G'day,
This is one for the thinkers or someone who might have seen something like this before, it is certainly testing my skills. Hopefully this yarn is pleasant reading.
Details;
1996 DT15C in good condition probably <100 hrs use, oil injection system long since removed by my late father (outboard mechanic) and running on 50:1 premix.
Little sister had been using it for years and on her last trip it got VERY hot and seized. Her story,.......she launched the tinny then 3/4 throttle for ~15minutes down the river, no issues, 1/2 an hour of trolling around dropping nets then a quick squirt down the river again at ~3/4 throttle and she thinks it sort of went bang (or something, can't recall exactly) and stopped. Restarted and idled around picking up nets, seemingly (??) no issues then back down the river at 3/4 throttle and again it stopped. She couldn't start it and smoke was coming off engine. She paddled to the bank.
Brother in-law started it in a water drum a week later and it runs but a bit sluggish he thinks, and he sends a video, I can see the tell tale is blocked but exhaust relief port has plenty of water coming out.
They buy another motor, I pick it up and set to investigating to see if it is worth repairing, it was my late dads so would like to keep it and otherwise it is a low hours engine.
Here is what I find;
The water pump looks fine, near new condition, it had been replaced a year or so ago, the pick up is tucked up under the cav plate and unlikely (but not impossible) to be blocked by a foreign object, eg plastic bag and no blockage present in the water pipes.
Cylinder head no longer grey factory color but burnt tan, wires and cdi mounted on top cylinder melted but still working, it clearly got hot and for some time. Powerhead removed and engine split, top piston has small scuff mark from when it seized, rings undamaged, bore had light scuff mark, piston crowns normal with light carbon all over and no sign of pre-ignition or detonation damage, no sign of water ingress to cylinder, in fact pistons looked like new and bore otherwise like new. Gudgeon, big end and mains look great. The tell tale well in the power head had half a tea spoon of calcium scale blocking the outlet, not an issue in anyway, normal sort of tell tale blockage for that design.
I cleaned and measured the piston for eccentricity, polished out the small scuff mark with wet n dry, a light hone to the bores, new gaskets (no easy feat as somewhat obsolete) replaced impeller as already had one to go and reassembled.
Spark was ok but due the melted cdi and some wires with the insulation melted off a second hand cdi was sourced and damaged wiring repaired. With a bit of coaxing it started and ran in the tank, it settled down and idled, started 1st/2nd pull thereafter and revved up in neutral promptly-ish. Pumping plenty of water from relief and now unblocked tell tale
Cracked a beer, looked up and toasted the old man for passing on some of his knowledge and skills, although as follows not enough.
Three weeks later I put the motor on the tinny and take it for a run, it started 3rd pull, the usual choke on /off coaxing as it warms up for the first 15 seconds then it idles. Into forward open the throttle and it dies back to an idle, typical of a lean mixture scenario. Idle back to the bank it stalls/stops and won't restart. Tank vent is open and bulb primed, no joy, back to the shed.
Spark is thin blue while I am pulling it over, I have seen better and worse, fuel is good. I get it running again in the tank, twice whilst idling it does that classic 2 stroke misfire and snuffs itself and it difficult to restart, but it does start and it runs ok -ish in the tank or awhile.
There is something going on, why did it get so hot, I would've thought even a stoichiometric fuel A/F ratio wouldn't cause the level over heating as it has clearly been exposed to. Timing drift leading to detonation would leave a signature on the piston crown yet it was fine.
Why did my little sis think she thought it went bang and stopped initially.
The thermostat hasn't been checked, but I'll check this on its first loaded run on the water and water is fine. Timing has not been checked yet, but I doubt it is an issue, the advance pick up works fine and I can test it at idle by overriding it with my finger against the spring bias.
Fuel pump has not been checked yet, but I'm not losing prime at it picks up fuel from a tank 5' below the carby, blocked main jet,.....maybe, but it has a good inline filter, probably an easy enough check though so that will be the next place to check.
Anyone want to have a crack at internet engine diagnosis?
Thanks for reading this far and making it through the waffle.
