Don't think I'm getting full hp out of my old Johnson 33hp outboard, except just before running out of fuel

Question

I have an old boat with a Johnson 33hp 2-cylinder 2-stroke outboard that I had resurrected a couple years ago after not being used for many years... Now it at least runs, with a couple problems, possibly related.....

This isn't the main problem I'm asking about, but it might be related, so I'll just mention it.... The bottom cylinder is either gummed up or scratched up, and only holding about 20% compression...

Now, it does run, but I don't believe that it's truly running at full capability at WOT, as I seem to remember it many years ago (as a kid).... In the last couple minutes before a fuel tank runs dry, it will randomly increase in RPM and speed, before sputtering out... This higher RPM is just about what I seem to remember the WOT speed being... I've been told this is "normal", but I'm looking for a second opinion....

Any ideas if this truly is normal? How might I go about getting the higher HP out of it? Is the HP loss entirely due to the compression loss?

Thanks!

Answer 1

Your jets are set too rich and as you run out of fuel, the fuel/air mix is leaned out and causes the engine to develop the HP that you are missing from normal operation until the last few seconds.

Look at your plugs, if they are sooty and black, then you are running too rich of a mix.  Get someone that knows how to set them to look at it, too lean and you can burn holes in the pistons.

Good Luck

Answer 2

it'll gain RPM as it leans out when you run the carb(s) dry.  

but with bad compression, don't expect to get any kind of performance out of it.

you're really just tearing up the motor, running it out of balance like that.

Answer 3

yes on a 2 cylinder engine when you basically loose the one cylinder it almost cuts the power in half on it,anything over a 20 pound difference in compression and you,ll notice a big loss of power on it,my boat did this a few years ago and i had lost compression to one of the cylinders on it,and i being a v-4 i still noticed a great loss of power on it,it throws the cylinders out of balance on it and causes it to loose a lot of power,as to the part of the engine speeding up before it runs out of gas,not always,mine will choke down when it runs out,which has happened plenty of times some smaller engines though may speed up ,you need to find the loss of compression on it,id say that's due to the lack of compression,the cylinders have to be with in 5 pounds of each other or it throws it out of balance ,enough that you will notice it,and the smaller the engine is the more noticeable it is,a bigger engine will make up for some loss at high rpm,s but a smaller engine wont,good luck with it.