Unique 3ym20 problem Delaware Bay

[cyclebear]

Beneteau 323 with 3Ym20 engine. Works fine in neutral up through 3000 ram. Ditto in reverse gear BUT in forward gear engine loses power (rpm drops precipitously) almost immediately if at 3000 rpm. If kept at 1800 in forward engine operates for a while ( varies from 15 min to even an holur) as expected then rpm quickly drops to 1000 or slightly less quickly. After 30 secs to 1 min comes back to 1800. Cycle can repeat for hours. Filters checked and clean
Air bled twice (after checking filters). Engine runs fine in neutral for extended periods at various tpm from idle to 3200. When rpm does drop rarely does engine die. Checked prop to be clear of any fouling. Turns by hand easily. Prior to fist instance of problem loud chatter heard in stern. Occasionally somewhat reduced chatter occurred just before rpm dropped. In past 6 hours of operation no chatter seems to occur. Thoughts?
.

[capt jgw]

When it drops, is it making any unusual smoke? If it was overloading somehow, you'd likely get black smoke. Other than that I'd suspect something in the governor, maybe a weak or broken spring causing it to throttle down just like you pulled the lever. If that's the case the noise was unrelated.
Are you sure it's not just the throttle lever creeping back? That's real common.

[cyclebear]

No black smoke. Throttle doesn't creep. I might look at throttle links but it seems inconsistent with symptoms
Thanks for an idea I hadn't dully considered

[skipperpete]

Originally Posted by cyclebear

No black smoke. Throttle doesn't creep. I might look at throttle links but it seems inconsistent with symptoms
Thanks for an idea I hadn't dully considered

It really sounds like either fuel starvation or suction leak (air) in the supply side of the system. Do the easy stuff first, install a length of clear plastic hose on the delivery side of the lift pump and see if there are any bubbles visible while running the engine. The system is self bleeding so it will tolerate a suction leak to a point. While you’re hooking up that clear hose, put the output end in a container and operate the priming lever. You should get a decent flow of clear bubble free fuel but if you only get a trickle, start looking for a crook valve in the lift pump OR an obstruction to flow in the tank to lift pump plumbing.

[cyclebear]

Doesn't explain why the problem only occurs in Forward not Reverse. In reverse, even at 3200 rpm, no tach variation or loss of propulsion power. For 2 days I was convinced fuel/ air problem but tests of power in reverse make think I have been on the wrong track but skipperpete's suggestion is a good one for diagnosis.

[AA3JY]

Quality and quantity of transmission fluid??

[leecea]

Grasping at straws a bit here, but I wonder if the design of the prop means engine load is less for given rpm in reverse. Maybe 3000 rpm in reverse and 1800 rpm in forward are similar load, and since you don't run in reverse for extended periods, you don't get the same issue.

[Nicholson58]

The chatter from the prop might be a clue. Folding prop? Worn cutlass? Something fouled on the prop or shaft? Make sure all is proper back there. I use a bicycle spoke to clean marine growth from the grooves in the cutlass. Growth in the grooves can wear the shaft under size. Undersize shaft and worn cutlass can allow the shaft to chatter at certain RPMs. This chatter can impact the load the transmission sees.