#7 cylinder 0 compression
 

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.

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.

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

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.

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!

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.

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.

u meant rubber mallet right ????:eek:

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

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. :party-smiley-004:

hammers and duct tape, what cant they fix?

DO NOT RUN THE MOTOR! Pull the valve cover and check the operation of each valve, hopefully one of the rockers will be real loose. If not, check the keepers carefully, I dropped a valve once and the spring and retainer held the rocker tight. If rockers are tight and valves are still in their keepers then you probably have a hole in the piston. If one of the valves is hanging, it could be rust, or it could be bent, pull the appropriate manifold and inspect it. Either way I would pull the head and fix the problem. Even if it's only rust, if it's bad enough to hang the valve through a compression check of the motor, it's bad enough to score the valve guide. I'm only telling you all of this because I have a similar situation. I bought a 1998 Velocity with 50 hours on it a couple years back. I ran a leak test last month after running it two seasons and 2 cylinders were over 50%. When I pulled the heads, the 2 bad cylinders had a rust print on the cylinder walls. The rust was heavy enough to destroy the rings, now I'm looking at a rebuild. It would have been a lot cheaper had I checked it sooner. I hope you have better luck then me.

If a valve is hanging and you would like to find out before you pull a valve cover, hook up a air hose to the spark plug hole and put some compression to her and listen for air out the exhaust or intake. Simple trouble shooting to give you a idea whats going on. if you hear air out the oil breathers bummer!

i put air in the cylinder and found the intake valve was open and blowing out carb. Got valve cover off and the rocker was tight so i backed it off and filled it with air again and no more blowing threw carb. So im gonna adjust valves ( maybe ill get it right this time) and check comp again. Looks like i had no comp in #7 since july of last year.

If that is the case that engine would have been backfiring pretty badly.

I thought the same thing boat1. Towards the end of season i had to let it warm up a little before backing of trailer or it would die. Maybe valve sealed when warm? I adjusted valve and rechecked comp, getting 164 now. Pushrod didnt look bent and valve tip and spring look in good shape. Should i check anything else? is it ok to run and see how it does?

For clarification, you now have compression on all eight cylinders, fresh spark plugs, and the valves adjusted correctly? When you did the compression test was the carburetor at WOT or idle, was the battery fresh, were all the spark plugs in except the cylinder being tested? I asked because it will tell me alot more about how far off the seat the valve was. If you have all the things done in the first sentence, along with fresh oil/filter, I would run the engine and get it up to temperature and see how it sounds. Check the timing also.

:drink:

yes i now have comp on all eight cylinders, new plugs (i just had the cylinder i was checking out), Battery was just charged and carb was at idle,
Havent changed oil and filter yet, That was next and also have new cap and rotor ordered im waiting on.

Change the oil put on the cap an run it. The valve did not have to be off the seat much at all with the carb at idle. You should have the carb WOT while cranking a compression test. I also remove all the spark plugs but that is not totally necessary.

And Vise grips!

What compression are you getting on each cylinder now?

If you had no compression on that cylinder you would have a noticeable loss of torque and about 5-10% loss of top end speed. I would think you would have noticed that last year if it was happening. More than likely the valve stuck over the winter

I bought the boat last year, and didnt run it but a few times before changing springs. My top speed was 69 on gps after having a prop labed by bblades. I saw where people with same set up was getting 71-73