Johnson 20 hp Bypass Cover Gasket blown . . . whY?

Hi all,

I have purchased a used 1990 20 hp Johnson two-stroke and and trying to get it in shape to use. Everything seems fine and it starts easy and it runs great at full throttle and will even idle great most of the time. However, occasionally it coughs pretty hard and during the cough, the motor slows down significantly and loses power, then it runs fine again for a little longer before coughing again. I noticed when running it in the dark that a orange blast is visible when it coughs right around the bottom right side of the bypass cover gasket. It is clear that the gasket has blown in that spot. I intend to replace the gasket, but I am concerned about why it is blown in the first place. Will this happen again? Is there something else inside the engine going on (like a bad valve or blow-by past the piston) that I should be concerned about? Will replacing the gasket solve all my problems? Thanks for your help!!!!!

Ryan

Re: Johnson 20 hp Bypass Cover Gasket blown . . . whY?

Which came first, the chicken or the egg? Seriously, the blown gasket will certainly cause the coughing (lean sneeze) you describe. Replacing it may be all that is necessary.

However, the question is why did it blow in the first place? Lean sneezing form an improperly adjust carburetor might possibley cause it. So could a bad lower crankshaft seal.

Replace the gasket and see how it does. If you can adjust the carburetor so it doesn't sneeze, you should be good to go.

Re: Johnson 20 hp Bypass Cover Gasket blown . . . whY?

Thanks for the advice! I bought a new gasket yesterday and will install it this weekend. I'll post a follow-up to let you know if it solves everything.

Thanks again,
Farris

Re: Johnson 20 hp Bypass Cover Gasket blown . . . whY?

I replaced the bypass cover gasket and took the boat out yesterday. It ran a little better, but it was still lean sneezing. It would really only do it when I began to slow down after running it full throttle. The sneezing wasn't as loud (since it no longer had a leaking bypass gasket), but it was still pretty bad and it would lose power during the sneeze. Any further comments would be appreciated. It seems peculiar to me that it only happens during deceleration from high rpm. I've cleaned the carburetor. Is it possible that an improperly adjusted float could cause this? Should I be concerned about a lower crank seal? Is that difficult to replace (sounds like it would be). To complicate things, after running the engine for about 20 minutes, it died altogether, and I could only start it by choking it, and even then it would only run for a few seconds then die again. Plenty of gas. No idea. I'm hoping this was a fluke and is not related to the lean sneezing issue. In any case, any help would be appreciated! I'm a mechanical engineer and like to work on this stuff myself, but don't have much experience with outboard motors. At what point should I take it in to the shop and have them deal with it? Any idea how much something like that could cost me? I am clueless.

Thanks!
Ryan

Re: Johnson 20 hp Bypass Cover Gasket blown . . . whY?

You need to open up your low speed needle a little more. Go an 1/8 of a turn out, till the lean sneezing stops.

Re: Johnson 20 hp Bypass Cover Gasket blown . . . whY?

Ok, I have gotten the motor running reliably, but I still have a lean sneezing problem. It NEVER sneezes at idle, and it never sneezes at wide open throttle. It only sneezes when I am letting off the throttle to slow down after having run wide open for a few seconds or minutes. During this deceleration, it keeps sneezing every couple seconds until I get it all the way back down to idle. Does this mean I have a bad lower crankshaft seal? If so, how complicated is that to replace, and how do you know that it is the lower seal, not the upper? Thanks for all your help! P.S. I should mention that the slow speed adjustment needle does NOT take care of this problem. It eliminates sneezing at idle, but does not eliminate sneezing during deceleration.

-Farris