See full version: 3 Reasons Boat Motors Lose Compression
Your engine won’t be starting with the plugs removed; but there will be fuel in the cylinder so be careful with any smoking or sparks that could ignite those gases. If your engine is one that is fuel-injected, this could be very dangerous vapor that gets into the cylinders unless your engine has a way to prevent that. here
Here are some of the major reasons that boat motors lose compression:
One thing that you can do to prevent this issue is to add a product that prevents this to your fuel or they also make products in spray form as well. If the carbon residue has hardened or is built up to an extreme, you may need to completely disassemble your engine and get it completely cleaned and rebuilt. [links]
You’ll need to use the starter or the pull cord through around five flywheel revolutions. You’ll know you’ve done this enough because the compression gauge needle is going to stop going higher. here
If you do want to check your engine when running warm, you simply need to start it up and let it run for awhile so you can get an accurate reading of what your engine performs like during the boat’s actual use. more
In a car engine, the intake and exhaust valves are typically located on top of the cylinder. The intake valve opens and lets fuel and air into the cylinder for combustion. After combustion, the exhaust valve opens to expel the exhaust gases. Bad valves are caused mainly by overheating. Overheating can heat the valves' stems to the point of warping. The warped valve stems do not allow the valves to seat properly, allowing gases to leak out. This results in compression loss. The exhaust valve fails most often because it is exposed to the hot exhaust gases, which reach up to 1,200 to 1,350 degrees. The heated valve breaks or warps, causing leaks in the cylinder and compression loss. Valves can also acquire carbon buildup. Carbon buildup usually occurs on the exhaust valve due to the continuous passing of burnt gases. These burnt gases foul the valve seat, preventing a good seal on the exhaust valve. Gases then escape, which in turn causes compression loss. more
Engine compression is measured in the cylinder, where the air/fuel mixture enters and is then ignited. The burning, expanding gases push the piston, translating that energy into forward movement. The ability to confine this explosion in the cylinder is measured and called compression. Any leak out of the cylinder is defined as compression loss, which can severely lower the power output of the engine. more
Pistons are housed in the cylinder. When combustion occurs, hot gases push the piston, which in turn operates a crank shaft. Piston holes lead to leakage and low compression. Overheating is one cause of piston holes. When overheating occurs, hot spots form. These hot spots eventually wear holes in the pistons. Piston leaks result in low compression because the gases do not build up in the combustion chamber. Bad spark plugs, inferior gas with low octane and defective fuel injectors can all trigger hot spots on the pistons and eventually lead to holes.
The cylinder head is located at the top of the engine block. It enables combustion by allowing fuel and air to pass into the cylinder and then escape after the mixture is burned. The cylinder head is bolted to the engine block with a gasket in between. If the gasket breaks down, a small hole develops between the cylinder head and the cylinder. This is known as head gasket failure. The leak in the gasket allows gases to escape and leads to compression loss and poor engine performance. The most common cause of head gasket failure is overheating. Extreme overheating leads to warping of the head or engine block, which then results in a breakdown of the gasket seal.
Re: Cavitation problem, O/B pontoon boat
Nav, <br /><br />The ventilation plate sits just about flush with the surface of the water while running---MAYBE an inch or so under, can't remember exactly.<br /><br />The motor is a 75ELPT---I am assuming that means "Extra Long, Power Trim", maybe I'm wrong on that. The motor was original equipment with the boat when new, and that's documented. <br /><br />I realize that new pontoon boats ship without motors, and perhaps dealers throw whatever motor they have a surplus of on the boat, but I find it hard to believe the boat has had this problem since new. In person, I'm a fair judge of character, and this fellow would have told me about it for sure, although he did say that his son did most of the driving while taking the grandchildren out on the lake, so perhaps it was always a problem, and the seller wasn't even aware of it. Maybe they just accepted the problem and didn't think about it.<br /><br />OTOH, this prop looks almost, if not brand new, so I'm wondering if it was a recent replacement.<br />And if so, what did it replace?<br /><br />But more importantly, what now?<br /><br />Dean
Re: Cavitation problem, O/B pontoon boat more
E = Electric Start<br />L = Long Shaft (20")<br />PT = Power Tilt/Trim<br /><br />Have you examined the weight distribution? (Wondering if it is possible that you have a bow-heavy situation thus lifting the stern)
I'm pretty sure weight isn't the problem. Shows the problem the same with two aboard seated in back, and six aboard distributed all around the boat.<br /><br />Yeah, it's a Merc part number on the prop. The motor is labeled "Force---by Mercury Marine", and the lower unit bears a very close resemblance to a Merc, might even be a Merc lower unit.<br /><br />AMD: in regards to the translation of "ELPT". <br />any ideal if they made an "Extra Long" (longer than 20") shaft model ?<br /><br />The idea of trying different props out is a great one, I'll start asking around my local marinas.<br /><br />Where should the AV plate be while running? more
Re: Cavitation problem, O/B pontoon boat
This will assist in diagnosing, what is going on inside of the engine.
Every car engine needs a timing belt or chain; to keep the camshaft in correlation with the crankshaft. When these parts fail the camshaft stops turning; which causes the intake and exhaust valve not to open and close. Without the camshaft rotating, the engine cannot make compression.
Valve keepers are two half moon pieces of metal that; lock into the valve retainer holding the valve in place. If these pieces become dislodged, they can fly out of the retainer. Consequently, allowing the valve to drop into the cylinder contacting the piston. [links]