Hi all! I'm a long-time reader but first-time poster. Anyway, I have a 1987 V-Master bowrider with the 3.0 MerCruiser w/ Alpha Gen I drive. I've owned this boat almost four years, but for the last two I've had a persistent issue that I can't seem to resolve.
When cold, the engine starts and runs with little or no hesitation. After a minute or two of warming up, it'll idle, accelerate, cruise, and idle again smoothly. However, after cruising for about 20 minutes and then backing the throttle down to ~1500 RPM or lower, the engine stumbles and stalls within 10 seconds.
When this happens, the ignition coil and fuel pump are both quite hot to the touch, and the engine is extremely difficult to restart. I have to wait several minutes, pump the primer bulb (added by a previous owner), open the throttle almost fully, and crank it over several times. On the first few attempts, the engine either doesn't fire or runs briefly and dies, sometimes backfiring. Eventually, it will rev up and stay running as long as I keep giving it throttle. If I'm in open water, I do this process with the boat in forward gear, so when it finally revs up, I can continue on my way. As long as I don't back off the throttle, it'll continue cruising just fine.
A few things worth noting:
1) When the engine stalls and I squeeze the bulb, it seems like the carb bowl is NOT full. The bulb isn't stiff, and it sounds like there's additional fuel entering the carb as I squeeze.
2) HOWEVER, spraying starting fluid in the carb doesn't help the engine start. If anything, it makes things worse. My amateur knowledge tells me it's flooding, but how would it flood with seemingly no fuel in the carb?
3) Cooling system seems fine, but to be clear, I'm no marine mechanic. What I do know is that when I run the boat on muffs, I see a substantial flow of hot water out of the exhaust port at the transom, so there's obviously water flowing through the hot engine. When cruising on the lake, the temp gauge never exceeds around 140F, but I don't trust it completely.
4) The boat has always been a little hard to start after warming up fully and then sitting for an hour or so. I had always attributed this to vapor lock, but it wasn't enough of an issue to bother me. It certainly has NOT always shut down when backing off the throttle, like it's doing now.
This trouble seemed to start after I rebuilt the carb due to issues with the accelerator pump (which is no longer an issue). However, I'm borrowing my friend's almost-identical boat with the same engine for the purpose of swapping parts over to troubleshoot. When I swapped my friend's carb onto my engine, there was no improvement.
Here's everything else I've tried, not necessarily in order, to no avail:
-- Opened and inspected the carb at least twice after rebuilding.
-- Checked timing and dwell.
-- Replaced plugs (gapped to spec) and plug wires.
-- Changed from points to Pertronix. Initially used 3-ohm coil with resistor wire bypassed, then tried two other 1.5 ohm coils with resistor wire reconnected. Pertronix module was always connected directly to 12V per instructions, NOT after the resistor wire.
-- Replaced mechanical fuel pump with the one from my friend's boat.
-- Installed a marine electric fuel pump instead of the mechanical one (I was sure this would help; it didn't!!) The electric pump is mounted to the port side of the block, just behind and below the alternator, and wired to come on with the ignition.
-- Replaced questionable electrical connectors under the helm, thinking voltage to ignition system may be low.
-- Checked engine temps with IR thermometer immediately after it stalled on the water. Hottest I saw was ~210F on the lower starboard side of the engine block, which to me, isn't alarming. Most other areas of the engine were ~150-180F.
-- Checked condition of circulating pump (no obvious issues there).
The two things that seem to help are:
1) Running the boat without the engine cover (doghouse). For some reason, the breeze blowing across the engine seems to make a difference.
2) Continuously squeezing the primer bulb while backing off throttle and idling afterward. This makes it run much longer before stalling, or sometimes not stall at all.
Here are my theories:
1) Something is wrong with both my carb and my friend's carb (both are Mercarbs) that's causing a flooding condition when coming off the throttle, only when the engine is hot, and this is what empties the bowl. (My friend's boat needs some other work to be lake ready, so I can't just take that one out to compare.)
2) My engine is running hotter than it should for some reason, boiling the fuel in the line and causing vapor lock at idle (less fuel flow than during cruising). I'm considering installing a mechanical temp gauge just to confirm.
