5.7 Mercruiser Surging at high RPM

[bout time]

Took the family out fishing and my boat started to surge after I got it up on plane. It would drop 300-400 RPM and come back. I thought it might stall a couple times. It is fine on low rpm and only does it while up on plane.

 

I checked the inline fuel filter and checked for debris. It was clean. I replaced it any way and it still does it.

 

I only run non-ethanol 87 gas. Could this be bad gas ? Water in gas?

 

 

'Bout Time

Mike

[180ccf]

Chased a problem like this on my fathers boat (350 Ray with twin 454's). Would you believe after a lot of jerking around, it was the stock tach's. We discovered it while testing. I forgot to re-hook the tach's. Then it ran perfect. We had changed points, caps, rotors, plugs, wires. Full electrical tuneup. By forgetting to hook the tachs backup, we found the problem. Something to think about if you are at a loss or looking for something to try that does not cost any money.

[lakebound88]

Replace cap and rotor.. common cause here, if problem persists investigate deeper.

Making your stories, Reel Stories, since 2011!!

[Bozeman Bob]

Besides the inline filter do you have a remote filter that looks like a oil filter ? Many boats will have two or even three filters. When was the carb last overhauled ? You could have some dirt in it, but my first thought is the filters or kinked hose that isn't letting the correct amount of fuel in. I would think if you had water in the gas it would work its way out or not run at any speed, along with you changing the inline filter which is probably close to the carb where the water would end up.

[L&M]

Have Scott look at it in the morning

 

[bout time]

Have Scott look at it in the morning [emoji4]
 

That's always the plan if I can't figure it out on my own. You know me and I like to turn some wrenches first


'Bout Time
Mike

[flyboy7]

That happened to me one time. It was a loose battery terminal.

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[Valinote]

My port engine suffers the same sporadic loss of rpm every couple trips with the added symptom that it might backfire once or twice. Been happening on and off since last season and doesn't seem to matter if the engines are hot or cold. I was going to have Scott take a look at it but with the 5 mph limit in the bay...well it'll have to wait till the water goes down. I hate to say it, but it would be a lot easier to diagnose the problem if the motor would just die and not start!

-Steve D

[GAMBLER]

12 hours ago, Valinote said:

My port engine suffers the same sporadic loss of rpm every couple trips with the added symptom that it might backfire once or twice. Been happening on and off since last season and doesn't seem to matter if the engines are hot or cold. I was going to have Scott take a look at it but with the 5 mph limit in the bay...well it'll have to wait till the water goes down. I hate to say it, but it would be a lot easier to diagnose the problem if the motor would just die and not start!

-Steve D

Sounds like water in your fuel. 

[bout time]

Water separator changed, fuel filter changed, new cap and rotor, K100 added to fuel, carburetor sprayed and cleaned......5 mph ride to lake to give it a try


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[lakebound88]

Sometimes it's nice to do one at a time so you can learn from it when the repair actually fixes it, and knowing what the cause/symptoms are for future references. Good luck

Making your stories, Reel Stories, since 2011!!

[Xxx]

If all above fails I had a float in the bowl of the carb hang up and not let enough fuel get in to the bowl which is like a reservoir for extra fuel to keep a consistent amount of fuel supplied to the the motor. I've also had the bowls half full of water which also makes for thee same surging mode on a high demand for fuel over a long distance.

[bout time]

Just returned to dock. She ran flawlessly. I was able to get 25 knots / 3700 rpm into the waves and it never hiccuped. Ran it for about 15 minutes or so just to let it get a good test. Stopped and accelerated to cruising speed 3 or so times.


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[lakebound88]

Good to hear a basic preventative tune up fixed your issue. Happy fishing!

Making your stories, Reel Stories, since 2011!!

[bout time]

Worst part is the cap and rotor were done last year along with plugs and wires. Fuel separator was new this May. All in all I'm pretty anal about my maintenance and servicing. I'm not complaining at all. It cost me 100 bucks for all the new parts and additives and I'm back in action now

Thanks everyone for all the suggestions and help


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[Xxx]

Alway nice when a little TLC does the trick!!!  Congrats on the self repair!! 

[Bozeman Bob]

If you got some water on one of your fill ups that probably would of ended up in your spin on filter and be the source of your issue.  One of my two filters per engine has a drain petcock on the bottom, and since gas floats on water it is easily checked with a clear glass catching it when drained periodically for inspection. Good to hear you got her purring !

[woody 184]

I had the same thing happen on my Penn Yan and thought it was water in the fuel. It ended up being the coil. I replaced it and never had a issue again..... Woody