Mercruiser 350 Intermittent RPM drop while cruising

[alldodge]

Its probably still a carb issue, its either getting to much or not enough

If timing is correct, compression is good, advance works as required, it should run as designed

 

[achris]

I rebuilt the carb again. It idles perfectly now! BUT, now I am having trouble with rpm just off idle. When I give it a little gas, rpm moves as expected to 1500 rpm, then drops down to 1100 and runs rough. I slowly give it more gas and it stumbles along until 2200 rpm where it will suddenly surge up to about 3k. I can back it off and the rpms match throttle position until about 2k rpm, where it then starts to run rough again and drops to 1100 rpm. I have tried all 3 settings of the accelerator pump and spent a lot of time dialing in the idle screws, but no matter what it does the low rpm stumbling. Any other tips?

Still sounds like a carb problem. Did you do a timing light check to make sure the timing is advancing properly. (and make sure the ignition isn't in base mode).

Chris.....

 

[siriussr]

I checked timing today (while in base mode). It was at 4degress btdc. I set it to 8 per the service manual. It helped a bit, but I’m still getting surging from 1100-2500 rpm. I am checking fuel pressure next. If that doesn’t reveal anything I will soak the carb in an ultrasonic cleaner for a day or two, and replace the knock sensor since the connector is slightly rusty. I also cleaned the connectors in the engine compartment with DeOx-it.

 

[siriussr]

After some more reading on this carb, I think the ‘transfer slot’ may be clogged. Going to try 1 more rebuild.

 

[siriussr]

ok, I rebuilt the carb and the engine runs really good. idle and revs are smooth. BUT, I have a slow RPM creep that happens from 1800-2400rpm, only in the upward direction. Here is a video:

https://youtu.be/LXpTrzCuqGE

 

[siriussr]

Could this be the throttle cable sticking and then slowly coming into adjustment?

 

[alldodge]

Might need the throttle cable adjusted, but doing it in neutral doesn't tell me much

 

[siriussr]

Since this seems to be a new problem that the boat is running poorly, I think it is from a different issue. 2 weeks ago we barely made it to a gas dock since we were almost out of fuel on our trip, but the dock was closed. My father in law brought a few 5 gal jerry cans and we got the boat filled up enough to get us home.

I just checked the fuel filter and it’s full of crud. And when I looked in the carb, there was some of the same looking stuff, just a bit smaller. They looked larger than 10 microns so I’m not sure how they got through the filter. I think these bits are clogging the idle and transfer circuits and giving me the new problems.

do I drain the tank or just run it through and change the filter often? Or is this not likely the problem? The tank has about 20-25 gallons in it now (~1/3)

 

[Rick Stephens]

I just got a boat in the shop that I am going to pull the fuel tank out of and flush the tank. It's a mess. Nothing worse than recurring problems that require carb rebuilds - especially on an outboard with 4 carburetors

 

[siriussr]

Update: this is finally fixed! There was water and dirt in the tank. I pumped all the crap out of the tank, flushed the fuel lines, replaced the filter, and rebuilt the carb. Between that and adjusting the timing, I think that finally got rid of the problem. Went for a 4 hour cruise today and didn’t have a single problem. Thank you all for your help! I hope this information can help some unfortunate soul in the future.

 

[Rick Stephens]

Wonderful! You done right getting it all out!

Rick

 

[siriussr]

update of the update of the update: This problem persisted, so I replaced all fuses in the 12v block, and cleaned the kill switch (on my boat the switch completes the circuit to the ignition instead of grounding it). 20 hours of cruising later and 0 problems!

 

[Slangen]

update of the update of the update: This problem persisted, so I replaced all fuses in the 12v block, and cleaned the kill switch (on my boat the switch completes the circuit to the ignition instead of grounding it). 20 hours of cruising later and 0 problems!

Not sure if you still have the issue but I’m having the exact same issue. I have the Mercruiser full maintenance manual and there it says. A drop in RPM is caused by high oil pressure. This can be caused by a faulty oil pressure sensor, a clog or the oil being too thick or too much oil in the engine.
When I first noticed the issue I released some oil and it went away. However it came back after the winter. So I’m going to do an oil change and actually flush the engine oil and then put in new oil.
I’ll let you know if that helps.