nass-t-habit
New Member
- Aug 30, 2010
- 21
- Boat Info
- 87 270 Sportfish
Rigged for scuba diving
- Engines
- twin Merc 260's
I need serious help, from somebody who knows Merc 260/5.7 carb engines inside and out, or at least has knowledge and experience working with them. Knowledge of these as twins, in the Amberjack configuration would be even better.
I've got a 87 270 Sportfish (Amberjack flybridge) with twin "260's" in it. I picked it up like 10th or 12 hand, both motors have obviously been replaced at least once.
The starboard motor is a 4bbl carb with v-belts. The port motor is what appears to be newer, 2bbl carb with the serpentine belt.
The starboard motor was running with perceivably far less power. The throttle had to be much, much further ahead to match the RPM's. It also built RMP's much slower than the 2bbl port motor. I found it had one cylinder with absolutely no compression, so I replaced it with a known good motor of the exact same configuration. But I still have exactly the same issue. It has far less power. And after installing Flowscans Saturday, I found out the 4bbl motor is burning TWICE as much fuel! But still providing less performance.
The 2bbl motor is burning 9 GPH, the 4bbl motor is burning 16! That's at 3400 RPM, with a heavy load, tabs down, drives tucked, in 2-3 foot seas, 22 mph on the GPS
The 4bbl motor has good and equal compression and leakdown on all cyliders. New plugs gapped properly. All 8 plugs fire equally (visually) when pulled out. If you look down the carb with the hammer down, you can see all 4 jets working, equally side to side, more from the back 2. It's not a throttle cable problem, I get full linkage motion with the full stroke of the throttle lever. The drives are matched. the props are indentical. It burns no oil. No leaks, no has gas fumes anywhere, no sheen on the water. It starts instantly, idles perfectly and runs great - unless you compare it the other motor. It's timed dead nuts on, the timing advance works within spec. New fuel in both tanks, new filters, problem is persistent no matter which motor(s) pull from which tank.
The tachs are on, the engine synch works. When the synch centers and the tachs are equal (one set anyway) you can feel that it's right. The ride smooths out, the vibration disappears, and the boat picks up speed slightly. I even did that "blind", when it felt right, the synch was centered.
WTF is going on here?
The ONLY thing re-used from the old motor is the distributor (new cap and rotor though), and the y-pipe. I replaced one flapper in the y-pipe, the other one is still rattling.
I will take any and all input I can get on this issue. However, what I'd really like to know is:
1. Can these 2 motors really be that different in performance?
2. Can the 4bbl really burn nearly twice as much fuel as the 4bbl.
3. Does it sound like there's something fundamentaly wrong with the 4bbl motor? (it does to me)
4. Any clues as to what the problem might be?
5. What SHOULD this boat be consuming for fuel? Assuming properly running 260 Mercs with Alpha drives, swinging 17x13.5 AL props. ( I'm guessing on the diameter, the pitch is right)
6. IF that 2bbl serpentine belt motor really is that much better of a motor, anyone have any clue as to what it might be as far as year/model? All serial numbers are gone.
I have no access to anyone local who knows what they're talking about. Any help will be greatly appreciated.
Thanks for taking the time to read this insanely long post, let alone respond.
And while I may be an idiot in some regards, I was Merc hack for 2 years about 20 years ago. I've owned 4 Searays since 1986, all with V8's that I've run long and hard, and never had enough money to hire anyone else to fix anything that broke. So I do have some reasonable hands-on experience and knowledge of the older ones.
I've got a 87 270 Sportfish (Amberjack flybridge) with twin "260's" in it. I picked it up like 10th or 12 hand, both motors have obviously been replaced at least once.
The starboard motor is a 4bbl carb with v-belts. The port motor is what appears to be newer, 2bbl carb with the serpentine belt.
The starboard motor was running with perceivably far less power. The throttle had to be much, much further ahead to match the RPM's. It also built RMP's much slower than the 2bbl port motor. I found it had one cylinder with absolutely no compression, so I replaced it with a known good motor of the exact same configuration. But I still have exactly the same issue. It has far less power. And after installing Flowscans Saturday, I found out the 4bbl motor is burning TWICE as much fuel! But still providing less performance.
The 2bbl motor is burning 9 GPH, the 4bbl motor is burning 16! That's at 3400 RPM, with a heavy load, tabs down, drives tucked, in 2-3 foot seas, 22 mph on the GPS
The 4bbl motor has good and equal compression and leakdown on all cyliders. New plugs gapped properly. All 8 plugs fire equally (visually) when pulled out. If you look down the carb with the hammer down, you can see all 4 jets working, equally side to side, more from the back 2. It's not a throttle cable problem, I get full linkage motion with the full stroke of the throttle lever. The drives are matched. the props are indentical. It burns no oil. No leaks, no has gas fumes anywhere, no sheen on the water. It starts instantly, idles perfectly and runs great - unless you compare it the other motor. It's timed dead nuts on, the timing advance works within spec. New fuel in both tanks, new filters, problem is persistent no matter which motor(s) pull from which tank.
The tachs are on, the engine synch works. When the synch centers and the tachs are equal (one set anyway) you can feel that it's right. The ride smooths out, the vibration disappears, and the boat picks up speed slightly. I even did that "blind", when it felt right, the synch was centered.
WTF is going on here?
The ONLY thing re-used from the old motor is the distributor (new cap and rotor though), and the y-pipe. I replaced one flapper in the y-pipe, the other one is still rattling.
I will take any and all input I can get on this issue. However, what I'd really like to know is:
1. Can these 2 motors really be that different in performance?
2. Can the 4bbl really burn nearly twice as much fuel as the 4bbl.
3. Does it sound like there's something fundamentaly wrong with the 4bbl motor? (it does to me)
4. Any clues as to what the problem might be?
5. What SHOULD this boat be consuming for fuel? Assuming properly running 260 Mercs with Alpha drives, swinging 17x13.5 AL props. ( I'm guessing on the diameter, the pitch is right)
6. IF that 2bbl serpentine belt motor really is that much better of a motor, anyone have any clue as to what it might be as far as year/model? All serial numbers are gone.
I have no access to anyone local who knows what they're talking about. Any help will be greatly appreciated.
Thanks for taking the time to read this insanely long post, let alone respond.
And while I may be an idiot in some regards, I was Merc hack for 2 years about 20 years ago. I've owned 4 Searays since 1986, all with V8's that I've run long and hard, and never had enough money to hire anyone else to fix anything that broke. So I do have some reasonable hands-on experience and knowledge of the older ones.
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