2004 Mariner/Mercury 40hp 2-Stroke idling rough and won't rev up in gear

[11855]

Hi Folks,

This is my first post and I would greatly appreciate some advice. I have a 2004 Quicksilver 460 with a Mariner 40hp 2-stoke from the same year. I got the engine professionally serviced earlier this summer after purchasing the boat and it was running perfectly fine. I have spent approx 30-40 hours on the lake.

I moor the boat in a private harbor but over the past 2-3 weeks the water levels have dropped and for about 200-300 meter into the harbor and berth there is a lot of weed near the surface. I usually have had to remove some from the prop when I get in. To mitigate I try to keep the trim up leaving and entering and it seems to work.

On Tuesday evening I was coming back to the harbour and about 500 meters out (before I hit weeded area) the engineer was running at slow speed but all of a sudden the revs dropped as if the trottle had been eased back then the engineers started to sound different (rougher). I tried increase power ingear but the engineer would not rev up. I was afraid it might shutdown so I just cruised back into the harbour. When I tied up the boat there was a lot of weed/grass around the prop which I pulled off. Probably the worse I have seem over the past 2-3 week. While berthed I restarted the engineer in idle but it still didnt sound right and called it a day.

I went back to the boat yesterday evening to check it out and use my electric outboard to get out to 4-5 ft water. My electric also had some weeds I then started the outboard but there was no change from Tuesday and it still idled rough and would not rev up in gear so I returned to harbour and captured a video of the engine running
20210721_183106000_iOS.MOV

NB! Cooling water is flowing freely from the engine.

From what I have research first step seems to be to check the water pump impeller. I checked the water intakes and they seem fine. Any help would be greatly appreciated. I am not sure the root cause here was the weeds but it is definitely a possibility as the prop was quite well wrapped.

 

[Rappey]

Sounds as if it's not firing on all cylinders.
Weed and water blockage will have no effect on the engine other than extra loading when it's wrapped around the prop dropping the revs, or overheating the engine. Does it have an overheat alarm?
Could be as simple as a plug failure or wire dropped off.

 

[11855]

Hi Rappey, Thank you for taking time to watch the video clip and reply back. I am not sure if there is an overheat alarm, I didn't hear anything. . I didn't check the plugs but I will definitely do that. Perhaps the odd sound is the engine running on one cylinder instead of 2 and the slight knocking sound. When getting it serviced I had the engineer do a compression test and the reading was 130psi on both cylinders.

 

[Rappey]

That sounds like a healthy outboard.. Its quite surprising how well some outboards run when down one cylinder.. I had new outboard whos spark plug failed within 20 hrs ..
There is a huge list of "could be" but plugs is the quickest and easiest to check.. It may be that in neutral when revving up the other plug is kicking back in making it sound smooth but in gear under load it could be failing.. I dont think the weed and water flow has anything to do with it..
Next could be ht lead, coil pack or a carb blocked..
Pulling the plug cap off whilst its running is a very quick test, leading to some "shocking" results

 

[11855]

Thanks again Rappey. I am going on holiday this weekend for 2 weeks so I might not get to the boat until I get back. Great that you think the engine sounds healthy. It was running great until this happened. I will follow your advice and start with the plugs. Will let you know.

 

[11855]

Hi Rappey,

I got down to the boat this evening and it looks like you are on the money. I started the engine no problem and let it run for a few mins. I then took the lead off the top plug and the engine kept running on the bottom plug. I then pushed back on the top lead and pulled out the bottom plug and the engine cut out immediately. I can start the engine on only the bottom plug so it looks like the top plug has failed as you suggested. I will order a new plug.
Is it by design that an outboard will limp home on one cylinder. I was chatting to someone who was really surprised an engine would run on one plug.

 

[Rappey]

I would not have thought your outboard has any clever features due to its age. Limp mode appeared on 4 strokes . Some engines just seem to still sound ok with a cylinder down.. until you put them in gear..
From a past experience one thing to keep a good eye on is the cable that pulls on the oil pump to allow oil from the two stroke tank into the cylinders.. I didnt on mine and allowed it to go very slack, resulting in no oil on prolonged slow speed until the eninge seized
Many used to disconnect the oil injection system altogether and mix the oil with the fuel in the tank to avoid any problems..
This forum is full of very helpfull and knowlegable people who love a challenging question. Having no solution to a problem is a rare thing !
You will never be ridiculed for asking what you may think is a daft question... some very simple questions can end up 20 pages long as every angle is explored.!
Its unusual for no one else to comment on this thread . Can only assume others Thought the same as me and had nothing else to add...

