Low speed vibration

[gradydude]

I have a 1996 S250TRXU that starts and runs perfectly - except for a vibration that occurs at about 1300 - 1500 rpm. The boat (Grady 232) noticeably shudders at that speed. When driving the boat I just avoid that speed range but since I have a significant no-wake area it would be great use it. Idle to 1200 and 1600-WOT at 5800 is perfectly smooth.

All six carbs were rebuilt with a powerhead replacement a couple years before I bought the boat last year and I haven't touched them. I did put in new spark plugs with no change. Compression is spot on.

Any suggestions? Is this just the nature of the beast or can they run smooth through the entire rpm range?

Thanks,
Art

[rodbolt17]

they can, they typically don't.
without being able to hear/feel it it may or may not be normal.
that motor runs on 5 cyl to about 800 RPM, then it runs on 4 cyl to about 1800 RPM. then it picks up the other two dead holes and runs on all 6.

[gradydude]

Thanks - I didn't realize the carb'd engines had cylinder deactivation. I thought that was an OX thing. That could be all it is and I'm a firm believer in "if it ain't broke, don't fix it"

-Art

[rodbolt17]

come to think of it it may be just on OX66 using CCS.

that carbed engine used a rudimentary auto enrichment a TPS sensor and a coolent temp sensor and a knock sensor.
all feed info into the ECU and the ECU can alter fuel trim via the 6 solinoids.
test enrichment rail pressure and make sure the stbd head is coming up to temp.

if the orings on the solinoids don't seal, they don't come in the carb kit, it will run rich.