Johnson Seahorse - no high speed

[Mad Max1]

I have a 1980 Johnson 7.5 which starts well and trolls great. However when I open it wide open I am unable to obtain any fast speed. This motor is on a 12' aluminum boat. Is there possibly something out of adjustment or out of tune?

 

[5150abf]

Re: Johnson Seahorse - no high speed

Sounds like a carb thing.
Before you start pulling things apart try running a good shot of Sea Foam through the gas tank, the stuff is awesome.
I get mine at Speedway gas stations but it is probly at autozone too or any marina, red and white can,I swear by the stuff.
If you are lucky you have a bit of gunk in your carb and this will clean it out,good place to start before you pull stuff apart.

 

[tashasdaddy]

Re: Johnson Seahorse - no high speed

dirty carb. what type load is in the boat?

 

[Mad Max1]

Re: Johnson Seahorse - no high speed

Sounds like a carb thing.
Before you start pulling things apart try running a good shot of Sea Foam through the gas tank, the stuff is awesome.
I get mine at Speedway gas stations but it is probly at autozone too or any marina, red and white can,I swear by the stuff.
If you are lucky you have a bit of gunk in your carb and this will clean it out,good place to start before you pull stuff apart.

I will see if I can find some "Sea Foam". Thanks.

 

[Mad Max1]

Re: Johnson Seahorse - no high speed

dirty carb. what type load is in the boat?

Two guys, assorted fishing tackle, gas can, the usual. Not really a lot of weight.
It doesn't sound like it's bogging or anything; just doesn't go very fast.

 

[HighTrim]

Re: Johnson Seahorse - no high speed

You never said what type of speed you were talking about, with only a 7.5 and over 400lbs of gear, you definately wont be breaking any speed records, lol If it runs good at trolling speed and high speed, just not alot of speed, im not so sure it is carb related, more set up possibly. A picture of the stern would help.

 

[lexkyboater]

Re: Johnson Seahorse - no high speed

You might also try turning the idle speed mixture screw in till it seats, then back it out 3/4 - 1 1/2 turns. Those needle jets wear out with time and it's probably letting too much gas throgh. Even after I rebuilt and cleaned my 71 Johnson 9.5 HP carb (twice!) it still bogged at WOT and wouldn't rev out. That's even after adjusting the low speed mixture according to http://forums.iboats.com/showthread.php?t=167352. I was at WOT at the lake, and turned the low speed mixture in about 1/8 of a turn and it took off! This is because low speed mixture affects high speed as well. But, in my case, I think I need to look at my float adjustment again. I remembered it being higher than the recommendation and I just read tonight that float height is critical to proper operation. I originally thought that gas only drained out the bottom into the high speed jet, but that's not true. I think with the gas level too high in the float chamber, it's pulling too much gas as a result. Still, likely you need a rebuild like the guys are suggesting. Everyone here says that's needed about every 3 years if it's been in operation, or after it's been out of use for a while. --Steve

 

[tashasdaddy]

Re: Johnson Seahorse - no high speed

are you running a long shaft on a short shaft boat?

 

[freddyray21]

Re: Johnson Seahorse - no high speed

are you running a long shaft on a short shaft boat?

been there done that. Had a 14ft fiberglass short shaft transom long shaft motor could not figure out why it would not get me up on ski's

 

[Mad Max1]

Re: Johnson Seahorse - no high speed

Well, thank you all for the great input. I will try all of the suggestions and will look into whether I am using a long shaft or not. I did put a carb kit in and had to tinker with the float adjustment a few times to get it right. It doesn't stumble or anything; just sounds like it is revving up good but not really pushing me very fast. Next time out I'll also try the low speed adjustment suggested.
Again, many thanks all

 

[solomon]

Re: Johnson Seahorse - no high speed

I have the same problem on the same motor. 1980 7.5 Seahorse. I ran some sea foam through it yesterday and it runs jam up in a barrel, but in the lake stalls at mid throttle, and the top end just isn't great. I know it 'will' go faster because it was running a little better last year. After reading this thread I think it must be the mixture screw. It's a little aggravating because the knob is gone from it so I have to use needle nose. Also at wot if you play with the choke it surges. I guess it could only be the mixture screw. I also put new plugs in it.