Compression - Good or Bad

just did a compression test on my 1985 Johnson V90 and came up with 120-118-120-111... What causes one cylinder to fall out when the rest of them are top notch. Plugs look good (consistant) and spark and fuel are getting to all cylinders. I was given the motor a couple of months back and I know the powerhead was replaced a few years back. Is it possible for me to retorque the powerhead or am I running a motor that is burning down? <br /><br />If you are looking at the motor from the stern of the boat the low compression cylinder is the one on the bottom right. <br /><br />Thanks for all your help... I also lost my fuel pump the other day, or atleast think I did because I replaced everything else (fuel lines, hand pumps, vents, etc) and it still only runs above 2500RPM when I hand prime myself...

Re: Compression - Good or Bad

Welcome!<br /><br />We also assume the next lowest cylinder was right above it. This is not uncommon for the starboard cylinders to coke (a sticky carbon like substance) up and the rings will start to stick in the groove which allows the static compression to read low. A good decarb job will tell and is a good idea too.<br /><br />It could also be a sign of a head gasket starting to leak. If the heads havent been off in a while, it may not be a bad idea to change the head gaskets and water deflectors anyway.<br /><br />Since its a replacement powerhead it could have a set of non-matching heads. <br /><br />I would start with a good decarb job and recheck the compression. This is a typical scenario especially if the motor is set up to turn in the low to mid 5000 rpm range. It should be at 5800-5900 with your average load.<br /><br />Good luck!

Re: Compression - Good or Bad

Welcome to iboats Robert.<br /><br />My compression should be so good on MY 90hp. <br /><br />Your compression is great. If you want to try and get it closer together, do a decarb and test it warm. You could have a bit of carbon sticking a ring. <br /><br />The cylinder your refering to is #3. The top right is #1. Left side is #2 on top and #4 on bottom. <br /><br />Your year is the first year for the VRO pump. If your engine is equipped with the pump, might be time to replace it if it's not pumping. Sounds like you covered all the other bases in the fuel system. Take a look at the wire harness coming from the pump. If it's a three wire harness, it's an oldy. You would do well to replace it.<br /><br />Keep in touch....thanks.

Re: Compression - Good or Bad

BTW -- it wasnt easy to pass on the comment about priming yourself by hand..........

Re: Compression - Good or Bad

Careful Dhadley.....Mods are lurking with the "poof" button.....but.....never mind.

Re: Compression - Good or Bad

I have this exact motor ! last year i ran a comp test and had 130 130 120 115 #3 being 115, well i thought that ain't too bad, so i go to use it this year for the first time and it is running crappy, and i had to choke it every time i started it, I thought it just needed to be opened up a little, so the next time i take it out, it did the same thing and i had to choke it and keep it running about 3 grand or it wouldn't stay running so i get out where i can open it up and it was just balking i would go full throttle and it would just wallow there and then pickup rpm and then cut out and then pickup and on and on, well i finally got her up on plane and was runnin about 5200 which is wot on this setup i run her that way about 5min and shut her down to fish but everytime i went to start her i would have to use the warm up lever and choke it and even then she would barely stay running. So i take her to the mechanic who did the comp test before and he does another and this time it is 110 110 100 75 he pulls the head and and the #3 with 75 psi the head is pitted like somebody took a real small screwdriver or punch and put a bunch of real small nicks in the top of the head! the top of the piston is kinda scuffed up , the mechanic says a small piece of piston must of broke off. Well the cylinder is not scored to my naked eye it looks like the other cyl on that side ,smooth as a babies butt. So i am wondering why the other 3 cyl comp would go down? also is it for sure a piece of ring? If so could the mech pull it down and dust replace the ring on that cyl and put anther head on her and rebuild all 4 carbs! If he did just that what should it cost and would that work, as I can't even afford a total $2000 rebuilt. What is everyones guess on the cheapest i can get by with .<br /><br /><br />thnx<br />mark

Re: Compression - Good or Bad

Thats exactly what can happen from the rings coking up. They stick in on the side opposite from the end gap and as more and more of the ring sticks in the piston loses support. Finally the piston rocks enough that the end of the ring shears off. <br /><br />On motors propped out in the low 5000 range, like yours at 5200, the coking appears much quicker than on a motor set up at 5800.<br /><br />I think you will also find that the other cylinders are enough out of round that they need to be bored. Also a product of the increased combustion temps.

Re: Compression - Good or Bad

THANKS for all the responses... my motor as well pushes 5200 at WOT throttle... can i pull the head check it out and then replace with a new gasket? I sure hope so... as far as the carbs go, what exactly is a decarb? is it a rebuild or a good cleaning... Thanks once again for all your help...<br /><br />PS: two more things - is there a check valve in my built in fuel tank? and if i were to replace the fuel and disconnect the oil injection, would I have to adjust my carbs? THANKS

Re: Compression - Good or Bad

Hi Robert, I also have a built in fuel tank and it has a check valve at the tank. Also, in the pick up tube going into the tank I found a small screen filter that was clogged. That was the cause of my fuel restriction problem.