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Author Topic:   High Speed cutout on 40hp merc

15ftlover posted 02-18-2004 12:43 PM ET (US)
Wanted to solicit input, I have a '91 4 cylinder mercury that had new plugs, new stator and both carbs rebuilt about 6months ago. I have since put about 15 hours on the motor when it started to lose power at near WOT. No alarms but I noticed it was spitting unburnt fuel through exhaust and leaking fuel around lower carb. I can't tell if fuel is flowing out of venturi or wether it is some other type of leak (around bowl?). With engine off the fuel still seems to be leaking from carb. Any ideas as to the next step? The filter is new and no debris, fuel lines checked with no apparent leaks. Boat will wide open initially but then loses power after several minutes and bogs down off a plane. All cylinders are getting spark. I think it is getting fuel starved but can't explain the fuel drops that show up on the water surface during idle. Our local mechanic wants to rebuild his 'rebuilt carbs" but I don't want him touching my engine again since he just did such a swell job 6months ago. Advice appreciated
Hen
Clark Roberts posted 02-18-2004 06:28 PM ET (US)
15ft, sounds like a hole in fuel pump diaphram allowing fuel to be pumped into crankcase of lower cyl. which drives the fuel pump. Once fuel is in the crankcase it floods out through carb throat. Check it out, get rebuild kit and rebuild and that should do it. You must remove the starter motor to get at the fuel pump but it's a very, very easy job(remember to disconnect battery before dinking around with that starter motor)... happy Whalin;/// Clark... Spruce Creek Navy
15ftlover posted 02-19-2004 05:18 PM ET (US)
Clark, thanks for the reply. Mechanic says the lower piston has lost some compression (it's reading 90 psi vs. 120psi in other cylinders) I imagine the fuel starve may have left piston running with little lubrication. I think the diaphragm is a good surmise-am going to have a look (replaced it years ago) It appears it may have been fuel line deterioration above the internal fuel filter (the line had some crumbling black stuff on the inside after cutting through it-that may have fouled the carb jet)
I've read of people running motors with scored pistons but am concerned about unbalanced power taking toll on cranks/shafts ect.
The motor has seen alot of use in 13 years and I think I'm finally going to repower with 60hp mercury EFI 4stroke-anyone's thoughts on that motor? I hope to use the same cables and perhaps prop (stainless, may be too small) to save some money. Anyone out there looking for a good lower unit from a 40hp merc?

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Author	Topic: High Speed cutout on 40hp merc
15ftlover	posted 02-18-2004 12:43 PM ET (US) Wanted to solicit input, I have a '91 4 cylinder mercury that had new plugs, new stator and both carbs rebuilt about 6months ago. I have since put about 15 hours on the motor when it started to lose power at near WOT. No alarms but I noticed it was spitting unburnt fuel through exhaust and leaking fuel around lower carb. I can't tell if fuel is flowing out of venturi or wether it is some other type of leak (around bowl?). With engine off the fuel still seems to be leaking from carb. Any ideas as to the next step? The filter is new and no debris, fuel lines checked with no apparent leaks. Boat will wide open initially but then loses power after several minutes and bogs down off a plane. All cylinders are getting spark. I think it is getting fuel starved but can't explain the fuel drops that show up on the water surface during idle. Our local mechanic wants to rebuild his 'rebuilt carbs" but I don't want him touching my engine again since he just did such a swell job 6months ago. Advice appreciated Hen
Clark Roberts	posted 02-18-2004 06:28 PM ET (US) 15ft, sounds like a hole in fuel pump diaphram allowing fuel to be pumped into crankcase of lower cyl. which drives the fuel pump. Once fuel is in the crankcase it floods out through carb throat. Check it out, get rebuild kit and rebuild and that should do it. You must remove the starter motor to get at the fuel pump but it's a very, very easy job(remember to disconnect battery before dinking around with that starter motor)... happy Whalin;/// Clark... Spruce Creek Navy
15ftlover	posted 02-19-2004 05:18 PM ET (US) Clark, thanks for the reply. Mechanic says the lower piston has lost some compression (it's reading 90 psi vs. 120psi in other cylinders) I imagine the fuel starve may have left piston running with little lubrication. I think the diaphragm is a good surmise-am going to have a look (replaced it years ago) It appears it may have been fuel line deterioration above the internal fuel filter (the line had some crumbling black stuff on the inside after cutting through it-that may have fouled the carb jet) I've read of people running motors with scored pistons but am concerned about unbalanced power taking toll on cranks/shafts ect. The motor has seen alot of use in 13 years and I think I'm finally going to repower with 60hp mercury EFI 4stroke-anyone's thoughts on that motor? I hope to use the same cables and perhaps prop (stainless, may be too small) to save some money. Anyone out there looking for a good lower unit from a 40hp merc?