Have Questions on Engine Leak Down Test

[prayers1 153]

This is a first for me,though it seems simple there is always that area of doubt if Im doing it right.

 

I have the Proform dual gauge leak down tester. Obviously I start with cylinder #1 since its a sure beat I can tell when it's TDC with the harmonic balancer. I have the valve covers off and all plugs pulled. Instructions say to have the engine warmed up, but it's not. I rotate the engine by hand, double check on TDC with a pencil in the Cylinder and both valves are up.

 

When I perform the test, I hear air moving within the engine, I check for all obvious exits and hear a faint rush of air coming out of both tail pipes. Note that I have an H-Pipe after the collectors. I'm thinking it's leaking past the exhaust valve or maybe my harmonic balancer is off, so I redue the test 5* advance and retarded. More air rushes through when advanced 5* advanced or retarded. I triple check my work on TDC and same scenario.

 

You turn the regulator until the right hand gauge needle sits in the middle of SET, then plug it into the hose off the cylinder. My readings on both 1 & 5 were in the 20% mark, it's 1 a.m. and I call it quits.

 

Other thoughts, last week I readjusted my valves. My builder had used home made shims, flat washers grinded down on the sides for Scorpion Pedestal Rockers. I had a set of 0.20 shims from Scorpion Racing that I installed on top of the flat washers. Instruction were to tighten down 18 to 20 ft. lbs after zero lash. I had either 7/8's or 3/4's turned as an average on all 8 to reach 20 lbs.

 

I believe by adding shims loosens the valves? I don't think that can have any effect on the leak down test or could it.

 

Is the 20% acceptable? The motor might have 2000 miles on it.

 

I thought a fairly new motor will not let any air pass by. I think of it as if you were adding air to a tire, if no leaks you wouldn't hear any air pass by.

[1969_Mach1 333]

At least you purchased the better 2 gauge type and not the single gauge type. 

 

First make certain you are on compression stroke and not the overlap stroke when both intake and exhaust valves are slightly opened at the same time.  From my experiance 15% is normal on a good relatively new warmed up engine.  And that is usually leaking past the piston rings not past a valve.  I'd see %20 on a good warmed up engine with a 100K or so miles on it.  But still past the piston rings and not a valve.  I would back off the rocker arm bolt and retest.  If the leakage goes down you know the valves are still adjusted too tight.

 

Have you tries any other cylinders?  Or, done a compression test?  Typically a compression test is done first.  Then if something seems out of sort, a cylinder leakage test is performed to futher help pinpoint the cause.  Definitely make certain all 8 cylinders are good.  Relating to drivability, a  cylinder with low compression will cause rough idles and stumbles on acceleration.

 

You are correct, adding shims will loosen the valve adjustment.  Do know what valve adjustment your cam mfg suggests?  It ranges from 1/4 turn past zero lash to 1 full turn past zero lash.  Most common is 1/2 turn past zero lash.

[prayers1 153]

I'm having one heck of a time finding TDC on compression stroke.  With these Cleveland heads and the Shock Towers its almost impossible to fit some type of rod in the spark plug hole to watch its travel for TDC.  Its a lot easier with #1 since I can glance at the timing pointer.  When I'm over at #3 or #5 it's a lot harder to watch the rocker arms and turn the crank.

 

It seems when I think I'm at the Compression Stroke the rocker arms stay motionless for some time while I'm still spinning the crank, so I never know when I'm at true compression stroke at TDC.

 

Scorpion states after achieving zero-lash, set the torque wrench to 18 to 20 ft. lbs. Turn the adjusting nut until torque is reached, either 1/2 or 1 full turn. I was able to get them all in with with a 1/2 or 1 full turn.  Note: If less than a 1/2 turn a longer push rod is needed, if more than a full turn shimming would be needed.  In my case I shimmed on top of another shim.

[prayers1 153]

Been at it for a while and can't seem to get over that hump of where TRUE TDC is. As for an example, on #1 I watch the intake valve getting ready to close, as it closes and exhaust is closed, I should be on my compression stroke, the harmonic balancer will rotate 20 degress until it reaches 0, If I didn't have timing marks I would never know where true TDC is.

 

Any answers?

[Rsanter 152]

Remove the plugs

Put your finger in the spark plug hole

Turn the engine with a ratchet

In the compression stroke you will be able,to feel the engine trying to blow your finger out of the hole

 

Bob

[prayers1 153]

Yes I know that, its easy, from there it gets you in the ball park on the timing pointer then rotate the crank until it lines up with TDC.

But how do you know when there is TDC on any given cylinder.

 

Case in point, when doing cylinder # 2,  I have a 2' air hose in the hole with my thumb on top, I crank the motor around and wait until I start feeling compressed air, when at that point do you stop rotating the crank so that your at TDC, because I still feel air escaping after a few more turns of the crank.

 

Someone set me straight, Thank You!

[DocWok 30]

Your TDC points for each cylinder are 90 degrees apart from each other.

 

If you don't have a fully indexed harmonic balancer you can either buy a timing tape and stick around your balancer which will give you the 90 degree separation points.

 

Or you can get a strip of paper place it on your balancer starting at the TDC mark and wrap it around the balancer till it comes back to the TDC point and mark or cut it off there.

 

Remove the paper strip, divide and mark its length into 4 equal segments.

 

Return the paper strip back to the balancer and transfer your 4 marks onto the balancer with a thin white marker pen, these will be your TDC marks for each piston as you follow your firing order.

