I POSTED A FEW WEEKS AGO ABOUT MY 1980 25HP JOHNSON RUNNING FINE AT 5MPH BUT SPUTTERING AND WANTING TO DIE AT A HIGHER SPEED. SOMEONE SUGGESTED MAKING SURE THE FUEL TANK WAS VENTED WICH IVE NEVER DONE BUT IT WORKED.....UNTILL NOW. IT DOES THE OPPOSITE THING, RUNS GREAT OPENED UP BUT WANT TO DIE AT IDLE AND SLOW SPEEDS. THE CARB IS WHAT COMES TO MIND FIRST,COULD IT BE THE FUEL PUMP? I CHECKED THE FILTER AND ITS FINE. I WANT TO TRY TO FIX IT MYSELF BEFORE TAKING IT IN TO GET LOOKED AT. IM BY NO MEANS A MECHANIC BUT AM VERY MECHANICALLY INCLINED. ANY SUGGESTIONS?
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MOTOR TROUBLE.....CARB?
- Thread starter shawnfish
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What does a fuel pump do? It's sole purpose is to keep the needle and seat in the carburetor closed. If it can supply fuel to the carb at WOT then it can certainly supply it at idle. Disclaimer.....You may have a very small air leak in the fuel line that can affect pump operation (it would rather suck air than fuel though may overcome a very small leak at higher RPM) so check this at all fittings. The carburetor would be where to go next. It has very small passages in it for idle operation which can be restricted easily. I f you attempt to clean it yourself it would be wise to purchase a new kit for the carb. New kits have alcohol resistant flexible fuel components in them. Clean all small passages with compressed air or a piece of tag wire or equivalent.
huntinfool
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First off the sole purpose of the fuel pump is to pump fuel to the carb, not to keep the needle in the seat. The float's sole purpose is to keep the needle in the seat, when the float bowl is full. But I digress. That is really not important.
It sounds like you could be having a problem as stated above. However my guess is that you have some build up in the carb. The carb has several jets that the fuel has to pass through. Some are for idle and some are for WOT. If there was some trash in the float bowl and you sucked it up into the idle jet it would not run at idle. If it was in the main jet it would not run at WOT. When you opened your vent to your tank did it make a sucking sound, or did it blow? If the tank was in the sun and expanded and was forcing the fuel into the carb, it might have run at idle, but not enough pressure to run at WOT. Having said that if it made a sucking sound it could be that the fuel pump was able to pump enough fuel into the carb at idle, but not at WOT. Now that the pressure is gone, your having trouble. Next I need to ask, is your filter before the fuel pump or after. If it is before, then I would say your probably ok. However if it is after, I would suggest looking at the fuel pump. I just did one that was full of trash.
See here.
So It could be a carb problem or it could be a fuel pump problem, but either way I would bet that one or both are dirty. Good luck and ask more questions
It sounds like you could be having a problem as stated above. However my guess is that you have some build up in the carb. The carb has several jets that the fuel has to pass through. Some are for idle and some are for WOT. If there was some trash in the float bowl and you sucked it up into the idle jet it would not run at idle. If it was in the main jet it would not run at WOT. When you opened your vent to your tank did it make a sucking sound, or did it blow? If the tank was in the sun and expanded and was forcing the fuel into the carb, it might have run at idle, but not enough pressure to run at WOT. Having said that if it made a sucking sound it could be that the fuel pump was able to pump enough fuel into the carb at idle, but not at WOT. Now that the pressure is gone, your having trouble. Next I need to ask, is your filter before the fuel pump or after. If it is before, then I would say your probably ok. However if it is after, I would suggest looking at the fuel pump. I just did one that was full of trash.
See here.
So It could be a carb problem or it could be a fuel pump problem, but either way I would bet that one or both are dirty. Good luck and ask more questions
huntinfool
Well-known member
Here is a pic of one of the carb jet types. There are several different types, this is just one.
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huntinfool said:
And how does the float bowl get full...........from the pump! So I guess the sole purpose of the fuel pump is to fill the float bowl......which just happens to close the needle and seat when full.
the filter is before, i started to remove the silencer and noticed the starter needed to be removed to get to a bolt on the carb and decided i didnt have time(dinner for fathers day)so i stopped..after i ate my boys wanted to go fishing so i put the bolts back on and got it all together when i realized enable to remove the silencer i had to unscrew the low idle adjustment knob(had a needle that went into the top of carb)and had no idea how far to screw it in,so i screwed it in till i could bareley feel a little resistence. after wearing out some crappies on a brush pile for a bit i decided to take off the motor cowling and start it to see what would happen(i used the minn kota to get to the pile). a few tries and nothing so i screwed the low idle adj. knob a lil more and it started and died immedeiateley, finally i got it just right and it idled great, got it up on plane and it ran fine. did whatever might have been clogging something up pass through? that adjustment had'nt been touched since my gramps gave me the boat. the knob has leaner and richer arrows, is that for air mixture? oh yeah the little mark on the knob is in the same place it was before(middle of leaner/richer)so im not sure that was the cause. also it was same gas. just would like to know if its something that might occur again?huntinfool said:
That knob just pulls off.No need to mess with adjustment. #-o
