115 Johnson Losing Power

[AUtigers]

I just recently bought a new power head for a '78 115 horsepower Johnson. I have been down at the lake the last couple of days breaking it in. It ran solid for the first four hours of idleing time required, and it also ran great for the first couple hours in gear. Then all of the sudden after about two hours of run time in gear at 2500 rpms it started losing rpm's sputtered and missed a little bit then cut off. I started it up again, and after about 300 yards, it did it again. So I had to idle in, but as I idled in for nearly an hour, motor didn't flinch once.
Does anybody know why it would have run so well for as long as it did, then start doing this, and then be able to run fine at idle. Any advice would be greatly appreciated.

 

[Rockfall55]

Re: 115 Johnson Losing Power

Checked for fouled plugs,,,probably running rich....

 

[Chinewalker]

Re: 115 Johnson Losing Power

You said you bought a new powerhead. What happened to the old one? Did you nail down a reason why it blew in the first place? Were the carbs rebuilt prior to reassembly? If a single carb throat is fouled enough to run it's related cylinder lean, you may be damaging the new powerhead. It is possible your carbs are gummed up. Could also be water in the gas. As Rockfall mentioned, a look at the plugs may tell the story...
- Scott

 

[Dennisanoka]

Re: 115 Johnson Losing Power

You may have done this but I would inspect the plugs to see if there is a clue as to if one is not firing, re-check the torque on the head bolts and the spark plugs, confirm each cylinder is indeed firing(pull one plug wire at a time-listen for change), Re-check the link and sync on the carbs, re-check the compression on each cylinder, pump the primer bulb to see if the rpm's pick up. These are the things I'd try first, about 15-20 minutes of tests in all.

 

[AUtigers]

Re: 115 Johnson Losing Power

I checked the spark plugs, they we're a bit oily, only because I have been running double oil 25:1. They also appeared to be equally oily so it would seem like they are firing correctly. The carb was cleaned before it was put on the new powerhead so that should be ok. I'm going to go down to the lake this aftertoon to see if the problem lies in the fuel pump/system. Could the problem have anything to do with timing or anything electrical.

Thanks for the help.

 

[Dennisanoka]

Re: 115 Johnson Losing Power

I don't know that you can rule anything out at this point. I'll state the obvious, "you never know what the problem was until you know what the problem was." Regarding cleaning the carbs. First of all as you can see I am a second class petty officer, I am not a sea Captain or Antient Mariner, but there is one I want to share with you. Every single short cut or item that I have ever taken, I have had to go back and do every single one anyway. That includes full carb kits and includes cleaning every needle valve, idle jet, port and orifice with a wire just big enough to fit through them. Air can't clean out sticky gunk. When we don't do a job 100% and then it doesn't work right all we can do is begin to wonder is it something I didn't do correct or is it something I didn't do the first time. If I do it right the first time at least I should be able to ellininate that one. I'm not saying it is the carbs but they are still on the list of possibilities at this point.

Proper compression+correct fuel/air mixture+ spark at the right time= a smooth running engine That's what I have heard here the most.

 

[ezeke]

Re: 115 Johnson Losing Power

Don't forget to check your flywheel and retorque your cylinder heads.

 

[Dennisanoka]

Re: 115 Johnson Losing Power

I'd hate to be the guy who forgot to put the key back in the flywheel and then burned alot of brain cells wondering why it ran so horribly. Man that would be a tough lesson to learn.

 

[AUtigers]

Re: 115 Johnson Losing Power

I took the boat out today to see if I could figure anything else out. I noticed fuel dripping out between that piece of plastic on the very front of the engine on the carbuerator. I also tried to choke the engine when it began to lose rpms, and that seemed to bring the rpms up for a second. Tried to squeeze the fuel bulb and that did not seem to have any pressure. I'm still stumped though. Anybody have any ideas?? Could it be the fuel pump?

 

[Solittle]

Re: 115 Johnson Losing Power

Dennis has alreday pointed you to the most probable cause at this point - crud in the carbs - rebuild em.

 

[AUtigers]

Re: 115 Johnson Losing Power

I'll check the carbs real good again, but the carbs had supposedly been cleaned by the Johnson dealer.

 

[Dennisanoka]

Re: 115 Johnson Losing Power

I may get in trouble for saying this but the reality of "some" businesses is to do just what is needed to get the product out the door. If they didn't think they had to put carb kits in they didn't because it would cut into their bottom line when selling that engine.

 

[Dennisanoka]

Re: 115 Johnson Losing Power

Maybe I'm alittle confused here. In your first post you said you bought a powerhead. I "assumed " that you then re-built/put it back together yourself using parts from a previous engine. Then just a post or two above you said you thought the dealer would have cleaned the carbs, so did you buy a finished engine from a dealer or did you rebuild it? Just trying to get the picture here.

 

[AUtigers]

Re: 115 Johnson Losing Power

Here's my story...I took my engine to the dealer to have it repaired, not yet knowing that I had blown a piston. Before the dealer told me that I had blown a piston they cleaned out the carbs. It was going to be too expensive to have them rebuild the engine, so I decided to do it myself. As I got further into a rebuild it turned out that I had a cracked engine block, so I ended up having to order a new powerhead. So in short, I put my carb off my old engine that was cleaned on a new powerhead.

 

[Dennisanoka]

Re: 115 Johnson Losing Power

Thank you-

I suggest spending 2 hours MAX. and $50 to rebuild the carbs. It should be done no matter what anyway by someone who REALLY cares about the engine and that is you. You have spent alot of money already and you want an engine that will give you several more years of enjoyment. Even if this doesn't cure your issue you can cross it off the list and it just isn't that painful.

 

[Dennisanoka]

Re: 115 Johnson Losing Power

Search for Joes Reeves article on carb cleaning/rebuilding. Read it 2x and follow it all.

 

[Dennisanoka]

Re: 115 Johnson Losing Power

sorry- Joe Reeves

 

[AUtigers]

Re: 115 Johnson Losing Power

I'll give that a try. Thank you for all of your help.