54 sea horse question

Bearhunter08

Cadet
Joined
Feb 6, 2016
Messages
11
Hey guys, I got a 54 Johnson Seahorse 5.5 hp and I am having a bit of trouble with it. Recently I had someone rebuild the carb and I also converted it to run on the mikuni fuel pump. However once it gets running it will idle fine then all the sudden it revs up to wide open throttle and then dies out. If anyone has some info on what could be causing this issue it would be great for your input.
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
Running out of fuel. Better make sure your pump conversion is working. One thing that bothers me though--you say it revs up to WOT. That seems unusual. An increase in RPM just before it runs out of gas is normal. But wide open????
 

Bearhunter08

Cadet
Joined
Feb 6, 2016
Messages
11
I went through all the steps to the conversion I believe. I took the carb off and plugged up one of the holes that makes the positive air pressure for the old gas tanks to make it a pulse vacuum. What is the best way to run the fuel line?
 

Redstern

Cadet
Joined
Nov 18, 2013
Messages
17
Sounds like fuel starving. Sounds like your fuel pump isn't working well. It seems like it is revving up, and then it loses fuel pressure and dies. But why it revs up to max rpm without touching the throttle in the first place is very strange. Does it happen at random or does it always happen a certain amount of time after starting?
 

Bearhunter08

Cadet
Joined
Feb 6, 2016
Messages
11
Last night it actually ran normal without it going into high rpms on its own. 95 percent of the time it runs for a few after starting then it revs and dies. I figured it would be a fuel issue just wasn't for sure on what would be causing it.
 

Redstern

Cadet
Joined
Nov 18, 2013
Messages
17
Last night it actually ran normal without it going into high rpms on its own. 95 percent of the time it runs for a few after starting then it revs and dies. I figured it would be a fuel issue just wasn't for sure on what would be causing it.

That is weird. The only thing I can think for what that might be is that maybe the governor is either not set correctly or is sticking. That is, if your engine even has one.
 
Last edited:

Redstern

Cadet
Joined
Nov 18, 2013
Messages
17
How would I go about and checking the governor?

You would have to look it up. I'm not sure if your engine even has one. I know they are common on small engines like lawn mower engines, but only some larger engine have them. I can't really tell you what to look for because they don't all look the same.
 

Bearhunter08

Cadet
Joined
Feb 6, 2016
Messages
11
Okay. I will check online. I just wish I could figure this out since it will be time to go out on the lake in a month
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,047
????----No governor on that motor.------Is the fuel pump getting an alternating pulse of positive pressure and a slight negative pressure ?
 

HighTrim

Supreme Mariner
Joined
Jun 21, 2007
Messages
10,486
No governor on her.

You are running out of fuel, after the fuel in the bowl runs out.

How exactly did you do your conversion. Something is wrong with it. Either pulse line too long, intake manifold mod done incorrectly, hose routing backwards, etc...

What pump are you using?

Fill the carb bowl, then pull the fuel line going to the carb. Run the motor, with the line aimed into a bottle. Ensure the pump is working, fuel should be spraying out of the line into the bottle. If not, you know there is an issue with the pump/modification.

Ig it is pumping good, the issue is with the carb. Float sticking closed, seat clogged, etc...
 

Bearhunter08

Cadet
Joined
Feb 6, 2016
Messages
11
I got pictures of how the fuel is run. The fuel pump if a knock off mikuni (sp?) my buddy gave me. To try to get the pulse vacuum I took the carb off and plugged one of the holes behind the check values with a vacuum plug.
 
Last edited:

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,047
???---But the pressure tank only uses air pumped into the tank as needed to keep fuel flowing to the motor.---That is not a pressure pulse as required by a fuel pump.--Review the installation of this MIKUNI pump.
 

HighTrim

Supreme Mariner
Joined
Jun 21, 2007
Messages
10,486
You also would need to remove the actual check valves as well. Then plug one of the channels. That way, you are getting pulse to run the pump. Trim the vacuum plug so that the intake manifold holds it in place. If one of the holes is not properly sealed, the vacuum and pressure pulses will cancel each other out. If you do not remove the check valves, you will only get a pressure pulse, not a vacuum pulse.

I prefer to plug the old pressure line completely, then add a barb to the bypass cover for the pulse to run the pump. I understand though that the bypass covers are not removable on the 5.5hp, that is why a lot of guys resort to modifying the intake manifold on them.

Following article may help

http://www.outboard-boat-motor-repair.com/Johnson/Pressureized Fuel Tanks.htm
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
First things first. Is the pump pumping or not? Simple to find out. If you can keep it running by squeezing the primer bulb, the fuel pump probably is not pumping. You are doing the pumping with the primer bulb.
 

Bearhunter08

Cadet
Joined
Feb 6, 2016
Messages
11
I tested the pump by priming the motor, disconnected the gas line to the motor and let it run. I stuck the gas line into a clear bottle to see if it was flowing and it was pumping.
 

Bearhunter08

Cadet
Joined
Feb 6, 2016
Messages
11
I tested the pump earlier today. I primed the engine and disconnected the fuel line to the motor and stuck it a clear bottle. Started the motor up and I watched as the fuel starting pumping into the clear bottle. I also shorted the vacuum line to roughly 14 to 15 inches. I would post pictures but the forum won't put them u.s.
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
Ok, so with that in mind, my best guess is the sintered fuel filter on the bottom of the carburetor is clogged and not allowing gas to go through as fast as the engine consumes it. An extremely common problem on those motors. As a test, remove the filter and see if the motor will keep running without it.

BTW, a 14" pulse line is awfully long. It might work ok or it might not. Can't say as I've never tried it.
 

HighTrim

Supreme Mariner
Joined
Jun 21, 2007
Messages
10,486
Yeah I think that pulse line is too long. Run into that before. Better to have a shorter pulse line, and longer fuel lines BH. Will your pump fit under the carb?

If the filter is not the issue, which It likely is, your float may not be floating properly or improperly set, it might be getting caught on the float bowl gasket, etc..
 

Bearhunter08

Cadet
Joined
Feb 6, 2016
Messages
11
Okay. I will cut the vacuum line some more. What would be the best length? And unfortunately there isn't much room to mount the pump close to the motor on the the 5.5, if it was the 7.5 I could do it. I may just go ahead and order another pump.
 
Top