floyd7811

I decided to do a comp check while changing spark plugs today on my hp500 carb motor. It has about 250 hrs on it and springs were replaced last summer with comp valve springs. Motor ran fine, no issues when i put it up for the winter.
I got 150 to 160 on compression test on all cylinders until i got to the last one #7, got a reading of 0. I even checked the #2 cylinder again to make sure guage was still workin, then tried it again and nuthin.
So im trying to find a leak down tester, no luck yet. Was going to take valve cover off and look things over and adjust valves on #7 again to make sure they are right.
Should i go ahead and check valves or should i wait until i get a leak down test first?

P.S. also the motor was cold when tested as i have drive off and havent started motor yet.

picklenjim

Leak down isn't going to tell you anything different if you are getting zero compression. It wouldn't leak enough past the rings to give you a zero reading so it almost has to be a valve is hung up. Take the valve cover off and crank it over and see if one of the valves is stuck in the open position or not closing completely. Crank until that cylinder is in firing position and see if one of the rocker arms is real loose. If so that valve is probably not closed all the way.

Griff

If you got 0, then a valve is most likely stuck open.

ezstriper

spin it over a few times and try it....or better yet fire it up run it to operating temp..if runs bad recheck that cyl.

ECeptor

With a zero reading, I'd go ahead and plan on pulling the head off. No harm in rolling it over to see how the valves are moving. But even if that looked "ok" I'd pull the head.

Several years ago I had 1 cylinder at about 1/2 pressure of all others. My next step was to pull the head and have a look-see. It was a cracked head. I doubt that's your problem.

Hopefully nothing is bent/broke/cracked and all you need to do is freshen up the heads and bolt them right back on. Good luck!

Knot 4 Me

Most likely a valve siezed in the guide. If that is the case and you've turned the motor over, you may also have bent the push rod.

Ted G

iTrader: (1)

If you do find a valve stuck down take a hammer and smack it a few times, often that will loosen it up and it will reseat, then run your compression test again.

RunninHotRacing163.1

u meant rubber mallet right ????

ECeptor

If it passes after that would you pull the head and go through all the valves? Or would you run it?

Ted G

iTrader: (1)

I would run it, this would normally be caused by rust between the valve and guide, 10 minutes of running will clean it right up. And yeah, a rubber mallet would be preferable but I have used a hammer and piece of wood, sorry I should have elaborated further.