8.1's running rough at cruising speed

Hey Ray

Member
Jan 11, 2016
62
San Rafael, CA
Boat Info
1997 330 Express Cruiser
Engines
Twin Horizon 8.1
I have 8.1's in a 1997 330 EC. Engines were installed in 2014 and have about 200 hrs on them. Took her out yesterday and it ran rough when attempting to get up to cruising speed and wouldn't get close to expected MPH at cruising speed of 3400 - 3600 RPM. Usually can run at 25 - 30 MPH in that range depending on tide, but yesterday could barely get it to 20 MPH. Seemed like it wasn't getting up on plane. Felt a slight vibration in my feet at the helm, but nothing alarming. When I got back to the dock I pushed it up to 3000 RPM in neutral and engine's sounded very smooth and strong. Beginning of last summer did the whole Cool Fuel filter replacement and rail/injector replacement/cleaning, and at end of last summer had struts replaced and shafts realigned. Also just had bottom cleaned before this last ride. Any ideas?
 
Update: I reached out to the guy who cleaned the bottom today because I just got his invoice last week. I noticed a lot of barnacles on the trim tabs and he said they are "going crazy right now." Turns out he cleaned my bottom on 3/30 and is really behind on billing, and said the barnacles on the running gear could have definitely slowed me down. He is going to be in my harbor today and will clean it again, and I'm taking it out on Friday so I'll see if I get back to my regular performance and will report back. Still, if anyone thinks it could be something else please chime in!
 
Those engines are tough on spark plugs - first place to look.
 
Thanks for the spark plug suggestion. I'm going to see if the cleaning of the running gear helps, as the spark plugs only have about 100 hours on them, but will try that next if the cleaning doesn't do the trick.
 
Diver said all the running gear was "encrusted" with barnacles and tube worms, even sent me some cool videos. Taking it out on Friday to see if performance is back to normal, and if it is will wait til end of season to change out plugs.
 
Depending on the number of hours you have on the current plugs I'd change them anyway. It's shocking what a difference it can make on these 8.1s. And if the plug wires are pretty old I'd do those at the same time. I used the Volvo part number to buy my wires as they were a fraction of the mercruiser price (ripoff) and were IDENTICAL to what was on there.
 
Carter, do you think 100 hours are a lot for spark plugs on the 8.1s (specifically the Delcos)?
Seadawg -Copb8tx is spot on. I had a set that only lasted 15 hrs before one electrode was completely gone and started a misfire. And the wire change advice is good as well...seems like when I had my 8.1's (it was a while back) that everyone had much better service from the Champion plug. There are plenty of posts about the cross ref. I can look for it - but I did give the next owner all of my records...
 
Depending on the number of hours you have on the current plugs I'd change them anyway. It's shocking what a difference it can make on these 8.1s. And if the plug wires are pretty old I'd do those at the same time. I used the Volvo part number to buy my wires as they were a fraction of the mercruiser price (ripoff) and were IDENTICAL to what was on there.
Would you happen to still have the part number for the Volvo wires? I getting ready to change my plugs and wires.

Thanks
 
Took the boat for a ride on Friday and it ran perfectly, so the growth was the obvious cause. When I had the struts and shaft work done last summer the yard recommended Prop Speed for just this use case, but I was already too many $'s in so I declined. Has anyone out there used it and if so what are your thoughts? Might consider it next time it is hauled out.
 

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