cant reach max RPM's at WOT

[davejnz]

I have an '87 Johnson 20hp/tiller that i've had for 3yrs.Last yr,i rebuilt the carb and did the lynch and synch(didn't set the WOT timing,no timing light).At first it would cough at about 1450RPM,i backed out the low speed needle to 1.5 turns from lightly seated and the coughing stopped.My manual says to prop it for 5000RPM and with my 10/13 prop i hit 5020 even with a buddy in the boat with me.Problem is,when i advance the twist grip throttle all the way,the RPM's drop to 4800.If i back off just a tad they climb back up to 5000.any suggestions?

 

[walleyehed]

Re: cant reach max RPM's at WOT

Need to make sure your throttle plates are not going past center, as in closing back down slightly..

 

[davejnz]

Re: cant reach max RPM's at WOT

just checked,the butterfly is at the horizontal position when the throttle is advanced all the way.Before the carb rebuild/lynch n synch,the butterfly only opened up about 3/4.

 

[TwoBallScrewBall]

Re: cant reach max RPM's at WOT

Perhaps then it's a high speed jetting issue, too much gas at full WOT? Or too little?

 

[davejnz]

Re: cant reach max RPM's at WOT

When the carb was rebuilt,i didn't check to see if my buddy set the float properly.Could this cause it?How would i be able to determine if its a high speed jet issue?I have noticed oil depositing in the compartment and mainly in the carb throat around the bottom of the vertical tube that is perpindicular to the butterfly.I checked all fuel/primer hoses and the plastic ties are tight and i dont see any noticeable leaks.

 

[walleyehed]

Re: cant reach max RPM's at WOT

Those verticle tubes are "emulsion" tubes and that's where the fuel comes up from the bowl...being wet with fuel/oil is normal.<br />If the floats were set too low, it "could" cause a lean condition at WOT. A weak or leaking fuel pump can do the same thing.

 

[davejnz]

Re: cant reach max RPM's at WOT

After breaking the drain plug on my lower unit,i took it into the shop to have them insert a helicoil and do the lynch and synch for me.It didn't help.I've tried engaging the manual primer when running at WOT to see if its running lean but the engine quickly drops RPM's which i was told is normal.You might be on to something with the weak or leaking fuel pump,I occasionally will lose prime after trolling at slow speeds and have to pump the ball.I have a cheap Tempo gas tank and i think that it leaks vacuum where the male connector threads into it.

 

[angus63]

Re: cant reach max RPM's at WOT

Borrow another tank and fuel line and you can prove your theory. I had a Tempo fuel line and bulb for about 2 days once, never will again!!