Surging / losing rpm

[achris]

... The switch is needed when shifting into gear. It momentarily cuts the power to the ignition system.

Not required when shifting into gear. Required when shifting OUT of gear.

Once the shift is made, the ignition is restored. Momentarily killing the ignition keeps the lower unit gears from meshing with power applied to them.

Again, no. The dog clutch is undercut, and with load on the propeller while in gear, the dogs will not un-mesh. The momentary engine 'stall' unloads the dogs when they are being pulled apart. That takes the load off them and allows them to be un-meshed.

Killing the engine just as a load is applied is a very bad idea, just ask someone who's got a damaged/un-maintained shift system!

Chris.......

 

[Grub54891]

Don't think it's your issue, but a bad tachometer will cause running issues. Try disconnecting it and see if any thing changes.

 

[jws123]

Don't think it's your issue, but a bad tachometer will cause running issues. Try disconnecting it and see if any thing changes.

^^^^^
I had something like this before my tach was old a and shorting out was doin all kinds of funny stuff was shocked it was the issue.

 

[tank1949]

Hey this is my first time on here but I have a 2001 bayliner with a 5.0 mercruiser. It will randomly lose rpm and jump right back up. The boat will pull back a little then speed back up. I’ve been chasing this problem over 2 years. Sent it to many mechanics and tore it down twice. I will attach a video of the problem and a list of all things I have changed. Almost a completely new motor at this point. It won’t let me post the video but the best way I can explain is I’ll be cruising and everything’s fine. Rpm’s will randomly drop and shoot back up by itself and the boat will slug back a little then go back to the same speed

You may have an electrical wire leaking current. Start with muffs on and at night. Be sure to have good fire extinguisher near by. Using rubber gloves, exercise electrical wires and look for leakage. However, I suspect fuel blockage issues. My tanks had inline fuel filters inside the tanks' pickup tubes. I removed and rely soley on water fuel separator/filter. Old tanks accumulate crud and crud gets sucked up.

 

[nola mike]

Throw a timing light on your coil wire. You'll be able to see if your ignition is momentarily cutting out, and if it's before or after the distributor cap. Agree that it looks like a wiring problem. Can also try wiggling wires when underway to see if you can reproduce.

 

[Dylangrimsley]

Okay I know I’m super late but I still have the exact problem. It still has the random cut out sometimes it’s very often sometimes it’s further between but taking it in by myself the other night it cut out twice and completely cut off. Then fired right back up and continued to cut out randomly all the way to the dock. I did try to make a test run the day before and I pushed off from the dock before cranking cause it’s always crunk but this time it didn’t. It would start up and shut off soon as it cranks like it wasn’t keep fire. So I pulled the carb cover and watched gas shoot in but it still wouldn’t stay crunk. Then I took off the fuel line going into the carb and turned it over and watched gas shoot out. So I screwed it back in and fired right up. Idk if that helps any. And I looked at the Serial number plate on the side of the motor and I’m 100% sure it reads M001615 but I can’t find anything on google. No number even close. Idk what I’m getting wrong. Any help would be very helpful thanks ahead

 

[nola mike]

Okay I know I’m super late but I still have the exact problem. It still has the random cut out sometimes it’s very often sometimes it’s further between but taking it in by myself the other night it cut out twice and completely cut off. Then fired right back up and continued to cut out randomly all the way to the dock. I did try to make a test run the day before and I pushed off from the dock before cranking cause it’s always crunk but this time it didn’t. It would start up and shut off soon as it cranks like it wasn’t keep fire. So I pulled the carb cover and watched gas shoot in but it still wouldn’t stay crunk. Then I took off the fuel line going into the carb and turned it over and watched gas shoot out. So I screwed it back in and fired right up. Idk if that helps any. And I looked at the Serial number plate on the side of the motor and I’m 100% sure it reads M001615 but I can’t find anything on google. No number even close. Idk what I’m getting wrong. Any help would be very helpful thanks ahead

Please start your own thread...

 

[itsathepete]

Please start your own thread...

This is his thread.

 

[nola mike]

This is his thread.

Well. Then carry on!

 

[Dylangrimsley]

Please start your own thread...

Please pay better attention next time

 

[Scott06]

Okay I know I’m super late but I still have the exact problem. It still has the random cut out sometimes it’s very often sometimes it’s further between but taking it in by myself the other night it cut out twice and completely cut off. Then fired right back up and continued to cut out randomly all the way to the dock. I did try to make a test run the day before and I pushed off from the dock before cranking cause it’s always crunk but this time it didn’t. It would start up and shut off soon as it cranks like it wasn’t keep fire. So I pulled the carb cover and watched gas shoot in but it still wouldn’t stay crunk. Then I took off the fuel line going into the carb and turned it over and watched gas shoot out. So I screwed it back in and fired right up. Idk if that helps any. And I looked at the Serial number plate on the side of the motor and I’m 100% sure it reads M001615 but I can’t find anything on google. No number even close. Idk what I’m getting wrong. Any help would be very helpful thanks ahead

I think you need to put a 0 in front of your serial number here is a parts break down fir it https://www.marineengine.com/parts/mercury_marine_parts/sn/M001615

i would suspect this is electrical loose or dirty connection. Do you loose your dash power when this happens ?
thoughts are look at positive and negative battery and wire connections make sure they are clean and tight.
try disconnecting the shift interrupt (assuming its an alpha) see if it goes away.
inspect the cannon plug for loose or corroded connection
jumper around man overboard or kill switch

 

[dubs283]

Sounds to me like you definitely have an ignition issue. Cutting out cold then firing back up immediately usually rules out fuel supply. From previous posts you mention new spark plugs, have you inspected/changed out any other tune up parts?

Judging by provided info I'll guess you have a thunderbolt ignition system. Number one cause of the issue you describe as related to ignition is the sensor in the distributor. Three wire, oem, grounded to same stud as main battery is the only way to rule out this component when dealing with an intermittent issue like yours