Losing RMP on Mercruiser 383s - HELP

[Mizerita]

Well...took delivery of the Double Bogie (now "Hei Hei") and all was good until yesterday. Starboard engine took some time to start, then while in idle quit at least 6 or 7 times. "Revved" it a bit and got it working; however RPMs were about below port engine at WOT (5000) and about 400 rpms off at 4000. While running back to marina at 3800, the engine lost rpm to below 3000, but then a few seconds later (with a bump in throttle) gained power. Since this is my first with a gas inboard, my first thoughts:

1. Condensation in gas - boat was shipped from Maine two weeks ago to sunny Florida. But guessing both engines would be acting up with 1 fuel tank?

2. Fuel filter - going to check this today but was just changed at the end of last year with maybe 10-20 hours of running since.

3. Distributor Cap - I checked both caps and did see some particles in the cap on the faulty engine side. Was black...not corroded but the cap on the port/good side was nice and clean/shiny. Not sure if this would cause the issue?

4. Yachting solutions tech said maybe "choke adjustment"...he said these Mercruisers have a starting sequence that starts with the automatic choke. Now I know the sequence great, but not sure if this would be the issue as again, one engine runs fine.

Was supposed to run her to the Bahamas this weekend. Not sure if that will happen, but any help is greatly appreciated. Thanks guys...

[Carl]

If carburated I'd say you have dirt or water in carb.

Low hours on fuel filters but changing can't hurt.
BUT, when having issues I like to see whats in those filters. For that reason a CLEAN clear glass bottom and funnel to drain filter into. Let it sit and settle then see what you see.

Hard to have water with ethanol these days as it gets soaked up...but

A few particles getting caught by filter is fine...but if you have lots...maybe a problem.

Clean shows fuel looks to be ruled out...at least to that point.

[aussierob]

In addition , i would check your spark plugs.

A single mis/non-firing cylinder would explain a lot of what you noted (poor cold idle and not running at full power).

We don't have ethanol in our marine fuel here (it's all 95+ premium) in Aus. If i've ever found water in the lines it created stalling, not a "low power" type condition.

Rob

[Tony Meola]

Could be a week fuel pump.

Before we repowered, we were forced to wire the choke on the carbs open. Otherwise when the engines were warm the starboard engine would die on acceleration if you did not let it run at idle for a few minutes.


Check that the chokes are not closing on that engine.

[Carl]

In addition , i would check your spark plugs.

A single mis/non-firing cylinder would explain a lot of what you noted (poor cold idle and not running at full power).

We don't have ethanol in our marine fuel here (it's all 95+ premium) in Aus. If i've ever found water in the lines it created stalling, not a "low power" type condition.

Rob

With my Carters now Edelbrock style carbs a little bit of water in the bowl would have me idle and run slow ok, then goose it up to 2600-2700 would cause lack of power. Sometime I could push it and she'd clear, other times it was slow it down, remove flame arrestor, place hands over carb a few times and she'd suck whatever it was through and be good...other times I had to go back in. Pull carb, open and clean out the bowl. Or if I didn't have spare gaskets with me...flip her over a bucket, give 3-1/3 good shakes and put her back on.
Anyway, when I had the problem I'd be sure to look and pretty much always some water was found. Now I'm talking droplets like amounts as some, not a layer of water.

[Mizerita]

Fuel from filter was clean. No water found in fuel and pulled carb, nothing. No droplets and no water to be found. Distributor cap had a ton of soot in it so had one overnighted and putting on this afternoon.

On interesting thing. While on a work rack, we wanted to start the engine and see what it was doing while at idle...we set up a hose through the strainer and closed off water out...as engine was running, the water we were supplying was not being sucked through. The other engine was perfect when we tried it. I ran the boat the day before for at least an hour but didn't overheat? Pulled the housing and was cracked and impeller was slightly destroyed. Would this have anything to do with the loss of RPMs or an entirely different issue?

Hoping to find out more today. Thanks for the input.

[Tooeez]

Check if you have anti-siphon valves on your fuel lines--they are located on the tank pick-up fittings, and go between the pick-up and the fuel line to the engine. They have a spring loaded ball inside, could be sticky or partially blocked. Also check the pressure on your fuel pumps--the ethanol kills them, and caused me the same kind of problem you describe.

[Tony Meola]

Water pump would be a different issue. If the housing was cracked you were probably sucking air and not drawing water and that might have destroyed the impeller. Are you missing any pieces off the impeller. If you are chef
Co the transmission cooler they will be stuck there.

[Mizerita]

Yep, found the pieces of the impeller...and top of sea strainer basket. Replaced impeller and housing.

RPM issue could have been the distributor cap. I replaced it and if fired up and seemed to idle fine. Going to run her today and see if it was the culprit. Thanks guys.

[Carl]

Yep, found the pieces of the impeller...and top of sea strainer basket. Replaced impeller and housing.

RPM issue could have been the distributor cap. I replaced it and if fired up and seemed to idle fine. Going to run her today and see if it was the culprit. Thanks guys.

Fingers crossed for you!

[Mizerita]

Thought we fixed the problem, but the problem came back after a few runs. If I remove the distributor cap / rotor (see picture below), the "arm" moves down and up if you pull / push on it. I think it gets sucked down which is causing grinding and the metal shavings I talked about earlier in the post. I checked the port motor and there was no movement. I tried to add a washer to see if it would prevent it from touching, but it didn't work.

Engines again lose RPMs, sputter (which sounds like a backfire / clicking sound), then eventually stall at idle. Doesn't appear to be a fuel issue...seems to be a spark issue. Hoping someone has had this problem and fixed. Thanks guys.