Okay, here's one for the motor heads...454 X 320 '91 1000 rpm power loss

I,ve owned my 3400 Luhrs for eleven years. The last three we run 1150 hull speed 95 % of the time. Could do 3000 cruise at 19 mph, 4100 wot. I am noticing a gradual loss of power til it will now barely go above 3000. Everything seems fine till then. Done a full tune including wires, fuel and filters checked and replaced, all seems good there. Little dirt in the carb screens but no change. Carbs seem to be working as far as mechanical. I am also now seeing an almost immediate increase in water temp when load is applied. Pretty sure that means a lean condition, and starting to think the est ignition has ceased to advance enough. However,both engines run identical, very smooth to 3000 at just above half throttle position, but full advance results in nada ! 50 to 100 additional rpms. I,m puzzled ?
Anyone have an idea where to go next ?

Do the carbs' secondaries open?

Mechanics appear to be okay as to carb linkage.....what I can't tell is if enough fuel is being delivered. Next item is to make certain of, is that the est distributors are working properly as to full advance ect. Strange that both engines are identical in operation.

lots of barnacles on the props?

Did ask if the linkage appears ok - asked if the secondaries open. specifically under load, with the hull planing. The sound change is rather unique once recognized. Besides the throttle plates, the air valve needs to open. The throttle plates always open long before the air valve does. Air valve controls fuel flow thru the secondaries.

chances of both modules failing randomly with identical failure modes is hard to imagine.

Just for grins....what is the condition of the bottom of the boat?

makomark said:

chances of both modules failing randomly with identical failure modes is hard to imagine.

Just for grins....what is the condition of the bottom of the boat?

Click to expand...

Does 19kts still happen at 3000rpm? That would tell a lot.

Performance IS off a bit now at 3000 and seems to be laboring....17 - 18 mph. Bottom has been checked and cleaned. From the bridge, you really can't hear much engine sound, except exhaust, but that is the only other thing I can think of besides the ignition. Visually, fuel system looks good and filters have been changed with zero change.
Loving this site, thanks much for the responses, It will get figured out ! Hate it when something is not right with the boat !

Replace the carbs. It's time.

Do the Fastjeff Dyno Test:

1. Set both throttles at 3,000 rpms.

2. Advance one only all the way, then note the rpms achieved

3. Return that throttle to 3,000 rpms and repeat the above on the other engine.

If one motor is weaker than the other, concentrate your effort on it.

Jeff

Thanks, I'll try that....don't know why it did not accur to me already !

Have the marinas gas mechanic coming with me on Monday to test and whatever. He's pretty sharp, I'll let you know what is found.