120 hp power surge

[jeffh1973]

I recently replaced my 90 hp Evinrude with a 1990 120 hp Johnson. It starts and runs great up to 3500 rpms but when the throttle is opened full it will run great for about 20 seconds and then die off and then pick back up. It will do this 4 or 5 times and then die . It will start right up and run good up to 3500 rpm. Could the 90 hp fuel line be too small? Thanks for any help.

 

[hotshot123]

Re: 120 hp power surge

i dont know if the fuel line could be too small but i bet iff you pump the bulb while running, it will cure your problem. then you have another problem of having to have someone pump that bulb while you ride.

my 120 was doing the same exact thing and i replaced all the lines. its a pain working on those smaller ones near the carbs but i had an air leak in a hose and it was doing this same thing.

 

[archcycle]

Re: 120 hp power surge

That small V4 90 should have had a 5/16 fuel line, which is the minimum recommended for the big V4 120/140, with the recommended being 3/8. I kept the 5/16 from my 90hp when I switched to a 1990 140 and haven't had any problems.

This is very likely a fuel issue. Check:
Fuel tank vent could be closed
Primer bulb check valve could be faulty, bulb could be dry rotted, etc.
Pump the primer bulb while running and see if it gets better
Dry rotting around fuel line connectors
Dirty fuel filter or fuel/water separator full of water. Dump it out.
Tiny fuel lines around the carbs for cuts or dry rotting
Rebuild the carbs because you could have a sticky float needle or 4
All places where fuel lines hit connectors

Because the 90hp ran fine on that fuel line though I'm really going to lean towards the carbs or their feed hoses.

 

[jere1972]

Re: 120 hp power surge

I would agree with others, on sounding like a fuel prob, don't think fuel line size is a prob, I'm running 5/16 from my seperator to my motor which is a V6 (yeah big no-no) fuel delivery has not been an issue though, I would suspect either clogged filter, or bad/leaking VRO on fuel side, or like others have stated clogged tank vent ,I recall seeing a thread some where here where vacuum/pressure testing a VRO was mentioned (how to) I believe the VRO uses a Diaphram and they dry out crack corrode away with the ethenol just like you fuel lines!