Looking at another thread here on iboats (https://forums.iboats.com/threads/3-5-mercury.777945/#post-5966759), and doing research online I have discovered that this motor has a de-compressor? Where? How to fix? Mine is very hard to turn over. I originally thought it was a valve timing issue, but the valves are driven by direct gear off the cam shaft. I took off the oil pan after draining the oil as it was due for an oil change anyway, and the cam shaft timing mark I found aligned with the gear shaft timing mark. The rocker arms and valves appear to be ok, nothing stuck. Using a feeler gauge the exhaust valve appears to be slightly tight at 0.1mm instead of between 0.11m and 0.19mm. I had actually thought it was much more out of spec days ago but I had mistakenly measured the gap. A second measurement showed me it was only slightly out of spec.
This all developed in front of my eyes one day while testing in a test tank about a week ago. I started, it was running at idle, in gear, purring along just fine. I throttled up a bit and it sputtered and died. I suspected the fuel pump, but wanted to open the gas cap first, but never got the chance. I could not restart. I pulled. And pulled. And pulled. And gave myself tendinitis. I also observed what appeared to be back firing or back-flow of air out of the carb while attempting to restart that I had never noticed before. Little puffs of air, sometimes sooty in appearance. I have no idea if that’s normal, but I doubt it. Eventually the motor quasi-hydro-locked on me and tipped over toward me quite a bit during one pull. Pulling was met with such resistance that I could not overcome sometimes. It would just jam. I removed the spark plug, which was also due for replacement anyway, and it was fairly black, but not encrusted, nor wet with anything. I cleaned it up a bit with a toothbrush. I have a new one but that one is still in the box. Ever since that afternoon, and it’s been over a week, I cannot turn the flywheel except with great difficulty on the compression stroke, unless I remove the spark plug. Without a spark plug, the flywheel and everything moves smoothly. I also think there is some new resistance due to my need to overcome a vacuum before the intake valve opens but it’s not as strong as the over compression.
I thought this motor had no cam shaft de-compression mechanism of any kind to assist starting. Looking at the service manual, I don’t see any reference to any kind of de-compression mechanism anywhere. I hope I have the correct manual: https://media1.svb-media.de/media/s...10-59-17_2cd641e2bd2fc2a699804065d133ada4.pdf
The valve timing is not adjustable. Nor is the ignition timing. It was possible that on that day I got too much fuel into the cylinder and that caused a quasi-hydro-locking situation, but it’s been many days since then, and many of those days I left the spark plug off the motor, so that gasoline is long gone. I am stumped why it’s so hard to pull or how to fix.
I have not adjusted anything yet. I have only drained the oil and removed the pan and since put it back, and peeked at the valve rockers, piston rods and cam shaft via the oil filling port. I did remove the head from engine and removed the pull starter so I could examine this indoors on a bench. I bought a feeler gauge and am looking into getting a torque wrench and bore scope to help me out here. I don’t think the flywheel is involved, but I haven’t remove it to check for shearing. Don’t see how that could be involved in over compression. I still suspect as I originally did that the valves are not opening and closing at the correct times unless there actually is some decompression mechanism that is failing me. But I know the valve timing is not adjustable, correct, and direct gear driven by the cam shaft. I also know hydro-locking is impossible at this point. I considered maybe there is carbon build up in the cylinder? But without a bore scope I cannot tell at present.
I should mention I had previously cleaned the carb and it is certainly possible I flooded the engine while trying to start it again in the test tank from all the pulling. But what is so strange is that it was working, then merely pulling ok but not starting, and then from one minute to the next, hard to pull at all.
And I just detached the fuel pump pulse hose from the intake manifold. But this does not help at all. I still have a lot of compression resistance. Covering the manifold intake while manually turning the flywheel I can usually feel vacuum on the intake stroke but instead I hear air being sucked in via the detached hose. Otherwise I left the fuel pump attached to the side of the motor, but minus the fuel tank obviously.
Any advice would be welcome. Thank you.
This all developed in front of my eyes one day while testing in a test tank about a week ago. I started, it was running at idle, in gear, purring along just fine. I throttled up a bit and it sputtered and died. I suspected the fuel pump, but wanted to open the gas cap first, but never got the chance. I could not restart. I pulled. And pulled. And pulled. And gave myself tendinitis. I also observed what appeared to be back firing or back-flow of air out of the carb while attempting to restart that I had never noticed before. Little puffs of air, sometimes sooty in appearance. I have no idea if that’s normal, but I doubt it. Eventually the motor quasi-hydro-locked on me and tipped over toward me quite a bit during one pull. Pulling was met with such resistance that I could not overcome sometimes. It would just jam. I removed the spark plug, which was also due for replacement anyway, and it was fairly black, but not encrusted, nor wet with anything. I cleaned it up a bit with a toothbrush. I have a new one but that one is still in the box. Ever since that afternoon, and it’s been over a week, I cannot turn the flywheel except with great difficulty on the compression stroke, unless I remove the spark plug. Without a spark plug, the flywheel and everything moves smoothly. I also think there is some new resistance due to my need to overcome a vacuum before the intake valve opens but it’s not as strong as the over compression.
I thought this motor had no cam shaft de-compression mechanism of any kind to assist starting. Looking at the service manual, I don’t see any reference to any kind of de-compression mechanism anywhere. I hope I have the correct manual: https://media1.svb-media.de/media/s...10-59-17_2cd641e2bd2fc2a699804065d133ada4.pdf
The valve timing is not adjustable. Nor is the ignition timing. It was possible that on that day I got too much fuel into the cylinder and that caused a quasi-hydro-locking situation, but it’s been many days since then, and many of those days I left the spark plug off the motor, so that gasoline is long gone. I am stumped why it’s so hard to pull or how to fix.
I have not adjusted anything yet. I have only drained the oil and removed the pan and since put it back, and peeked at the valve rockers, piston rods and cam shaft via the oil filling port. I did remove the head from engine and removed the pull starter so I could examine this indoors on a bench. I bought a feeler gauge and am looking into getting a torque wrench and bore scope to help me out here. I don’t think the flywheel is involved, but I haven’t remove it to check for shearing. Don’t see how that could be involved in over compression. I still suspect as I originally did that the valves are not opening and closing at the correct times unless there actually is some decompression mechanism that is failing me. But I know the valve timing is not adjustable, correct, and direct gear driven by the cam shaft. I also know hydro-locking is impossible at this point. I considered maybe there is carbon build up in the cylinder? But without a bore scope I cannot tell at present.
I should mention I had previously cleaned the carb and it is certainly possible I flooded the engine while trying to start it again in the test tank from all the pulling. But what is so strange is that it was working, then merely pulling ok but not starting, and then from one minute to the next, hard to pull at all.
And I just detached the fuel pump pulse hose from the intake manifold. But this does not help at all. I still have a lot of compression resistance. Covering the manifold intake while manually turning the flywheel I can usually feel vacuum on the intake stroke but instead I hear air being sucked in via the detached hose. Otherwise I left the fuel pump attached to the side of the motor, but minus the fuel tank obviously.
Any advice would be welcome. Thank you.
