We recently experienced a significant engine problem on our 1986 Bayliner with AQ131/275. During our last outing, after running normally for approximately 20 minutes at 4400 RPM's (oil pressure, temperature, power, etc. all normal), we went to idle for a few minutes. When I reapplied the throttle, a loud grinding noise, following by a loud knock was heard, with smoke emanating from the carb spark arrestor. I immediately turned off the engine and we were towed back to port. I've been diagnosing with the aim of deciding how far to go with engine repair. Here's what I've determined so far:
1. No oil, or engine coolant leaks observed.
2. Removed valve cover - no obvious damage to cam or hydraulic lifters. No obvious wear or damage to the timing belt.
3. All four plugs look perfect
4. When turning engine over with plugs removed - a knock is heard on each revolution.
5. Compression test: Cylinder - 1 (130), 2 (130), 3 (75), 4 (0)
At a minimum, it looks to me like a significant valve or cylinder head problem. What concerns me however, is the loud knocking noise when running, indicating perhaps a valve having been sucked into the #4 cylinder? At any rate, given the ominous symptoms, I'm not sure doing a leak-down test would provide further definitive information. At this point, I'm considering pulling the cylinder head and seeing what that reveals.
I've learned a great deal from the expert commentary available here, so I'd be most appreciative of any thoughts, comments or advice regarding further testing prior to pulling the head. Thanks.
1. No oil, or engine coolant leaks observed.
2. Removed valve cover - no obvious damage to cam or hydraulic lifters. No obvious wear or damage to the timing belt.
3. All four plugs look perfect
4. When turning engine over with plugs removed - a knock is heard on each revolution.
5. Compression test: Cylinder - 1 (130), 2 (130), 3 (75), 4 (0)
At a minimum, it looks to me like a significant valve or cylinder head problem. What concerns me however, is the loud knocking noise when running, indicating perhaps a valve having been sucked into the #4 cylinder? At any rate, given the ominous symptoms, I'm not sure doing a leak-down test would provide further definitive information. At this point, I'm considering pulling the cylinder head and seeing what that reveals.
I've learned a great deal from the expert commentary available here, so I'd be most appreciative of any thoughts, comments or advice regarding further testing prior to pulling the head. Thanks.