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1991 70hp Johnson outboard

C

Cshaw

Member
Looking for a little bit of help I’ve been trying to trouble shoot my new boat motor new to me I guess the problems it’s having is I will take it out to the lake it will start up fine runs idles good runs full speed just fine the. After I idle it down a bit and cruise the motor dies out and doesn’t want to start back up it’s still primed the only thing that helps it out is if I tilt the motor all the way up so I’m wondering if maybe I have a problem with the carbs? Or the fuel pump is getting weak and only works decent for about 10 mins any help would be appreciated
 
You have a bad compression gauge or a motor that needs work.----I would pull the cylinder head for an inspection of cylinders.----Note ----They all " bark and roar " on the hose and sound really powerful to a novice !
 
There are lots of bad gauges out there.----There are lots of weak starters out there.----There are lots of weak batteries out there.-----New head gasket is cheap.
 
Another thought....full agreement with my mentors here. However, could be flooding. When it does restart, after "draining carbs" by tilting it, does it smoke? When it restarts are you using the choke? Could be a foreign object stuck in the float needle valve on one carburetor. I had this on a 75 that I rebuilt last year. However, it cleared itself up and the guy is happy with it. It's possible that it is stuck in there and you may need to find out which one by pulling the spark plugs and note the cylinder that is rich. Then remove that carburetor and clean it. Make sure to synchronize all the carburetors on these as they may mount in slightly different position every time....... enough to cause rich or lean on the carb that was removed. If this is the case, you may get a rougher idle or if it's lean could even dry out that cylinder under extended low speed operation. Then you could end up with serious damage to the motor. Some operators and owners do not know that each cylinder is completely separate from the other and with a separate carburetor associated with each cylinder, you may develop a problem as aforementioned.
 
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timguy said:
Another thought....full agreement with my mentors here. However, could be flooding. When it does restart, after "draining carbs" by tilting it, does it smoke? When it restarts are you using the choke? Could be a foreign object stuck in the float needle valve on one carburetor. I had this on a 75 that I rebuilt last year. However, it cleared itself up and the guy is happy with it. It's possible that it is stuck in there and you may need to find out which one by pulling the spark plugs and note the cylinder that is rich. Then remove that carburetor and clean it. Make sure to synchronize all the carburetors on these as they may mount in slightly different position every time....... enough to cause rich or lean on the carb that was removed. If this is the case, you may get a rougher idle or if it's lean could even dry out that cylinder under extended low speed operation. Then you could end up with serious damage to the motor. Some operators and owners do not know that each cylinder is completely separate from the other and with a separate carburetor associated with each cylinder, you may develop a problem as aforementioned.
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before I even ran the boat on the water I took off all the carbs to make sure they weren’t dirt if they needed anything the carbs looked good freshly rebuilt And the floats seemed to be working correctly I did not synchronize them after re installing I will have to do that
 
timguy said:
Another thought....full agreement with my mentors here. However, could be flooding. When it does restart, after "draining carbs" by tilting it, does it smoke? When it restarts are you using the choke? Could be a foreign object stuck in the float needle valve on one carburetor. I had this on a 75 that I rebuilt last year. However, it cleared itself up and the guy is happy with it. It's possible that it is stuck in there and you may need to find out which one by pulling the spark plugs and note the cylinder that is rich. Then remove that carburetor and clean it. Make sure to synchronize all the carburetors on these as they may mount in slightly different position every time....... enough to cause rich or lean on the carb that was removed. If this is the case, you may get a rougher idle or if it's lean could even dry out that cylinder under extended low speed operation. Then you could end up with serious damage to the motor. Some operators and owners do not know that each cylinder is completely separate from the other and with a separate carburetor associated with each cylinder, you may develop a problem as aforementioned.
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and yes the last time I did a test run on the water when it stalled out after running for awhile I gave it a little choke to get it started back up then it ran fine until I get to the boat dock and idle back down
 
Okay then. Compression is low but adequate. Perhaps your fuel tank is not venting properly and starving for fuel at low speed....your pump cannot overcome the vacuum created in the fuel tank.....especially once it's built that vacuum, then you slow down.....(the pump is getting less action at idle/slow speed).
 
timguy said:
Okay then. Compression is low but adequate. Perhaps your fuel tank is not venting properly and starving for fuel at low speed....your pump cannot overcome the vacuum created in the fuel tank.....especially once it's built that vacuum, then you slow down.....(the pump is getting less action at idle/slow speed).
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It is vented properly kind of tested the vent out by pressing the plastic gas tank down by hand to make sure getting good ventilation
 
Is there any chance you can try another fuel tank? Some venting systems are unreliable on plastic tanks. At least give it a run, maybe with fill cap loose enough to allow certain venting.
 
timguy said:
Is there any chance you can try another fuel tank? Some venting systems are unreliable on plastic tanks. At least give it a run, maybe with fill cap loose enough to allow certain venting.
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yes I have another spare metal tank I will try here on Saturday and I plan on checking everything when or if this problem occurs again when I take it down to the lake
 
timguy said:
Is there any chance you can try another fuel tank? Some venting systems are unreliable on plastic tanks. At least give it a run, maybe with fill cap loose enough to allow certain venting.
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yes I have another spare metal tank I will try here on Saturday and I plan on checking everything when or if this problem occurs again when I take it down to the lake
 
Maybe is preset low speed......" Idiotproof".....which doesn't protect me any. Probably has around a #35 behind that plug in front top.
 
timguy said:
Maybe is preset low speed......" Idiotproof".....which doesn't protect me any. Probably has around a #35 behind that plug in front top.
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looked around online it says these carbs have preset jets so there’s no way to adjust the mixture it says I can adjust the idle at the idler arm
 
Not much you can do on the newer motors. Try the gas tank, making sure good venting for it, then when it stalls, and you restart it, check for excessive smoke. Is the VRO still functioning?......that's the oil injection system.
 
Coaching folks on outboard repairs can be frustrating.----The parts books clearly show slow speed mixture screws on the 91 models.----So look at the carburetors and find them !!-----Or perhaps the motor is not a 91 model or the carburetors are not original !!
 
timguy said:
Not much you can do on the newer motors. Try the gas tank, making sure good venting for it, then when it stalls, and you restart it, check for excessive smoke. Is the VRO still functioning?......that's the oil injection system.
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no the oil injection has been disabled and plugged not sure when I purchased the boat like that And not really and excessive smoke little bit when you first crank it up that’s about it
 
racerone said:
Coaching folks on outboard repairs can be frustrating.----The parts books clearly show slow speed mixture screws on the 91 models.----So look at the carburetors and find them !!-----Or perhaps the motor is not a 91 model or the carburetors are not original !!
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here’s what should be 91 carb diagram https://images.app.goo.gl/56Hba98qay41kh9z9

and the part number 15 is the adjustment screw right if so then it doesn’t have it
 
Correct ----So to settle this argument perhaps your motor is NOT a 1991 model .----Or carburetors have been replaced.-----Model # of your motor is ???
 
It should almost be a requirement on here.----No posting without model # or serial #.----Or pictures galore of motors in question !---Yes there are early and late production models for 89.
 
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