1977 johnson 4hp stalls at trolling speed

[hardwater fisherman]

i have a johnson 4w77. it starts well, runs at wot well. when i throttle back it runs fine. but when i run it at trolling speed for a long time it will stall. then i need to choke it fully to start it. if i throttle up when i hear it is going to stall it will stay running. this motor is always in gear so its hard to adjust the carb when its on the boat,i put the boat up on the bank and tried to adjust it. but it runs well for 5 mins before it stalls so its hard to know which way to turn the low speed adjustment. some times it just dies suddenly. i know its not my hose because i run my 40 hp off the same hose and tank. maybe the carb needs to be cleaned? if anybody has any ideas please let me know.

 

[Mercman2008]

Re: 1977 johnson 4hp stalls at trolling speed

Sure with all outboards you start with a compression check if that checks out look at the condition of the plug.

it sounds like you do have a carb issue but before you jump the gun make sure it is not just as simple as a bad plug.

 

[hardwater fisherman]

Re: 1977 johnson 4hp stalls at trolling speed

i checked the compression it was 112 psi on top and 110 psi on the bottom so i think thats ok. i ran it in a barrel and i found out the primer bulb was going soft and it would die. if i pumped it until the bulb was very hard it would run for 10 minutes then die. when it died the bulb was soft. so then i started and every few minutes i pumped the bulb and it kept running. if i pumped the bulb and then shut it down it would start right up. but if i let it die it was hard to restart. to restart i needed to pump the bulb and also choke it. does this seem like a fuel pump issue? it runs good at 1/4 throttle and more.

 

[Don Hansen]

Re: 1977 johnson 4hp stalls at trolling speed

It's normal for the bulb to go soft. Picture the fuel pump sucking fuel from the tank. May be obvious, but does your tank have a manual vent you need to open. If that's not it. Remove the cover from the fuel pump, the big head center screw, and check the screen. If the screen is clean then the fuel pump probably needs to be rebuilt.

DLH

 

[nwcove]

Re: 1977 johnson 4hp stalls at trolling speed

when was the last time the carb was cleaned properly and re-kitted? and even with the motor being always in gear, you still need to fine tune the ls needle while underway....not in a barrel or with the bow ashore.

 

[hidef]

Re: 1977 johnson 4hp stalls at trolling speed

The bulb should not go completely soft. It sounds like your fuel pump is going bad. I would recommend rebuilding or replacing it and also replace both fuel lines under the cowl with alcohol resistant ones if they haven't been changed out.

 

[hardwater fisherman]

Re: 1977 johnson 4hp stalls at trolling speed

i have an update to share. i cleaned and rebuilt the carb with a rebuild kit. then it ran better but not like it should. so i priced a fuel pump and it was 127 dollars from brp. i noticed they used this pump on my motor and then again in the early 90s until 2005. it fits various models up to 8hp. then i thought that pump looks alot like the pump on the old 9.5hp i have. then i checked the parts list and sure enough that pump was used also for many years mostly on the 9.5s and 10s also on 15s. but it was also used on 6s and 8s so it started looking at it and the only difference is the location of the outlet. so i put it on and pumped the bulb and it started first pull and i got it to run slower than i ever saw it run. i guess it never hurts to have some parts motors. i had to shorten the gas line going to the carb. on the original pump the outlet is on the bottom and the replacement is out the side facing forward. thanks all for the help and have a good day.