1985 Johnson 120 won't stay on plane

Titanium48

Petty Officer 1st Class
Joined
Apr 24, 2008
Messages
303
I have 16.5 ft bowrider with a 120 hp Johnson outboard (CJ120TLC0). It starts and idles well and runs fine under about 2500 rpm. Holeshot is good when I hit the throttle, but after a short time (10 - 30 seconds) the engine will lose power, surge a few times and then stall. Restarting it after it stall is like a cold start (need to push the key for "choke"). If I pull it back to idle before it stalls it will usually stay running. The primer bulb is a bit soft after it stalls, but it is not flat and is still full of fuel. This was a VRO engine but the previous owner had disconnected the wire running to the oil tank and run 50:1 premix. Initially, the problem was intermittent - the motor would stall on the first few attempts at getting on plane, but would then run at any speed for an extended period. It got worse the last time I had it out and I had to keep it at 2000 rpm to get back to the launch.

I suspect the fuel pump isn't delivering enough fuel to keep the engine running at high speeds. Any other suggestions? Good ways of confirming this? I couldn't find a fuel pump rebuild kit for my engine in the iboats store - do they exist?

Thanks.
 

Daviet

Fleet Admiral
Joined
Sep 24, 2008
Messages
8,958
Re: 1985 Johnson 120 won't stay on plane

If you have someone pump the primer bulb continusly, it will bypass the fuel pump. If problem goes away, you need to overhaul/replace the fuel pump.
Are you still using the fuel side of the VRO pump, or was it converted to a pulse type pump?
 
D

DJ

Guest
Re: 1985 Johnson 120 won't stay on plane

Classic fuel tank vent issue or anti siphon valve issue.

1. Run the boat with the fuel tank cap left loose. Does it improve?

2. There is an "anti siphon valve" located on the fuel tank outlet. In many cases, it is the outlet. That valve can be removed for TESTING only. If that proves to be the culprit, replace it. Cheap, about $15.00-tops.
 

Titanium48

Petty Officer 1st Class
Joined
Apr 24, 2008
Messages
303
Re: 1985 Johnson 120 won't stay on plane

Thanks for the replies. I'll try the continuous pumping test next time I take it out. It is still using the fuel side of VRO pump.

I will also check for vent or fuel hose restrictions. I would have expected the primer bulb go flat if the fuel restriction is upstream or if the tank is under vacuum from a blocked vent, is this not always the case?
 

Titanium48

Petty Officer 1st Class
Joined
Apr 24, 2008
Messages
303
Re: 1985 Johnson 120 won't stay on plane

Update: I checked the vent and it was free-flowing. I checked for fuel line restrictions and found none. There was no anti-siphon valve and when I disconnected the fuel line at the engine I could easily fill a 500 mL bottle with a few pumps of the primer bulb. I replaced the fuel line between the cowling and the filter and replaced the zip ties with actual hose clamps. It turned out that the oil side of the VRO was actually still connected, but previous owners mixed their fuel and left the oil tank empty, so I plugged the oil line. I finally got a chance to get the boat back out (work and rain were conspiring against me), but it still behaved the same (not running well over 2000 RPM). When I tried manually pumping the primer bulb, it ran just fine at WOT.

So, it looks like I need to fix the fuel pump. Can the first generation VRO pumps (P/N 174264, no wires attached) be rebuilt? What about using an older style pump? I've seen discussion here about using BRP 438559 or Sierra 18-7353 as lower cost replacements for the VRO pump, but generally for smaller motors. Would I need 2 for a 120 hp? If so, should they be plumbed in series or parallel?
 
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