83' 140hp Johnson

[captainmike]

I have a 83' 140hp Johnson that bogs down at 5000 rpms after about 10-15 seconds. It seems to run fine at lower rpms. My boat mechanic has rebuild the powerhead with all new pistons, carbs, etc and now we have this problem. He doesn't know if the problem is in the lower unit or maybe still in the powerhead. He plans on finding another lower unit to install to see if the problem is in the lower unit itself or in the powerhead. Does anyone have any suggestions on maybe why it bogs down at 5000rpms with a rebuilt powerhead. Is it possible for the problem to be in the lower unit? Send any suggestions this way.. Thanks <br /><br />P.S. We already replaced the tach(which was way off before) and the prop, which also was the wrong size, and we still have this loss of power problem.

 

[The Marine Doctor]

Re: 83' 140hp Johnson

Check your fuel lines for air. Or replace the fuel pump.<br /><br />TMD

 

[Dhadley]

Re: 83' 140hp Johnson

Does it slow down like you lose fuel or is it trying to lock up? When it starts to lose power what happens when you hit the primer?<br /><br />How much time did you have on the rebuild before you started making the bonzi runs?<br /><br />Good luck!

 

[captainmike]

Re: 83' 140hp Johnson

It slows down like it wants to lock up. Didnt try hitting the primer. I have about 5hrs on the rebuild

 

[Dhadley]

Re: 83' 140hp Johnson

Ah, OK. What did you bore it to? See if the shop can give you the finished bore size. We need to know the exact size they used, not just ".020 over". It should be something like 3.5201 or something like that. <br /><br />In the meantime its time to look in the cylinders. Pull the heads if your tech doesnt have a scope. Pay attention to the exhaust areas of the cylinder walls.<br /><br />Are you still on double oil? Why did it need rebuilding to start with?<br /><br />Let us know how you make out!

 

[captainmike]

Re: 83' 140hp Johnson

Yes, Im still on double oil. It was rebuilt because I had 0 compression on one cylinder and low compression on the others. Last time I spoke with my tech, he told me he pulled off the heads and the walls and pistons seem to be fine. Far as the boring goes, I only know it was .030 over and not the exact size. I'll keep you informed on what he finds out. Thanks!

 

[Dhadley]

Re: 83' 140hp Johnson

Let us know what you find out. Also you will need to know what caused one cylinder to have no compression. You dont need a repeat performance by whatever caused it. Youve replaced the damaged piston (effect), you need to find the reason (cause).<br /><br />Good luck!