scout-j-m
Chief Petty Officer
- Joined
- Jul 31, 2009
- Messages
- 641
A few weeks back I bought a Mercury (Tracker Branded) 25 hp long shaft. Ad said it's a 1994 model, which seems accurate but I'm not 100% sure how to cross reference the serial number (0G092027) to the year. I do know that the serial range means it is a chrome lined bore and not mercosil, per my shop manual. The motor is an electric start with pull start and controls but seller had no way of testing e-start and was in a very difficult position to pull it to get compression readings. I could tell the bottom cylinder was lower though as it climbed much slower after a couple pulls than the top one. Spark test was excellent though. I bought it anyways hoping it could be salvaged. Price wasn't a steal or anything but I can get my money back plus a little if I decide to take the time to part it out and list items, so a good project with not much financial risk.
Once I got it home I hooked up the electric start and got 120 psi on the top cylinder and a little over 90 on the bottom. Sprayed some wd-40 in both plug holes to see if maybe it was a ring sticking and retested later on and no change. Used a cheap borescope that feeds video to my phone and really couldn't make out any scoring to the cylinder walls but it was hard to tell....maybe that chrome bore is harder to inspect as it was like looking in a mirror with a flashlight pointed back at you. I could make out that the tops of the pistons were only about 60% carbon covered so rings sticking didn't seem too likely. I pulled the exhaust cover/water jacket plate and could see some light scoring on the bottom piston at the exhaust ports. The rings themselves looked fine though. I have yet to pull the other 2 covers on the exhaust side or the intake port covers to inspect.
I plan to go through it like I would if compression was good to see if it will run and to help verify what all parts are good and marketable. I already verified the electric start is working. The lower turns and shifts smoothly and I already pulled it and it passed a pressure test and just needs a new impeller, gear oil, and drain/fill gaskets. I tested the thermostat and it's working although one of the thermostat cover bolts had been broken off and someone had basically epoxied or RTV'd that broken bolt in place which is suspicious. The motor itself is clean in terms of being free of caked grime from evaporated premix fuel but is loaded down with dirt dauber nests and mess that need to be cleaned up. Motor included an unopened OEM carb kit which is $100 on most parts sites so hoping I can clean the carb and save that.
Now that I laid out the backstory, here are my questions for those more familiar with theses specific models:
Once I got it home I hooked up the electric start and got 120 psi on the top cylinder and a little over 90 on the bottom. Sprayed some wd-40 in both plug holes to see if maybe it was a ring sticking and retested later on and no change. Used a cheap borescope that feeds video to my phone and really couldn't make out any scoring to the cylinder walls but it was hard to tell....maybe that chrome bore is harder to inspect as it was like looking in a mirror with a flashlight pointed back at you. I could make out that the tops of the pistons were only about 60% carbon covered so rings sticking didn't seem too likely. I pulled the exhaust cover/water jacket plate and could see some light scoring on the bottom piston at the exhaust ports. The rings themselves looked fine though. I have yet to pull the other 2 covers on the exhaust side or the intake port covers to inspect.
I plan to go through it like I would if compression was good to see if it will run and to help verify what all parts are good and marketable. I already verified the electric start is working. The lower turns and shifts smoothly and I already pulled it and it passed a pressure test and just needs a new impeller, gear oil, and drain/fill gaskets. I tested the thermostat and it's working although one of the thermostat cover bolts had been broken off and someone had basically epoxied or RTV'd that broken bolt in place which is suspicious. The motor itself is clean in terms of being free of caked grime from evaporated premix fuel but is loaded down with dirt dauber nests and mess that need to be cleaned up. Motor included an unopened OEM carb kit which is $100 on most parts sites so hoping I can clean the carb and save that.
Now that I laid out the backstory, here are my questions for those more familiar with theses specific models:
- Given the compression numbers, the piston scoring, and this being a chrome lined block, is the block junk now? Or is it possible the rings may be partially stuck and the motor could be tuned up and a decarbing could maybe help those lower compression numbers? Since you can't bore or hone these chrome lined cylinders, could new rings possibly help?
- If the block is junk, what other year model powerheads would be interchangeable? There are a fair amount of used powerhead shortblocks from the previous generation motors that look very similar aside from lacking a thermostat. Is it possible to do a block swap with an older model? If it would all bolt up correctly would it then be able to use my ignition system which has the automatic electronic timing advance within the CDI?
- Whether the block is junk now or not, what can I look for as to the culprit of the low compression numbers on the bottom cylinder? Motor doesnt have any visual signs of overheating on the exterior. It's a single carb motor, so I assume it wasn't a lean condition...at least not from the carb. Are the crank seals prone to failing on these? The top few inches of the driveshaft had some unburnt fuel/oil on it but not enough to make me think lower seal was leaking. Current impeller has set up into a cupped position but is fully intact. Plugs that were included were brand new so can't infer anything from them. What else should I check out?
