Low compression (bottom starboard cyl)

[StriperChaser]

My 96 Johnson 112 SPL was running rough at idle and stalling going into gear. I rebuilt carbs and first time out it started easy and ran smooth at idle but still stalled going into gear. First time up on plane all ok but afterwards one cylinder cuts out but finally fires. Ran great everytime at WOT (5000 RPM) Pulled plugs and all looked the same. Dry tan colored insulator and tip, but damp electrode. Checked compression on cold engine, throttle in neutral. Three cyl at 115 – 120 PSI but bottom starboard at 40 PSI. Checked again tonight with warm engine, throttle wide open. Checked low cyl first and was at 100 PSI, others at 120 PSI. Re-checked low cyl again and it was back down to 40 PSI. Made sure the tester had a good seal each time. Why such a big difference in PSI in that one cyl (100 – 40 PSI) ? Runs good after it’s on plane. Will it do more harm to run it as is ? Any ideas / suggestions ?

 

[Hooty]

Re: Low compression (bottom starboard cyl)

Hot or cold, throttle open or closed, it doesn't matter, just so all are the same when you test them. You may have a stuck ring in the low cylinder. Give the low one a shot of oil and see if compression comes up. If it does, do a de-carb job on it. If it doesn't, pull the head and have a look. You can also remove the transfer port cover (I think) and get a good look at the cylinder walls.<br />Keep us briefed.<br /><br />c/6<br /><br />Hooty

 

[StriperChaser]

Re: Low compression (bottom starboard cyl)

Thanks for the tip. Saw a post about tilting up the motor and pouring in SeaFoam to decarbon a cylinder. It said to let it sit for a few days. How much should be poured in? Is it possible to pour in too much? How would you get it out or will it just burn out when the motor is cranked?

 

[Hooty]

Re: Low compression (bottom starboard cyl)

I'd follow the directions on the container. Some of that stuff is pretty stout caca.<br /><br />c/6<br /><br />Hooty