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Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 14299
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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2007 50hp mercury runs fine then will just idle down then rev

Customer Question

2007 50hp mercury runs fine then will just idle down then rev back up then down then up...
Submitted: 1 year ago.
Category: Boat
Expert:  Jason replied 1 year ago.

Hello my name is***** am I speaking with?

Can I get the serial number on the engine please.

Customer: replied 1 year ago.

I will go get it

Expert:  Jason replied 1 year ago.
Okay
Customer: replied 1 year ago.

1B503983 SER#


(###) ###-####D MODEL#
Expert:  Jason replied 1 year ago.
Got it. Now when you say "run fine then will just idle down then rev back up then down then up" how many rpms is the engine dropping? Is it going from 5k rpms down to 2? More, less?
Customer: replied 1 year ago.

It is hard to say because last season it would miss at around 3k to3.5k my mechanic disconnected the tach and the problem went away....but now it will be wide open (16ft Pontoon) running about 18MPH then go to slower speed like 13MPH then chug then back to full power

Expert:  Jason replied 1 year ago.

The mechanic just disconnected the tach, but didn't actually troubleshoot the problem?

Customer: replied 1 year ago.

Basically he stated that I didn't need a tach and it is running fine now. It did run like new last season after his work....new fule lines (deteriorated from ethanol) carb adjustments... and timing

Expert:  Jason replied 1 year ago.

I understand. The engine needs 3 things to run. Compression spark and fuel. Compression doesn't really change, you either have it or you do not. So when the engine rpms start going up and down. The engine is either starving for fuel. Or it is dropping spark on cylinders. In order figure out which you would have to take the boat out on the water, duplicate the problem. And then check for spark with a timing light. You will either have it or you will not. If you do, there is a fuel issue. If you do not, then the ignition system needs more troubleshooting.

In regards ***** ***** ignition. There is a compent under the flywheel called a stator. The stator provides the power for both the igntiion system and the charging system.

The stator drives the charging system. The charging system drives the tachometer. If this problem is related to the tach issue I would lean towards the stator. But it all needs to be tested as these parts are not cheap.

I don't know what what you have for tools, time, or ability. Some on water testing will be requried to get us pointed in the right direction. Did you have a timing light and can you check spark? How can I best be of help to you?

Customer: replied 1 year ago.

I guess I will start with the fuel....it is raining here today so hopefully tomorrow I can narrow it down...I will let you know. I can get a timing light....what am I doing with the light to check for spark?

Expert:  Jason replied 1 year ago.

For fuel testing what you want to do is duplicate the problem and then pump the primer bulb over and over and over and if it picks up or not. Also see if the bulb sucks flat or does not want to reinflate

On spark, you are hooking up the timing light to each plug wire 1 at a time and seeing if the light flashes consisitantly with engine rpms or not.

Let me know how you make out.

--Jason

Customer: replied 1 year ago.

OK thanks I will be in touch

Expert:  Jason replied 1 year ago.

You bet. Enjoy the rest of your night.

--Jason

Expert:  Jason replied 1 year ago.
Good morning. Just checking in to see how you made out. Any luck?
Customer: replied 1 year ago.

No spark cylinder #2

Expert:  Jason replied 1 year ago.
Got it. Your going to have a bad CDM module (what they call those coils )CDM's are a little funny, just because #2 isn't firing does not mean it is actually bad. Are you near the engine now in order to do a little bit of troubleshooting in order to determine which CDM is the bad one?
Customer: replied 1 year ago.

Not right now..... If one is bad they are all the same age.... Would it be logical to replace them all?

Expert:  Jason replied 1 year ago.

Not necessarily, they run about 100 bucks each. Why spend 300 when you only need to spend 100.

On the CDM modules CDM #1 gets its charging ground path through CDM #2 or #3

CDM #2 and #3 get their charging ground path through CDM #1

A shorted Stop Diode in CDM #1 would prevent CDMs #2 and #3 from sparking.

A shorted Stop Diode in CDM #2 or #3 would prevent CDM #1 from sparking

You have a couple of different ways ot go about this. You can swap 1 and 3 and see if spark returrns on #2 or not. If it does, CDM 1 is actually bad, not sending a ground signal to CDM 2. Or swap 2 and 3 and see if 2 starts sparking again, and if 3 dies, then it is a bad CDM #2. Or, just get 1 new CDM and stick it in #2 spot and see if it sparks. If not, put #2 back and then put the new on in the #3 spot.

It's a funny system and I know that sounds complicated. But does that hopefully make any sense to you?

Expert:  Jason replied 1 year ago.
Good morning, just checking in. Any luck?
Customer: replied 1 year ago.

Cold and rain all weekend how to try something after work today.

Expert:  Jason replied 1 year ago.
Sounds good. Let me know how you make out. --Jason

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