2003 115HP Mercury 2 stroke engine - cylinder cuts out when cold at full throttle

[Al Lounsbury]

So this is a 2+2 engine. That is under approx. 2500 rpm, only 2 cylinders in use, over 2500 rpm then the bottom two kick in with the high speed jet in the carburetor.
Now the problem: When the engine is cold (first time started during the day), when I accelerate to Wide Open Throttle (WOT), as the bottom 2 cylinders start to cut in, it's like an on/off switch and someone is flicking it for a good minute. And it's a hard cut out/cut in, that is no power then full power. This goes on for a good 1-2 minutes then after that - totally no issue. Everything works as expected, hit the hammer and all four cylinders kick in immediately.
My initial guess at the problem was the two check valves for the high speed cylinders since it was a hard on/off and maybe the check values were sticking, but alas replaced them and NOT the problem. In fact I think the newly design check values dropped 100 rpm off the top end, but not changing back to original until problem solved. And plugs have been replaced with new ones twice, no change.
So now I'm thinking it's one of the CDM's that maybe collect some moisture overnight and until flushed out - it's just intermittent? Or is there any chance it's the switchbox or trigger assembly? At this stage, looking for ideas, since just not sure. My next guess is to replace the bottom 2 CDM's but truly would love some additional thoughts and just hoping someone has seen this problem before.

And if there is anyway to troubleshoot this situation better, let me know.

Regards Al

 

[Dave1027]

I'm leaning toward a fuel pump issue. If you haven't rebuilt the fuel pump yet, you might as well try it.
Otherwise you can try this test. Take it back out to the water and have somebody ready to pump the primer bulb. Try to duplicate the issue. If pumping the primer bulb helps then it's probably down to the fuel pump.

Are you nailing it as soon as you cold start it? I usually have to idle away from the dock first. By then the motor is warmed up.

 

[Al Lounsbury]

I'm leaning toward a fuel pump issue. If you haven't rebuilt the fuel pump yet, you might as well try it.
Otherwise you can try this test. Take it back out to the water and have somebody ready to pump the primer bulb. Try to duplicate the issue. If pumping the primer bulb helps then it's probably down to the fuel pump.

Are you nailing it as soon as you cold start it? I usually have to idle away from the dock first. By then the motor is warmed up.

@Dave1027 So yes, I normally do let it idle on the two cylinders at the dock and even a little ways out into the lake before nailing it home. So typically a good 5 minutes of run time on the 2 cylinders before trying to get all 4 fired up. I'm not leaning to the fuel pump since it's a hard cut out/in, and I would have thought fuel starvation would affect all cylinders and not such a hard cut in/out. That said, I will certainly do the pump the bulb aspect when nailing it - easy test and just like eliminating as much as possible. Also, I've debated on the role of the accelerator fuel pump but since this happens at a steady full throttle, I suspect it's no longer in play.

 

[DeepCMark58A]

Do you pump the bulb before you start up? I know sounds simple but older motors can be fickle.

 

[Al Lounsbury]

Do you pump the bulb before you start up? I know sounds simple but older motors can be fickle.

@DeepCMark58A You bet. It's part of my standard operating procedure just to ensure an easy start versus running the starter for minutes to draw fuel.

 

[DeepCMark58A]

I have a 90 115hp mariner not the best motor for a ski boat but the motor just keeps on running. Luckily the motor is usually run warmed up. Kids come up and run that thing at full throttle all day sking and tubing. Thirsty motor.

 

[Dave1027]

@Dave1027 I'm not leaning to the fuel pump since it's a hard cut out/in, and I would have thought fuel starvation would affect all cylinders and not such a hard cut in/out. That said, I will certainly do the pump the bulb aspect when nailing it - easy test and just like eliminating as much as possible. Also, I've debated on the role of the accelerator fuel pump but since this happens at a steady full throttle, I suspect it's no longer in play.

It could be all carbs not getting enough fuel causing the cutting out. It could be right on the edge which might make it more temperature sensitive like cold starting requires a richer mixture.

The accelerator pump is only in play when increasing the throttle. Is meant to add additional fuel directly to the bottom two cylinders to aid them in transitioning from a lean not running mode to the high speed jet running mode. It basically gets the bottom two going when throttling up. It should have no bearing on top end power.

 

[flyingscott]

Put a timing light on the motor when it is doing that. Your transition ishould be at 1800 rpm not 2500. What is your compression and spark needs to jump a 7/16 air gap.

