2000 Mercruiser MCM 3.0 - Engine cycling / dies at mid RPMs - videos included

[MidwestFloater]

Hi all, I recently picked up a 2000 Sea Ray Bowrider 180 with Mercuriser MCM 3.0L a / L605518 serial variety. I've been having some issues both at idle and when trying to accelerate.

The prior owner didn't do much maintenance and hadn't run it much over the last five years. So far since buying it I've replaced the spark plugs and fuel filter and done a seafoam application to the carb and fuel tank. The spark plugs were blackened but otherwise appeared to be in decent condition. The fuel filter appeared dirty but not clogged or water filled.

After these changes, we're still facing issues, primarily when trying to accelerate or in mid RPM ranges.

The first video here is after a fuel filter change today. Without touching the throttle, the engine cycles in RPM. It sounds as if it is struggling to get enough air.

Video 1)

After that, we took off the air filter to watch the carb as it the cycling was happening. However, after taking off the filter it stopped the cycling process and ran more smoothly at the low RPM ranges. We expected the cycling to come back after putting the air filter put back on, but it did not. However, we then ran into the issue where upon increasing throttle, the engine starts to die around 1500 RPM. If I give too much throttle, it totally dies (video 2). If I slowly accelerate, it begins to die but eventually overcomes the 1500 RPM range and gets up to higher RPM ranges (video 3).

VIdeo 2)

Video 3)

We did some minor tweaking of the idle screw and it idles alright now but haven't been able to get past the 1500 RPM issues.

I'm wondering what to test next in the troubleshooting process. Thanks for the help!

 

[Bovegas]

I would check accelator pump. If you remove air filter and pump the throttle you should see gas squirting. If it doesnt accelerator pump doesnt work

 

[alldodge]

hadn't run it much over the last five years. So far since buying it I've replaced the spark plugs and fuel filter and done a seafoam application to the carb and fuel tank.

Is the gas clear and clean, and since it sat so much it might even be bad, with not enough power left in it?

For me its fuel system, remove and rebuild the carb, check fuel pump pressure (3 to 7 psi all rpms), check antisiphon valve. Run the motor from a gas can and not the tank. Also stop putting anymore snake oil until you get the motor running correctly again

 

[MidwestFloater]

We've burned through about three tanks of gas now so I think it should be ok, but I'm not 100% sure. Thanks for the thoughts though!

My current to-do list:

• Check accelerator pump
• Check fuel pump pressure
• Check antisiphon valve
• Rebuild carb (if others don't fix the issue)

Could the choke cause anything like this?

 

[alldodge]

Could the choke cause anything like this?

Only if its not opening

 

[MidwestFloater]

Only if its not opening

Ok thanks. Apologies for being a total novice, but can I see if it's opening or closing through the carb or externally at the choke location?

 

[Bondo]

Ok thanks. Apologies for being a total novice, but can I see if it's opening or closing through the carb or externally at the choke location?

Ayuh,..... Welcome Aboard,...... Remove the flame arrester(air filter lookin' thiny), 'n Look,......

 

[MidwestFloater]

Thanks team, will check back with updates!

 

[MidwestFloater]

Hey all, update on things here.

I was out checking on the boat by myself, so it was though to both look down the carb and handle the throttle at the same time. But I was able to do a bit more investigation.

So I started the boat from cold with the flame arrester off and right away noticed that the choke was full open. I'm not sure what sends the signal to control the choke position, but I figured it should be at least partially closed assuming it's getting an accurate temp reading. I was previously having some issues with temperature readings but just replaced the temp sender and it seems to be working now.

After getting it started (choke full open) but still pretty cold, I applied a little throttle and got to the usual spot it struggles. I went to the carb and pushed on the choke a bit and it seemed to pop free, as if it had been stuck in place and was not able to rotate again. (I wasn't able to let it cool down and restart to see if it would close but will do that soon).

I noticed as I applied some throttle that (what I think is the) accelerator pump was kind of just dripping down the carb. When I manually choked the engine with my thumb, the RPMs rose significantly. A video is below:

Again, I'm a novice so I don't really know what I'm talking about. But here's what I'm thinking.

The choke was probably stuck in place since the carb is probably pretty gummed up. That would explain why it has been difficult to start, but not necessarily the mid throttle range issues. The accelerator pump portion may also be gummed up and shorting it on gas in the mid RPMs ranges, giving it the dead zone. My suspicion is that I should clean and rebuild the carb.

What do y'all think? Am I anywhere close?

Thanks!

 

[alldodge]

The motor is barely getting enough gas to run. The rpm's going up when the choke starts closing, because its getting more gas. It won't idle at lower rpms because its not getting enough gas. To me the carb needs a rebuild.

 

[QBhoy]

Mmm. Probably best listen to the experts above like AD etc...but to me and thinking simply...on the first video when it’s surging...and if you had the throttle at the idle position. It shouldn’t be getting enough fuel to surge that high...no matter what.
I then skipped to your last video when you hold the chock manually. It makes me think that the idle speed has been messed with and way too high...or your throttle cable needs reset...along with perhaps other issues maybe.
I would take the throttle linkage off from the remotes and just maybe rule out any cables.