I could use some fresh ideas. I am troubleshooting my 4-cyl Merc. I was out Wed and Thurs enjoying the last nice days of the year and several times the engine stuttered between 90-100% WOT. A quick random mis-fire after reaching ~5000RPM from a hole shot, like 2 or 3 in a row then smooth for a second or two then another quick mis-fire fit, each time I'd pull back to 75% WOT and she'd settle down. Back up to 100% and she ran great from then until next hole shot. This happened 3 or 4 times out of the hole after either being off - cold then warmed up or trolling around 1200 RPM. All spring and summer she has ran great. Over 75 gallons of fuel this season - Canada, Kansas, Missouri, & Arkansas. Ran well. Starts everytime, idles great, accelerates quickly out of the hole and gets on plane just fine. Doesn't feel like a loaded up engine, no bogging down, more like a loose connection, very electrical in my thoughts. Air temp was ~45-50F and water was 54F.
Here's what I've done so far today...
I pulled the plugs and found #3 was carboned up much more than the other 3. See Pic - #1 is on the right. These were new plugs last spring. Sure looks like something is up with #3 cyl. Not just recently either. Does this look electrical or like something is goofy with the lower carb? Too rich on Cyl 3 or not enough heat?
Checked all the connections on the coil packs and switchbox, nothing loose.
Gap tested all four plug wires with the gap tester. Easily shot across the 3/4" gap.
Compression - all at 120 +-2 psi.
Pumped primer bulb until hard.
Some history.
I bought this boat summer 2010 as a father-daughter-son project boat. Previous owner had jump started the engine with a car battery then ran with no battery. Not a good idea. He had a marine shop replace the switchbox and trigger to get her running again. Also a new marine battery. Rectifier was still fried when I got her. Water leaked out of cyl block cover near #1 plug. Carbs were gummed up and leaking, gaskets leaked in multiple locations, lower unit was leaking, wiring was mouse chewed. She ran but smoked a ton and not all that smoothly. Flooded easily too. Well I got her cheap with the plan to repair/replace as needed. And have. The deeper I dug the more I found though. Fuel lines degraded, stator wiring insulation was gone, water pump impellor was shot. Oil/water green goo mix drained out of l/u exhaust. Signs of no routine maintenance and age. I pulled the powerhead last winter and found a ton of carbon buildup in the exhaust manifold and exhaust tube. Exhaust drop tube was nearly clogged. Since this was a winter project, we pulled apart the engine and honed the cylinders, replaced the rings and all needle bearings, reeds, oil seals and bearings, and carefully reassembled the powerhead. We installed a new CDI 174-5454K1 Stator, CDI 194-5279 Regulator, four new Sierra 18-5186 coils (two original ones were cracked), new starter solenoid and replaced starter brushes. Didn't replace the switchbox or the trigger since they were very new already. Also cleaned and replaced the carb parts, needles/ floats and all gaskets. New fuel lines, prime bulb and filter. New wiring as needed. Lower unit got a new shift-shaft seal, water pump housing and impellor. And a pressure/vacuum test. All went fairly well. Had to re-do the carb float adjustment once to get the needle to seat properly. The top carb would shoot gas out of air intake. We used our Merc manual and I-Boats forum ALOT including Clams "Link and Sync" to get her running nicely in time for spring thaw. We broke her in per the manual and re-propped to get RPM up around 5300. Shes been a good boat since. Runs strong and charges at 14.1V. Loads of fish hauled in! Lots of great memories and beautiful sunsets.
Sorry for the rambling. Just thought the history would answer some questions. I'm willing to try about anything to figure out what's up.
Thanks!
Troy
Here's what I've done so far today...
I pulled the plugs and found #3 was carboned up much more than the other 3. See Pic - #1 is on the right. These were new plugs last spring. Sure looks like something is up with #3 cyl. Not just recently either. Does this look electrical or like something is goofy with the lower carb? Too rich on Cyl 3 or not enough heat?
Checked all the connections on the coil packs and switchbox, nothing loose.
Gap tested all four plug wires with the gap tester. Easily shot across the 3/4" gap.
Compression - all at 120 +-2 psi.
Pumped primer bulb until hard.
Some history.
I bought this boat summer 2010 as a father-daughter-son project boat. Previous owner had jump started the engine with a car battery then ran with no battery. Not a good idea. He had a marine shop replace the switchbox and trigger to get her running again. Also a new marine battery. Rectifier was still fried when I got her. Water leaked out of cyl block cover near #1 plug. Carbs were gummed up and leaking, gaskets leaked in multiple locations, lower unit was leaking, wiring was mouse chewed. She ran but smoked a ton and not all that smoothly. Flooded easily too. Well I got her cheap with the plan to repair/replace as needed. And have. The deeper I dug the more I found though. Fuel lines degraded, stator wiring insulation was gone, water pump impellor was shot. Oil/water green goo mix drained out of l/u exhaust. Signs of no routine maintenance and age. I pulled the powerhead last winter and found a ton of carbon buildup in the exhaust manifold and exhaust tube. Exhaust drop tube was nearly clogged. Since this was a winter project, we pulled apart the engine and honed the cylinders, replaced the rings and all needle bearings, reeds, oil seals and bearings, and carefully reassembled the powerhead. We installed a new CDI 174-5454K1 Stator, CDI 194-5279 Regulator, four new Sierra 18-5186 coils (two original ones were cracked), new starter solenoid and replaced starter brushes. Didn't replace the switchbox or the trigger since they were very new already. Also cleaned and replaced the carb parts, needles/ floats and all gaskets. New fuel lines, prime bulb and filter. New wiring as needed. Lower unit got a new shift-shaft seal, water pump housing and impellor. And a pressure/vacuum test. All went fairly well. Had to re-do the carb float adjustment once to get the needle to seat properly. The top carb would shoot gas out of air intake. We used our Merc manual and I-Boats forum ALOT including Clams "Link and Sync" to get her running nicely in time for spring thaw. We broke her in per the manual and re-propped to get RPM up around 5300. Shes been a good boat since. Runs strong and charges at 14.1V. Loads of fish hauled in! Lots of great memories and beautiful sunsets.
Sorry for the rambling. Just thought the history would answer some questions. I'm willing to try about anything to figure out what's up.
Thanks!
Troy
