Motor Alternates Between Surge/Lull on Johnson 70hp

[buenanj]

I have a 70 hp 1999 Johnson outboard which has developed a problem that has me perplexed. The motor will run perfectly until it has been turned off and let sit for about half an hour (i.e. run out of the harbor without problems, turn the motor off to fish and then have issues on the ride back).

Once restarted, the motor will alternate between normal power and bogging down, this usually runs in a cycle of say 10 seconds at full power and then 10 seconds of an idle speed and then back to full power. This cycle repeats until the motor shuts off at which point, it won?t restart for a few minutes and then it?s back to the surge/lull cycle.

I have tried the following with no luck: replace gas, replace fuel line from tank to motor, new plugs, new filter, run seafoam through gas and sprayed carb cleaner in the carbs. If anyone has ideas on how to solve this issue, I?d really appreciate the help!!!

 

[racerone]

Run with a timing light to see if you are losing spark on one cylinder.-----Possibly test run with the cover off the motor too.

 

[buenanj]

Thanks for the tip, I'll give that a shot. I guess as the motor heats up it could cause issues with the electrical system?

 

[pn]

try pumping the primer bulb when it lugs, if it picks right up then i'd suspect fuel pump since you've replaced all the fuel lines, check your vent to your tank is clear too, blow some air thru it.

 

[buenanj]

Good thoughts, although I tried pumping the primer bulb while it was bogging down without any noticeable difference (also replaced the primer bulb). I also took off the gas cap to ensure that it wasn't creating a vacuum.

 

[pn]

check out your coils, pull em off look for cracks, clean up grounds, all grounds, sounds like somethings getting hot. is the motor heating up or getting hot, shouldn't be too hot to touch. you might taking your carbs off and cleaning them, might be something floating around the float bowl, debris.

 

[buenanj]

Pn thanks for the tips. I tried to duplicate my issue yesterday and noticed that at idle there wasn't any water coming out the back of the motor. If i gave it some extra throttle (in neutral) the water would start coming out and would be really hot. The water was too hot to touch and steam was coming off the water coming out of the motor which seems odd to me. The stream of water coming out of the motor was otherwise fairly strong once i gave it some throttle. At idle/3knt speed the water coming out the back would be kind of weak and then stop.

 

[racerone]

Replace the pump impeller.----Sounds like a cooling issue and motor going into S.L.O.W. mode.

 

[buenanj]

Today I replaced the thermostat which seems to have made a small difference in the consistency of water coming out of the motor. I tried to do the water pump and ran into an issue where after I took out the four mounting bolts and two bolts behind the prop, the lower unit wouldn't come off. Not only did it not come off, but despite my pulling and tapping with a mallet, it acted like it was still bolted on. Any ideas on why the lower unit won't drop? Even if I messed up releasing the shift rod, i'd think there would be some play between the lower unit and rest of the motor. I'm slightly stumped on this....

 

[buenanj]

Appreciate the insights into the motor problems. Redid the water pump and took the boat out for a test run. Boat ran great - I didn't have the time to let it fully sit for a few hours with the engine off, but so far looks good!

 

[buenanj]

update: Think it was a vapor lock issue. The motor had the same problems as before even with a new water pump and thermostat. I wound up installing an electric aux fuel pump which is able to pump gas when the motor starts choking out. I've done more research and it appears that a vapor lock is the root cause of the problem.

 

[joetheis]

Wow! I hate weird problems like that!
I replaced my impeller and thermo last fall, (thermo actually had a small shell stuck in it to keep it open)!
Even at idle my motor squirts water out like a drunk sailor peeing off the side, a real strong LOUD squirt!
So your fuel pump was shot? You'd think when you pumped the primer ball it'd pick up?? (I ran 45 minutes in on another boat with a craped out fuel pump, on the primer ball)!
Is it a VRO?