Motor Surging at 30 MPH

[TNeal]

97 Johnson 175 FastStrike - Went to Kissimmee to prefish for a tournament in two weeks. Motor ran great first thing in the morning, fished for about two hours. Started to move, had a hard time getting it started, it'd run a few seconds and die. Then I think I flooded it, so I waited a while and tried again. Once I got it started, got on plane, it ran fine up to about 30 MPH. Then it started surging. By surging I mean it was like pushing down and back on the throttle, push and let off. Then it would just die. It would start right back up but do the same thing over and over. We idled to the nearest boat ramp, didn't want to push the motor. Also, any suggestions for a mechanic in the Spring Hill/Brooksville, FL area

Thanks

[Fishbone]

Sounds like fuel/delivery problems.

[Ann-Marie]

Agree, clean or replace all the fuel filters.  Double check all connections for somewhere a small amount of air might be creeping in on the suction side.

[Wayne P.]

Did you get the motor overheated with vegetation blockage at the water pickup? Did your tach stop working or acted weird?

Been there, done that, my mechanic said I needed a rectifier replacement. That was done and it worked great.

[Way2slow]

Was that a carburated or DFI motor? I'm not sure how a rectifier could cause it if it's a carburated motor, I've never seen that happen. Might be possible with DFI because if the battery gets weak in one of those, they can do strange things. It will cause the tach to quit or do wierd things because thats were the tach signal comes from but once a Johnson or Evinrude carburated motor is running, it does not need a battery. You can throw the battery away and it would never know the difference, just won't have any tilt/trim.

On that FastStrike, my first check would be the fuel pressure, make sure it's at least 5 psi. Very good chance the fuel pump side of your VRO has started go bad. You could have a restriction but if your vacuum switch is working, it should give you a warning buzzer when you get enough restriction to cause loss of fuel. As mentioned it could be a suction leak from a bad/loose hose letting it suck some air. Suction leaks and be found by putting about five psi of pressure in the fuel tank vent and see if any leaks show up anywhere in the fuel line system. Make sure the vent is not blocked also. Do a fuel pressure check first, then you can work back from there if it's low.

Don't think you will just run down and buy a new fuel pump and try it. Your jaw is going to hit floor when you find out what it cost. There are other options (much cheaper and better) over buying a new CRO pump but they all will require you to start premixing your fuel.

[Wayne P.]

Way2Slow, the mechanic that fixed mine called the replaced part a rectifier. It may be a voltage regulator. It is water cooled and located at the top rear of the motor. When it gets overheated, it fails. The tach usually stops working when it starts to fail.  I had two replaced because of overheating when fishing in the Florida vegetation/mud and the vegetation on the Potomac River. The engines are carburated OMC Fastrike.

[Way2slow]

On that motor the rectifier and the regulator are one in the same, both are built into the same module and yes, it goes where you stated.  They have a bridge rectifier and the unfilter pulses across the bridge is what tellls the tach how many rpms the motor is turning.   I've just never seen one, or see how one can cause a surge.  All it's doing is keeping the battery charged and supplying a signal for the tach.   The motor runs off a CDC ignition that makes it's own ignition voltage and does not need an external voltage source to run.   The only way you can shut the motor off once it's running is by grounding out the power pack.  I've seen the igntion grounds break one or both of the connectors and people going nuts trying to figure out how to get the motor stopped.  The only thing you can do then is start pulling off plug wires or pull the air box off and cover the carbs.

[Chaz Hickcox]

I've got a 93 Johnson 150Hp and had the exact same problem.  It was overheating and Johnson has a failsafe in place to shut down your RPM so you don't blow it up.  All I had to do was replace the thermostat and it works just fine.  So in short I can say you're overheating, but that's just a symptom, I can't tell you why.

[Way2slow]

When it went into overheat mode, did the rpm just drop to 2,000 or 2,500 and hold at no more than that or did it surge between full throttle and 2,500 rpm?  Usually the over heat mode cuts the motor back so it can't turn over 2,500 rpm and does not surge between WOT and 2,500 rpm.  It will usually give you and overheat alarm also.

[Chaz Hickcox]

My alarm was broken so I never new what was going on by sound other than the engine.  When I would open the throttle it would sound like it would be surging back and forth causing the boat to cavetate in the water.  It was just as you described.