1975 Johnson 15hp wont idle

[browndog185]

hey guys

i've got a 1975 Johnson 15hp that will not idle. i've put a new fuel pump, new fueal lines, two new ignition coils, and rebuilt the carby, and it will not idle it'll run flat out no problem and will go down to about half throttle, but anything lower and it just dies. my dad played with the mixture screw and the idle screw is pretty useless it realy does nothing.
plz help i really wanna go fishing in the boat

 

[ultra353]

Re: 1975 Johnson 15hp wont idle

Usually when the mixture screw does nothing when adjusted it means the low speed circuit is clogged. Did you take out the core plugs and make sure there completely clean? Check your compression and see if there within 10 percent of each other. On your mixture screw start out from a lightly seated position and back it out 1 1/2 turns, with the engine up to temp slowly turn it in 1/8 turn and the wait about 15 sec for the engine to respond. keep doinging this procedure until the sounds like its goinging to die then back it out about 1/4 turn. That wil be your best setting.

 

[OptsyEagle]

Re: 1975 Johnson 15hp wont idle

Sounds like your carb did not get totally cleaned, specifically the slow speed jet. Did you use a rebuild kit or just clean it out? In the slow speed jet there is a very small plastic bushing that looks like a very small funnel. Did you change that? If you sprayed carb cleaner into that jet with the bushing in there, it might have damaged it since carb cleaner is hard on plastic parts. This bushing should be removed, then the jet should be sprayed with carb cleaner and blown out well with compressed air, and then the new bushing should be inserted. A new plastic retainer should also be used to ensure a good seal on that jet.

 

[browndog185]

Re: 1975 Johnson 15hp wont idle

hey guys,

thanks for the advice, however after pulling the carby off again and cleaning the main jet [its actually the only jet in there] and blowing air through all the pipes and holes, then rebuilding it again, it now does not even start

by the way i only cleaned the carby not with a carby kit

this may help, but i also replaced two bolts missing for the exhaust housing

i have an exploded view of the carby here:

any more is greatly appreciated

 

[Rick.]

Re: 1975 Johnson 15hp wont idle

Did you set the slow speed screw Properly? Every time you remove the carb. you need to perform a linc and sinc. Have you done that? Best of luck. Rick.

 

[AlTn]

Re: 1975 Johnson 15hp wont idle

Google...Leeroy's Ramblings...scrolled to his outboard motor related articles....his 9.9 to 15 hp. writings are extensive and a wealth of information

 

[OptsyEagle]

Re: 1975 Johnson 15hp wont idle

When you say you cleaned the main jet since it is the only jet in there, I am assuming you mean you cleaned the jet that houses the #8 needle. This is the slow speed jet. There is the high speed jet where #7 screws into. That should be cleaned as well but probably will not effect your idle.

The bushing I was talking about upthread is #10. That is a very small plastic piece that fits snuggly inside at the end of that slow speed orifice. It should not be sprayed with carb cleaner since carb cleaner and plastic bits do not go well together. I clean this area with seafoam if I do not plan to remove it. A good cleaning, however, would have that piece replaced. Blowing it out afterwards with a lot of compressed air.

What is happening at the very tip of that piece is what controls your idle (along with a few other things). The seal condition of #9 (called a retainer) is also important to the function of this carburetor. Was it is good condition?

 

[the machinist]

Re: 1975 Johnson 15hp wont idle

Also on those pre 93 motors, on the LH side of the lower motor cowling is a low speed adjustment knob. If this is set to far in (fast) you will never get it to idle. It has to be set in conjunction with the carburetor idle jet setting.

Then if this motor is like most of those old motors, the throttle cog gear linkage will be worn to the point that you may need to set the throttle twist grip faster than the indicated start position. If set to low, it will not start, but it will if set higher. Once it starts, you can slow it down.

LeeRoy from "LeeRoy's Ramblings"

 

[browndog185]

Re: 1975 Johnson 15hp wont idle

When you say you cleaned the main jet since it is the only jet in there, I am assuming you mean you cleaned the jet that houses the #8 needle. This is the slow speed jet. There is the high speed jet where #7 screws into. That should be cleaned as well but probably will not effect your idle.

The bushing I was talking about upthread is #10. That is a very small plastic piece that fits snuggly inside at the end of that slow speed orifice. It should not be sprayed with carb cleaner since carb cleaner and plastic bits do not go well together. I clean this area with seafoam if I do not plan to remove it. A good cleaning, however, would have that piece replaced. Blowing it out afterwards with a lot of compressed air.

What is happening at the very tip of that piece is what controls your idle (along with a few other things). The seal condition of #9 (called a retainer) is also important to the function of this carburetor. Was it is good condition?

no i actually cleaned th main jet #7 and the slow needle #8 and those bushings came with a fuel pump kit for some reason.
my dad has a theory that behind the Welch Plug #11 there is somthing there to be cleaned.
i went through LeeRoys Ramblings, lots of usefull info, gonna have a look at the timing, ill also try all the other suggestions today aswell.

Every time you remove the carb. you need to perform a linc and sinc. Have you done that?

how do i go about doing this btw

thanks for help guys

 

[AlTn]

Re: 1975 Johnson 15hp wont idle

LeeRoy covers that as well