1999 Yamaha F100 4 stroke Low compression and hard knocking.

rippinthewaves

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Aug 21, 2012
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The first time I took it out the engine sounded a solid alarm, knocked a little, and shut down. I inspected the impeller and replaced the corroded thermostat, suspecting an overheat and took it out again. Bogged, stuttered, slight knock as soon as I gave it WOT. Went home and found the negative cable was loose. Took it out a third time, got on plane for 10seconds, bogged, stuttered, alarmed, knocked, and shut down. Fired back up and had a consitent knock.
After reading up in the forums I performed a comression test with the engine at operating temperature. The cylinders tested the following; #1-100psi, #2- 129, #3-135, #4-144. After spraying some motor oil in cylinder number one, the psi jumped to 150. From what I have read this is indicative of failed piston rings. :facepalm: With a long handle screwdriver I can clearly hear the knocking toward the #1 piston head/rear engine.
Before I order parts and disassemble the engine, have I missed anything? Should I perform a leak down compression test to be sure? Could the carbureator rebuild be the cause of the failed rings? Could bad gas be the cause of the failed rings?
Thank you in advance for the help. I learned plenty of valubale knowledge by reading these forums for the last few weeks. Very enthused first time poster.
 

Capt Ken

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Jul 30, 2002
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Re: 1999 Yamaha F100 4 stroke Low compression and hard knocking.

You mentioned bad gas. On the inline fours, they do not have a knock sensor and when you feed it low octane fuel, predetionation will cause piston damage. I've been though a few due to this. I had to question Yamaha as to the damage I was finding and was told that was happening to many that was trying to run old fuel. Boats with white tanks that are not UV resistant are having the same problem.
 

Capt Ken

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Re: 1999 Yamaha F100 4 stroke Low compression and hard knocking.

BTY, you disassemble the engine then order parts. Not order parts then disassemble.
 

rippinthewaves

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Aug 21, 2012
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Re: 1999 Yamaha F100 4 stroke Low compression and hard knocking.

Thank for the quick response.
The boat originally sat for a while because of bad gas and clogged inline filters. Engine would not run correctly until the small red filters were pulled out of the fuel lines. Tink inch long screen filter imbeded in the fuel lines. After that the engine was back. We cleaned the inboard full tank, changed the fuel line running to the engine, and replaced the external water/fuel separator in June. Since then it has been ethonal free gas. The boat ran well twice and then I took it out and broke down. The gas in the tanks have been there two months.
My first request from grandpa is the service manual. Then I will diassmble and as you said order the parts.
 

99yam40

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Re: 1999 Yamaha F100 4 stroke Low compression and hard knocking.

knocking maybe bearings, and alarm may have been oil pressure not overheat.

You really need to pay attention to gauges and find out what alarm comes in.

Removing filters just lets trash into other places that plug up
 

rippinthewaves

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Re: 1999 Yamaha F100 4 stroke Low compression and hard knocking.

Your points are valid and pertinent. Unfortunately, the gauges have not worked in several years. It has annunciator lights for temp and oil pressure. I never made a point to hook them up. I was focused on bilge and light wiring. I figured if the oil was topped off and the pisser wasn't scalding hot, I would be good to go.
In hindsight, not replacing the screen filters may have been a bad idea.
 

99yam40

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Re: 1999 Yamaha F100 4 stroke Low compression and hard knocking.

always best to make sure everything is working like they designed it to, otherwise you are running blind with no indicators.

Tell tail is just an indicator that pump is pumping water, will not give indication of motor getting hot and oil level does not mean your oil pump is putting out the proper pressure
 

Silvertip

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Sep 22, 2003
Messages
28,758
Re: 1999 Yamaha F100 4 stroke Low compression and hard knocking.

The two most important alarms on an engine are temperature and oil pressure yet you were worried about a bilge pump and light wiring. That may have cost you an expensive engine overhaul. You also shouldn't order any parts even after you have the engine apart since a machine shop will need to mic the bearing journals and cylinder bores to see how much damage has been done.
 

rippinthewaves

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Aug 21, 2012
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Re: 1999 Yamaha F100 4 stroke Low compression and hard knocking.

I do not suspect a catastophic oil pump failure. Oil has been changed religiously and I would think to hear a low pressure alarm at lower rpm's. The engine ran good for 3-4 hours before first breakdown. There was quite a bit of corrosion inside the water jacket plate but i don't suspect it was poor circulation of cooling water. I do suspect that not syncronizing the carbureators after two cleanings may be the culprit for a burnt out cylinder. Lean fuel/air mixture would certainly cause a hot spot and predetonation. Could I be hearing a broken lifter or spring in the valve cover? Tap, tap, tap.
I have hole gauges and micrometers. The manual should have specs for the crankcase, am I right?
 
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