4.3 MPI Runs rough after putting in gear

[gwershing]

I have a 2003 4/3 MPI with Alpa 1 Gen 2.

My boat runs fine in the street on the muffs.
It starts fine and idles well.
I can get up on plane and it runs strong and fast.
After running for a while, and shutting down to sit for a while, it starts fine. SOMETIMES, after running/sitting/starting, when I put it into gear it runs rough and makes a loud knock. If I give it throttle too quickly it will die, but if I ease the throttle up slowly and increase the RPMs, it seems to "blow out" the problem and it slowly gets back to running strong. At that point, it will idle well, and runs strong and fast again. It's not always repeatable so I'm finding it hard to troubleshoot. But it does happen regularly.
I've replaced the IAC valve, and nothing changes.

Other issues that may not be related:
- Temperature gauge reads low, Not sure if it's a bad sender or stuck open thermostat. (I grounded the sender wire at the sender to test, and the gauge works)
- Throttle body makes a sucking or hissing sound, but it has been doing this for years before this new problem presented itself.

Also, the boat sat unused for a season before this problem presented itself.

 

[alldodge]

Has the fuel filter been changed?
Have you checked fuel pressure?

 

[dubs283]

When was the last full tune up?

 

[gwershing]

Has the fuel filter been changed?
Have you checked fuel pressure?

I have drained the fuel tank and replaced the filter. I have not checked the fuel pressure, not sure where to tap a gauge in. It just seems that if I had a pressure problem it wouldn't run perfectly, as it does mostly.

 

[gwershing]

When was the last full tune up?

I admit, it's been a while since a tune up. However, again, it runs perfectly most of the time. The problem only presents itself after it sits in the water awhile after speeding around, then putting it into gear. Then it's only a minute of easing the throttle up, and it's perfect again.

 

[alldodge]

Might be a bit of vapor lock but fuel pressure is measured at the fuel rail

 

[QBhoy]

Hi. On an mpi like this, if you are running cold, the engine will run very rich and over fuel, with it presuming it needs to and still in a state of warm up or cold start condition. Add to this, the extremely high possibility that the distributor cap is doing its usual mpi cap party trick…you’ll likely see such a behaviour.

 

[dubs283]

I admit, it's been a while since a tune up.

Yezzer

it runs perfectly most of the time.

Uh, huh

The problem only presents itself after it sits in the water awhile after speeding around, then putting it into gear.

When was the last tune up?

Then it's only a minute of easing the throttle up, and it's perfect again.

When was the last tune up?

 

[kenny7964]

I have just had a similar problem with my 4.3mpi, the bracket for the interrupter switch was loose and was causing the switch to trip on and off, it ran rough at idle and cut out if you gave it to much throttle but if you throttled slowly it would pick up and run fine at speed.