Wingnutt
Petty Officer 1st Class
- Joined
- Jul 16, 2003
- Messages
- 255
I'm having a bit of a time sorting this one out. Like the title says, I have low RPMs at WOT. Engine is a 383 stroker making about 340hp in a Bayliner 2455 weighing about 3900lbs (counting 30 gallons of fuel, water, pax and "stuff"). RPMs are: 4200 with 18 pitch prop (4 blade), 4300 with 15 pitch prop (3 blade). I made a WOT pass with the 18 pitch prop, then stopped and changed it to the 15 pitch prop. Same load, same conditions, and the runs were made about 15 minutes apart. Since I should have picked up at LEAST 800 RPMs, I'm scratching my head on this one since according to the manual, the stock 210hp 5.0 is supposed to spin between 4400-4800RPMs with a 15 pitch 3 blade prop.
So far what has been checked: tach checked and is good, boat weight...about right (4900lbs on an aluminum trailer), clean bottom (trailer boat), new distributor with 10 hours on it...initial timing set @ 8*, total advance is 32* @3500rpm, the carb (Rochester) has had a recent rebuild and has been rechecked three weeks ago...OK there, secondaries appear to be opening fully, secondary demand butterflys are opening, fuel pick up is clear, anti-siphon is free, 3/8 fuel line (steel) from tank to carb is clear, fuel vent is open, new manifolds and risers (less than 40 hrs use), no hot spots on engine or manifolds, flame arrestor cleaned, and the plugs are showing a slightly rich mixture, darker than a light tan, but not black.
The only think else that I can think of (and that has NOT been checked) is that the flappers have deteriated away and have fallen off the pins down into the "Y" pipe partially blocking the exhaust. I talked to the PO and he did NOT check the flappers when he put the new engine in. Soooooo I'm thinking that they have never been checked. However, I am not seeing the classic symptoms of a partially clogged exhaust such as hot manifolds and risers, melted exhaust hoses or a loud hissing noise. But the symptoms that I am seeing are a slightly rich fuel/air mixture (since the carb is jetted for the original 305 I should be seeing a lean condition when I pull the plugs which I am not) and not reaching max RPMs at WOT.
Anybody have any other ideas?
So far what has been checked: tach checked and is good, boat weight...about right (4900lbs on an aluminum trailer), clean bottom (trailer boat), new distributor with 10 hours on it...initial timing set @ 8*, total advance is 32* @3500rpm, the carb (Rochester) has had a recent rebuild and has been rechecked three weeks ago...OK there, secondaries appear to be opening fully, secondary demand butterflys are opening, fuel pick up is clear, anti-siphon is free, 3/8 fuel line (steel) from tank to carb is clear, fuel vent is open, new manifolds and risers (less than 40 hrs use), no hot spots on engine or manifolds, flame arrestor cleaned, and the plugs are showing a slightly rich mixture, darker than a light tan, but not black.
The only think else that I can think of (and that has NOT been checked) is that the flappers have deteriated away and have fallen off the pins down into the "Y" pipe partially blocking the exhaust. I talked to the PO and he did NOT check the flappers when he put the new engine in. Soooooo I'm thinking that they have never been checked. However, I am not seeing the classic symptoms of a partially clogged exhaust such as hot manifolds and risers, melted exhaust hoses or a loud hissing noise. But the symptoms that I am seeing are a slightly rich fuel/air mixture (since the carb is jetted for the original 305 I should be seeing a lean condition when I pull the plugs which I am not) and not reaching max RPMs at WOT.
Anybody have any other ideas?
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