Sync issue

[sarbog]

During the last hour of a 5 hour run today the stbrd engine (8.1 Horizons/'05 340 Sundancer) was requiring noticeably more throttle to turn the same RPMs. This was at cruise (3600 RPM and up). When I reduced speed they seemed to be in sync
with same amount of throttle. Pushed it wide open briefly to see how they responded and stbrd max'd at about 600 RPM less. Engines ran smooth and temp and pressures all good for whole trip.
thoughts?

 

[grayzone]

Do you have smartcraft?

Did you check the tacks against the smartcraft rpm's?

 

[jernberg]

Hi
Hade the same problem with one of my engines. Thought it was the IAC valve that had gone bad and put the engine in 90% mode. Sadly that wasnt the case. Two cylinders had bad compression due to two broken valves.

Hope its something easy!
/Henrik

 

[Bt Doctur]

if you had 2 broken valves the engine wouldn't be able to run.

 

[mainstream]

Hello when were the wires and plugs replaced last the computers on these engines will make adjustments so it seems all is good until it can't anymore and you get miss fires and raw fuel this happened to me with 8.1s

 

[sarbog]

I am having the plugs and wires checked as that is my suspicion as well.
Bought the boat from an older gentleman who was the original owner. Well taken care of but not been used a lot in the last year. Also going to have the fuel filters checked.

 

[jernberg]

If you have the plugs out do a compression test as well. Easy to do and then you know the engine health.
Had another friend with similar problem. 0 compression on one cylinder... Valve tune-up and he was good again.

/Henrik

 

[sarbog]

Beginning to wonder if it could be injector's carboned up. Both of the engines ran smooth for over 4 hours. Even when the stbd engine RPMS started to lag at the same throttle settings it was still running smooth, it just required more stbd throttle. No misfiring, sputtering, or raggedness.

 

[src]

I had a similar problem the starboard engine on my 310 with 7.4's, and thought it was the plugs/wires, since replacing some of them did improve the situation (couldn't access the starboard-side plugs on that engine at the time). However, the mechanic found an exhaust leak on the port side when winterizing, and a leak-down test found serious problems with several cylinders on the starboard engine. Ended up having to have the heads milled/valve job. Not a fun bill to pay.

I hope it's just your plugs and wires.

 

[Jimmy Buoy]

8.1's seem to have an appetite for spark plugs! What you've described is quite possibly just one or two problem plugs.

 

[Newbie]

I bet if you pull the plugs on that engine you'll find one that's fouled slightly or maybe even wet. Mine did the exact same thing to the T and it was an injector that quit firing above a certain rpm. It was not noticeable at all at idle and up to planing speed.