looks like you found it.