Is there anything I'm missing? I'm tearing my hair out, but I like this boat and I don't want to give up because of a seemingly simple problem. Any suggestions would be greatly appreciated!
When cold, the engine starts and runs with little or no hesitation. After a minute or two of warming up, it'll idle, accelerate, cruise, and idle again smoothly. However, after cruising for about 20 minutes and then backing the throttle down to ~1500 RPM or lower, the engine stumbles and stalls within 10 seconds.
When this happens, the ignition coil and fuel pump are both quite hot to the touch, and the engine is extremely difficult to restart. I have to wait several minutes, pump the primer bulb (added by a previous owner), open the throttle almost fully, and crank it over several times. On the first few attempts, the engine either doesn't fire or runs briefly and dies, sometimes backfiring. Eventually, it will rev up and stay running as long as I keep giving it throttle. If I'm in open water, I do this process with the boat in forward gear, so when it finally revs up, I can continue on my way. As long as I don't back off the throttle, it'll continue cruising just fine.
A few things worth noting:
1) When the engine stalls and I squeeze the bulb, it seems like the carb bowl is NOT full. The bulb isn't stiff, and it sounds like there's additional fuel entering the carb as I squeeze.
2) HOWEVER, spraying starting fluid in the carb doesn't help the engine start. If anything, it makes things worse. My amateur knowledge tells me it's flooding, but how would it flood with seemingly no fuel in the carb?
3) Cooling system seems fine, but to be clear, I'm no marine mechanic. What I do know is that when I run the boat on muffs, I see a substantial flow of hot water out of the exhaust port at the transom, so there's obviously water flowing through the hot engine. When cruising on the lake, the temp gauge never exceeds around 140F, but I don't trust it completely.
4) The boat has always been a little hard to start after warming up fully and then sitting for an hour or so. I had always attributed this to vapor lock, but it wasn't enough of an issue to bother me. It certainly has NOT always shut down when backing off the throttle, like it's doing now.
This trouble seemed to start after I rebuilt the carb due to issues with the accelerator pump (which is no longer an issue). However, I'm borrowing my friend's almost-identical boat with the same engine for the purpose of swapping parts over to troubleshoot. When I swapped my friend's carb onto my engine, there was no improvement.
Here's everything else I've tried, not necessarily in order, to no avail:
-- Opened and inspected the carb at least twice after rebuilding.
-- Checked timing and dwell.
-- Replaced plugs (gapped to spec) and plug wires.
-- Changed from points to Pertronix. Initially used 3-ohm coil with resistor wire bypassed, then tried two other 1.5 ohm coils with resistor wire reconnected. Pertronix module was always connected directly to 12V per instructions, NOT after the resistor wire.
-- Replaced mechanical fuel pump with the one from my friend's boat.
-- Installed a marine electric fuel pump instead of the mechanical one (I was sure this would help; it didn't!!) The electric pump is mounted to the port side of the block, just behind and below the alternator, and wired to come on with the ignition.
-- Replaced questionable electrical connectors under the helm, thinking voltage to ignition system may be low.
-- Checked engine temps with IR thermometer immediately after it stalled on the water. Hottest I saw was ~210F on the lower starboard side of the engine block, which to me, isn't alarming. Most other areas of the engine were ~150-180F.
-- Checked condition of circulating pump (no obvious issues there).
The two things that seem to help are:
1) Running the boat without the engine cover (doghouse). For some reason, the breeze blowing across the engine seems to make a difference.
2) Continuously squeezing the primer bulb while backing off throttle and idling afterward. This makes it run much longer before stalling, or sometimes not stall at all.
Here are my theories:
1) Something is wrong with both my carb and my friend's carb (both are Mercarbs) that's causing a flooding condition when coming off the throttle, only when the engine is hot, and this is what empties the bowl. (My friend's boat needs some other work to be lake ready, so I can't just take that one out to compare.)
2) My engine is running hotter than it should for some reason, boiling the fuel in the line and causing vapor lock at idle (less fuel flow than during cruising). I'm considering installing a mechanical temp gauge just to confirm.
Is there anything I'm missing? I'm tearing my hair out, but I like this boat and I don't want to give up because of a seemingly simple problem. Any suggestions would be greatly appreciated!