This is one for the thinkers or someone who might have seen something like this before, it is certainly testing my skills. Hopefully this yarn is pleasant reading.
Details;
1996 DT15C in good condition probably <100 hrs use, oil injection system long since removed by my late father (outboard mechanic) and running on 50:1 premix.
Little sister had been using it for years and on her last trip it got VERY hot and seized. Her story,.......she launched the tinny then 3/4 throttle for ~15minutes down the river, no issues, 1/2 an hour of trolling around dropping nets then a quick squirt down the river again at ~3/4 throttle and she thinks it sort of went bang (or something, can't recall exactly) and stopped. Restarted and idled around picking up nets, seemingly (??) no issues then back down the river at 3/4 throttle and again it stopped. She couldn't start it and smoke was coming off engine. She paddled to the bank.
Brother in-law started it in a water drum a week later and it runs but a bit sluggish he thinks, and he sends a video, I can see the tell tale is blocked but exhaust relief port has plenty of water coming out.
They buy another motor, I pick it up and set to investigating to see if it is worth repairing, it was my late dads so would like to keep it and otherwise it is a low hours engine.
Here is what I find;
The water pump looks fine, near new condition, it had been replaced a year or so ago, the pick up is tucked up under the cav plate and unlikely (but not impossible) to be blocked by a foreign object, eg plastic bag and no blockage present in the water pipes.
Cylinder head no longer grey factory color but burnt tan, wires and cdi mounted on top cylinder melted but still working, it clearly got hot and for some time. Powerhead removed and engine split, top piston has small scuff mark from when it seized, rings undamaged, bore had light scuff mark, piston crowns normal with light carbon all over and no sign of pre-ignition or detonation damage, no sign of water ingress to cylinder, in fact pistons looked like new and bore otherwise like new. Gudgeon, big end and mains look great. The tell tale well in the power head had half a tea spoon of calcium scale blocking the outlet, not an issue in anyway, normal sort of tell tale blockage for that design.
I cleaned and measured the piston for eccentricity, polished out the small scuff mark with wet n dry, a light hone to the bores, new gaskets (no easy feat as somewhat obsolete) replaced impeller as already had one to go and reassembled.
Spark was ok but due the melted cdi and some wires with the insulation melted off a second hand cdi was sourced and damaged wiring repaired. With a bit of coaxing it started and ran in the tank, it settled down and idled, started 1st/2nd pull thereafter and revved up in neutral promptly-ish. Pumping plenty of water from relief and now unblocked tell tale
Cracked a beer, looked up and toasted the old man for passing on some of his knowledge and skills, although as follows not enough.
Three weeks later I put the motor on the tinny and take it for a run, it started 3rd pull, the usual choke on /off coaxing as it warms up for the first 15 seconds then it idles. Into forward open the throttle and it dies back to an idle, typical of a lean mixture scenario. Idle back to the bank it stalls/stops and won't restart. Tank vent is open and bulb primed, no joy, back to the shed.
Spark is thin blue while I am pulling it over, I have seen better and worse, fuel is good. I get it running again in the tank, twice whilst idling it does that classic 2 stroke misfire and snuffs itself and it difficult to restart, but it does start and it runs ok -ish in the tank or awhile.
There is something going on, why did it get so hot, I would've thought even a stoichiometric fuel A/F ratio wouldn't cause the level over heating as it has clearly been exposed to. Timing drift leading to detonation would leave a signature on the piston crown yet it was fine.
Why did my little sis think she thought it went bang and stopped initially.
The thermostat hasn't been checked, but I'll check this on its first loaded run on the water and water is fine. Timing has not been checked yet, but I doubt it is an issue, the advance pick up works fine and I can test it at idle by overriding it with my finger against the spring bias.
Fuel pump has not been checked yet, but I'm not losing prime at it picks up fuel from a tank 5' below the carby, blocked main jet,.....maybe, but it has a good inline filter, probably an easy enough check though so that will be the next place to check.
Anyone want to have a crack at internet engine diagnosis?
Thanks for reading this far and making it through the waffle.