 

As you now have 4 marks starting at no.1 TDC separated from each other by 90 degrees it will take 2 revolutions of your balancer to cover the 8 TDC positions for each piston in your firing order.

 

Hope this helps.

[prayers1 153]

DocWok, I tried that same thing yesterday, I figured to continue in the firing order and turn the crank a 1/4 at a time starting w/ #1, but ran into trouble with all of the pulleys in the way.

 

That's a good ideal with the strip of paper around the balancer, I can try that. Thanks!

[ray1970 88]

In the name of boredom after a beer run why not run a CK on some of mine(hot motor)

Here is what I got on..1 and 2 cil.the rest were to hot for my hands..

apx 5000mi on the rings.....8000mi on the bore.

[ray1970 88]

As per normal I did not follow the rules.

I put the hose in and spun it over till I felt full compression at this point I know the valves are closed

and made made a reading..As long as the valves are closed and the pistons somewhere near top Im good with it..

351C are a bit tight getting the spark plugs in and out with the tall covers and the stiff hose for the leak down can be a bit of a chore at times

But I dont have anything else to do before supper...

[prayers1 153]

Ray1970- Per your comment "As long as the valves are closed and the pistons somewhere near top Im good with it.."

 

I've done that a few times on Cylinder #2 and it leaks like crazy out the Exhaust :(

 

Just got home from taking the family out to celebrate our daughters birthday, I'll see if I can get  a chance to check it with a compression tester.

[ray1970 88]

I realy hope your leaking is just a tight valve..How nice would that be....

If you haven't done a com.CK now would be the time.

Thinking you have the plugs out already??

[prayers1 153]

Yeah, gonna get at it tonight.

Valve Covers off & plugs already pulled the rest to be discovered. Keeping my fingers crossed!

[ray1970 88]

I dont know a lot about your motor but im thinking 170-180 psi would be a good number..

[prayers1 153]

I was able to finish the leak down test. The worst cylinder was #2, I did it several times to make sure.

 

Hear are the results:

1= 20% lightly coming out of tail pipe

3= 5%

7= 5%

2= 60% moderate on the gauge, heavy flow out tail pipe.

6= 5%

5= 10% could not detect where leak was coming from.

4= 5%

8= 5%

 

I also did a compression test:

1= 190

3= 185

7= 185

2= 155

6= 185

5= 190

4= 185

8= 178

[1969_Mach1 333]

Cylinders 1 and 2, the compression test is a little out of sort compares to the leakage test.  anyway, try leaking down those two cylinders with the exhaust valve adjustments loosened.  Curious, were hardened exhaust seats installed in those cylinder heads?

 

On a different note, with the cylinder pressures of 185 and 190 psi on the good cylinders, I am surprised you can run 16 degrees of initial timing without kickback while cranking the engine.

 

Lastly, when you crank over the engine with the ignition system disabled so it won't start, does it have an consistent even rhythm?  Or is the rhythm steady except for one or two spots where the cranking speeds up.  That's just a quick crude test for fairly even cylinder pressure.

[prayers1 153]

Has a consistent rhythm. I forgot what seats were installed. I tried several times to make sure both valves were loose on #2.  

In fact they were, I can lift both rocker arms and I even tried wacking the rocker to get the valve to seat.

 

Here's a compression test done on August 10, 2014 with a couple hundred miles on it. The motor wasn't broken in yet.

 

#1-175
#2-170
#3-175
#4-175
#5-170
#6-170
#7-175
#8-170

 

What should I do. The shorty headers were a bull to put on and I have a papers width between the right header and shock tower. I don't know if I should pull the head or motor.

 

I just did a valve adjustment and added 0.20 thick shims to all 16. This would of loosed the valve train.

 

Is it possible that caused this. Is it possible that I did some damage to the piston?

[1969_Mach1 333]

I doubt there is any piston damage.  You might have an exhaust valve that is slightly burned.  Or maybe some deposits on the seat preventing the valve from sealing properly.  Pull the head, pull the motor, I don't know at this time.  One thing for certain, get this ironed out befroe messing with the carb anymore.

[ray1970 88]

The last time i pulled the heads off mine with the shortys on it I unbolted the head first and moved it over to make more room to get at the header bolts.

If i remember right the RH side  I pulled head and header off all in one piece ..

That was two years ago..

[prayers1 153]

Early this morning I pulled the rocker arms off 1 & 2 and on each try I brought the piston up TDC.  I struck each valve a few times to free up anything up.

 

Each cylinder leaks past the exhaust, as I hit the exhaust valves on each, I see the gauge needle getting worse, so I know I'm getting movement each time I hit it.

 

Same results!

 

What to do?

[ray1970 88]

Time to pull the head to see what you have or dont have..

At least your nearing the end of your problem.. In a way..

[prayers1 153]

Pulling head, let ya know when the Monsters out!!!!!!!!!!!!!!!!!!!!!!!

[prayers1 153]

Well here its is. I turned the garage lights off and shined a small flashlight in the ports . I did this one valve at a time, I not see any light thru the valve.

I was expecting some sort of gap. I did not see any physical damage to either valve or piston.

 

 

[1969_Mach1 333]

You can try pouring solvent in the ports and see if any seeps out through a valve.  A machine shop will usually vacuum check them using a tool that seals over an intake or exhaust port and pulls a vacuum in that port.

[LoneWolf2U 136]

I would pull the valves and check the seat and face for proper seating. it looks like the angles may be off and are not matched.

While you have them out do a simple valve lapping using compound to insure the face and seat mate evenly.