 

[gordmac]

I would replace all the plugs rather than only one. If you can swap the plug leads over you could see if it was a lead rather than a plug.

 

[11855]

Hi Rappey and Gordmac,

I got back from vacation this weekend and the 2 new plugs arrived. I just replaced both plugs with the new ones and started the engine. Unfortunately it seems the issue may not be the plugs. As before with the engine running I disconnected the top plug lead and the engine continued to run ok. I then pushed the top plug lead back in and removed the bottom. as before the engine stopped.

One thing I did notice is I felt a small electric current when I went to remove the bottom lead but none on the top. I guess next candidate is the top plug lead.

thanks Declan

 

[Rappey]

It could be the coil pack. Your braver than me pulling off the cap with a running engine !

 

[gordmac]

Can you swap the leads over and see if the problem follows?

 

[11855]

Hi Gordmac, I did think about trying that but wasn’t sure it was a good idea. Is it safe to do that? My father in law who is more mechanical than me said you wouldn’t do that with a regular motor but he isn’t familiar with outboards at all.

If you are sure it’s OK I will give it a go before ordering a replacement coil. I found the part athttps://marineparts.ie/rec827509a10/

 

[Rappey]

You can swop coils, leads and plugs around without any drama . It helps to track down a faulty part.

 

[11855]

Thank you both. I will try that before ordering the replacement. I guess f the test is successful if will see the reverse ie. the engine will run on the top cylinder instead of the bottom pointing conclusively to the top HT lead/coil module.

 

[gordmac]

I perhaps wasn't clear, don't swap the leads over just at the plug end. Swap complete components.

 

[11855]

I think I got it but to clarify

Instead of lower HT lead connected to lower plug and upper HT lead connected to upper plug I will swap so the lower lead is connected to the upper plug and the upper lead connected to the lower plug.

I will then start the engine and I expect when I disconnect the lower lead the engine will continue to run. I will be then reconnect the lead and disconnect the upper lead connected to the lower plug and I expect the engine will cut out.

this should prove the lower HT lead/coil are good as it works fine on both plugs while either plug connected to the upper HT lead/coil does not work.

 

[BruceK]

If you can check the leads when it's dark to see if there is a carbon leader up one plug cap or if the HT lead is sparking directly to the engine rather than the spark plug. (Look for arcing to the engine body) HT leads can wear out especially if they have been exposed to oil, and mechanics will often spray the engine with oil to protect it from moisture.

 

[gordmac]

Swap both ends of the leads, plug and coil.

 

[11855]

Gordmac, apologies for the confusion but if I understand you correctly you are recommending I unscrew both of the CDM modules and both plugs. Then screw back in the CDM Module having swapped them and connect to the swapped plugs.

In the current situation it appears the problem is with the bottom CDM module as when I disconnect the HT lead from the plug the engine stops. With the swap test I should see the opposite i.e. if I remove the Top HT lead the engine should stop pointing to the faulty CDM module.

If however the engine does not stop then that would suggest the CDM Module is not faulty.

Also, my part number is 827509A7 and a replacement is available at Marine Parts here in Ireland - https://marineparts.ie/rec827509a10. for €110 including delivery. (Manufacturer RecMar)

However I also found an equivalent part on Amazon at Homyl Coil Ignition System 827509A10 For Mercury : Amazon.co.uk: Sports & Outdoors which is a lot cheaper at €37 including delivery (Manufacturer Homyl)

I am tempted to just go ahead and purchase the module from Amazon and replace the top CDM module on the engine.

I came across a video on Youtube showing a guy replacing a similar module he bought on Amazon https://www.youtube.com/watch?v=FXAN06xHvYg.

 

[gordmac]

Basically if the fault is a plug lead swapping over the leads on their own will mean the fault will follow the lead, if it doesn't then the leads are ok. Next would be to swap whatever the lead is attached to but if you know the lead is ok you can safely swap with those attached.
What I don't know is whether or not both plugs fire at tdc or if both pistons are at tdc the same time.