lifters clattering at 3,000 rpm

[xltier]

just bought a 1988 aquasport 290 sport fisher with twin 1991 454`s.supposedly with 600 hours on them. (no meter). boat came with 350`s.On my 1st trial it ran ok at lower speed running around 2200 rpm. when i decide to go faster it will take off ok and go up on plane. After about a minute of running around 3,000rpm i can hear the lifters start rapping and then it starts backfiring very loud.The oil pressure gauge for that motor starts twitching between 25 AND 35.but not steady.Then i throttle back to idle and the lifters are clacking away.If i leave it idle they will pump back up and go quiet again.Itried full load on just that motor and it only pulls about 2200 rpm by itself but will hold that rpm without lifter noise.The starboard only pulls 2200 rpm also by itself.So it appears to be a rpm related problem. I checked the oil for overfull. its clean and new filter.(quicksilver stuff). If it wasnt for the clacking i would be looking for a timing issue. Any help is greatly appreciated

 

[alldodge]

Oil level not going up so gas isn't mixing.
Oil pressure twitching could be a worn pump or suction screen clogging.
If oil pressure is accurate (35 psi) then bearings and or pump is worn

Backfiring valve guides are worn

Maybe rebuild.time

If your not able to get much over 3000 rpm then check compression

 

[Scott Danforth]

could be running really lean and you could have timing issues leading to detonation rattle

start by looking at the compression numbers

 

[xltier]

i think its definatly lifter noise because it continues back at idle and gradually quiets down. what should the oil pressure be.other engine is the same pressure but runs quiet.?

 

[alldodge]

Min oil pressure rule of thumb is 10 psi per 1000 rpm, so 35 and 3000 rpm is good. Most motors do higher then that and more in the range of 45 to 50

Your using 40W oil right?
20w-40, straight 40W, 25W-40

 

[xltier]

Min oil pressure rule of thumb is 10 psi per 1000 rpm, so 35 and 3000 rpm is good. Most motors do higher then that and more in the range of 45 to 50

Your using 40W oil right?
20w-40, straight 40W, 25W-40

just bought the boat and P O says it was done before lay up last year. i think he uses 25w40 mercruiser oil.