Looking for some direction here to get my GP1200 powered aluminum mini boat back in action. First a little history on it so we're up to speed, apologies in advance for the long winded post:
Purchased donor gp1200 last year dirt cheap, built mini boat, rebuilt the 3 carbs and ran the heck out of it until I overheated it sucking up sand and toasted top end. It would run WOT all day long, came off idle clean, it was on point prior to the overheating. Realized it was an SBT rebuild engine during tear down (sleeves were garbage, ports not lining up etc) so I had it re-sleeved. Rebuilt engine with new parts, ran back through carbs (not new kits, just made sure it was all blown out as it sat for a few months) and got it back together a couple weeks ago.
So on the maiden voyage, fired up, idles well, comes off idle well. I putted around and warmed it up, shut it off let it cool, heat cycled a couple times. I took off after the 3rd head cycle or so and after 1-2 minute of half throttle the engine shut down like it was starving for fuel. Plugs confirm, dry and white'ish. Fired right back up and idled to the dock, check plugs again and they look good (dark brownish) after the idle in.
I had planned to run a/m flame arrestors and switch up jetting a bit to follow the "oside bill" carb recipe so I decide now is a good time to do that. I pull carbs, recheck everything and jet according to his recommendation which is basically lowering pop-off a bit, running all 100 pilots instead of staggered stock jetting (95/97/100) and leaving the 135 mains. I also ran dual fuel feeds and removed the internal return restriction in favor of a 105 main jet in the return line, more tips from the couch forums from "oside bill". After some research I was expecting to find bad/creased check valves or fuel pump diaphragms worn out. All fuel lines are new, lines in tank have been pressure tested for leaks.
Retested this morning and I'm having the exact same issue, fuel starvation after running half throttle for short rip. Plugs aren't as white, but still showing lean.
So I am either not flowing enough fuel at half to WOT, or running too lean, either of which is causing the lean seize/shut down. I assumed it would have to be exceptionally lean to lean seize, but I may be wrong, maybe just slightly lean is enough to shut it down. I do not want do it again, I realize how bad it is on the engine.
So my theory is with this fresh top end and ports all aligned and cleaned up, perhaps I am just flowing that much better than I need more fuel from last year. I somewhat expected last year to have to change jetting a bit due to the increased engine load of the heavier hull compared to the GP1200 hull, but it was perfect. Could it be now that my engine is up to snuff, it's exposing the lean condition on top end?
I've ordered enough parts to rebuild fuel pump sides with all new diaphragms, check valves, and gaskets and I also picked up some 140 mains. That will be my next step. Hard to imagine needing rebuild on that side as they typically work for YEARS and all the internals are currently less than a year old, and of course all genuine Mikuni. Any advice is appreciated.
Purchased donor gp1200 last year dirt cheap, built mini boat, rebuilt the 3 carbs and ran the heck out of it until I overheated it sucking up sand and toasted top end. It would run WOT all day long, came off idle clean, it was on point prior to the overheating. Realized it was an SBT rebuild engine during tear down (sleeves were garbage, ports not lining up etc) so I had it re-sleeved. Rebuilt engine with new parts, ran back through carbs (not new kits, just made sure it was all blown out as it sat for a few months) and got it back together a couple weeks ago.
So on the maiden voyage, fired up, idles well, comes off idle well. I putted around and warmed it up, shut it off let it cool, heat cycled a couple times. I took off after the 3rd head cycle or so and after 1-2 minute of half throttle the engine shut down like it was starving for fuel. Plugs confirm, dry and white'ish. Fired right back up and idled to the dock, check plugs again and they look good (dark brownish) after the idle in.
I had planned to run a/m flame arrestors and switch up jetting a bit to follow the "oside bill" carb recipe so I decide now is a good time to do that. I pull carbs, recheck everything and jet according to his recommendation which is basically lowering pop-off a bit, running all 100 pilots instead of staggered stock jetting (95/97/100) and leaving the 135 mains. I also ran dual fuel feeds and removed the internal return restriction in favor of a 105 main jet in the return line, more tips from the couch forums from "oside bill". After some research I was expecting to find bad/creased check valves or fuel pump diaphragms worn out. All fuel lines are new, lines in tank have been pressure tested for leaks.
Retested this morning and I'm having the exact same issue, fuel starvation after running half throttle for short rip. Plugs aren't as white, but still showing lean.
So I am either not flowing enough fuel at half to WOT, or running too lean, either of which is causing the lean seize/shut down. I assumed it would have to be exceptionally lean to lean seize, but I may be wrong, maybe just slightly lean is enough to shut it down. I do not want do it again, I realize how bad it is on the engine.
So my theory is with this fresh top end and ports all aligned and cleaned up, perhaps I am just flowing that much better than I need more fuel from last year. I somewhat expected last year to have to change jetting a bit due to the increased engine load of the heavier hull compared to the GP1200 hull, but it was perfect. Could it be now that my engine is up to snuff, it's exposing the lean condition on top end?
I've ordered enough parts to rebuild fuel pump sides with all new diaphragms, check valves, and gaskets and I also picked up some 140 mains. That will be my next step. Hard to imagine needing rebuild on that side as they typically work for YEARS and all the internals are currently less than a year old, and of course all genuine Mikuni. Any advice is appreciated.
. Did you use the same Cylinder Head? What was the new rebuilt Engine Compression? Might have required higher Octane Fuel...