 

[Al Lounsbury]

Put a timing light on the motor when it is doing that. Your transition ishould be at 1800 rpm not 2500. What is your compression and spark needs to jump a 7/16 air gap.

@flyingscott I can certainly check the actual RPM is around 1800, since when I posted 2500 that was just where I felt it was. Not sure a timing light can help much since at the higher RPMS >3000, missing a flash or two could be un-noticeable. One suggestion made to me was place an inductive RPM meter on all four cylinders to see which ones are cutting out, if any. Basically a hard core test to see if an electrical problem or fuel problem. Now I need to order in the meters but I like that approach. Plugs we're gapped twice already this year when I replaced them so all good there, and compression test had all cylinders within 10psi of each other - so happy with that.

 

[flyingscott]

Good luck with your motor

 

[Texasmark]

Check your 3 and 4 plugs for oil fouling...if they look fouled replace them.
The longer you stay below the kick in speed for 3 and 4 the better chance of plug fouling you have.
Mercury has proven and posted information including building the Mark 5-20 with a slanted forward drive train to prevent puddling of fuel at the spark plug for these fishing motors that spend a lot of time at slow speeds. When you are running at low speeds trim the rear of the engine up slightly.....it does matter and you can get immediate results with smoother engine performance at low speeds.....don't believe me just do it.
Replace the small fuel filter (for 3 and 4) behind the rear of the oil tank.
When you clear the no wake zone and are ready to get on plane, (the hole shot....make it a shot) firewall the throttle...this squirts fuel into 3 and 4 as compared to other throttle movement options. Then as the boat comes up to the desired speed, reduce your throttle to the desired speed.

91 octane E0 runs good in that engine with Pennzoil Super Premium TC-W3 (synthetic blend) in the oil tank....helps to prevent plug fouling and little to no smoke. If you are a Marine Sea Foam fan put that in the fuel tank too. I use it in every tank running my 2002 115 2 stroker.

 

[Dave1027]

BTW, this is my 2+2 Merc. I think it's pretty smooth.

 

[Al Lounsbury]

Check your 3 and 4 plugs for oil fouling...if they look fouled replace them.
The longer you stay below the kick in speed for 3 and 4 the better chance of plug fouling you have.
Mercury has proven and posted information including building the Mark 5-20 with a slanted forward drive train to prevent puddling of fuel at the spark plug for these fishing motors that spend a lot of time at slow speeds. When you are running at low speeds trim the rear of the engine up slightly.....it does matter and you can get immediate results with smoother engine performance at low speeds.....don't believe me just do it.
Replace the small fuel filter (for 3 and 4) behind the rear of the oil tank.
When you clear the no wake zone and are ready to get on plane, (the hole shot....make it a shot) firewall the throttle...this squirts fuel into 3 and 4 as compared to other throttle movement options. Then as the boat comes up to the desired speed, reduce your throttle to the desired speed.

91 octane E0 runs good in that engine with Pennzoil Super Premium TC-W3 (synthetic blend) in the oil tank....helps to prevent plug fouling and little to no smoke. If you are a Marine Sea Foam fan put that in the fuel tank too. I use it in every tank running my 2002 115 2 stroker.

@Texasmark Plugs 3&4 actually look good and have been replaced twice this year just to be sure they are not the problem. And that sneaky small fuel filter that feeds the check valves was replaced as well as the main fuel filter and the check valves. Also totally a sea foam fan and yes, it's in every tank which is why my carbs are still so clean. Also having the engine trimmed up a bit at slow speed does result in a smoother engine - no debate there. I'm currently awaiting for the inductive RPM tach meters to come in, so I can at least isolate this to either a fuel problem or electrical problem. Hoping to get from Amazon this week and test over the weekend. Stay tuned but I'm still leaning to an electrical issue - either trigger assembly, switchbox or CDM module itself.

 

[Texasmark]

Looks like you are on top of the fuel as a suspect. On my 2002 90 I had a thermally sensitive CDM that would do what yours is doing. It didn't have the 2+2 function so it was easier to trouble shoot. I don't remember how I isolated it but here is an idea.

Start the engine and run it the way you run it and about as long as it will run without the problem. Shut it down and pull the plugs. If you have a wet one you have your smoking gun. Or get a helper and go out with the cowl off with a battery operated timing light. That might be a better method of isolation as you can quickly go through the cylinders looking for the culprit.