Last winter I put a new long block in my boat, the old motor broke a ring and did significant damage to a cylinder. Other than this damage, the boat ran great, was able to pull full nameplate RPM (with current prop, of course), and otherwise had no operational issues.
Fast forward to the present time. Long block replaced, transferring all associated parts from old motor. Manifold and riser looked wonderful, low hours and fresh water use only. Working my way through the 10 hour break-in cycle that the vendor recommended, everything seemed fine until the stage where higher rpms were allowed. There was a severe misfire about 3500, sounded like a coffee can full of bolts rattling. I did the following:
- Rechecked the timing and advance curve, 1 degree after TDC as stated by engine manufacturer (new from GM Marine). Also checked the advance at higher RPMs against the curve in the book. All good.
- Fresh fuel in tank, 91 octane
- New fuel filters
- New cap and rotor
- Rebuilt the carb, being sure to run a soft copper wire through all passages. Main jet and power valve should handle high rpms, ensure they were cleaned and clear
- Built a test hose with fuel pressure gauge and clear section to test for bubbles. Fuel pressure was low, bubbles at high RPM. Replaced pump. Rechecked, no bubbles, 5-6 psi.
- Now I can get to almost 4000 before misfire happens, sounds horrible above that though.
- Plugs gapped correctly and look fine.
- Engine serial number 0W042772, has electronic distributor.
I don't want to start throwing parts at it, but am at a bit of a loss here.
Any advice would be greatly appreciated!
Thanks,
Pete
Fast forward to the present time. Long block replaced, transferring all associated parts from old motor. Manifold and riser looked wonderful, low hours and fresh water use only. Working my way through the 10 hour break-in cycle that the vendor recommended, everything seemed fine until the stage where higher rpms were allowed. There was a severe misfire about 3500, sounded like a coffee can full of bolts rattling. I did the following:
- Rechecked the timing and advance curve, 1 degree after TDC as stated by engine manufacturer (new from GM Marine). Also checked the advance at higher RPMs against the curve in the book. All good.
- Fresh fuel in tank, 91 octane
- New fuel filters
- New cap and rotor
- Rebuilt the carb, being sure to run a soft copper wire through all passages. Main jet and power valve should handle high rpms, ensure they were cleaned and clear
- Built a test hose with fuel pressure gauge and clear section to test for bubbles. Fuel pressure was low, bubbles at high RPM. Replaced pump. Rechecked, no bubbles, 5-6 psi.
- Now I can get to almost 4000 before misfire happens, sounds horrible above that though.
- Plugs gapped correctly and look fine.
- Engine serial number 0W042772, has electronic distributor.
I don't want to start throwing parts at it, but am at a bit of a loss here.
Any advice would be greatly appreciated!
Thanks,
Pete
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