Force won't pump water after 2000 rpm's

Last month I bought a 1998 Force 40 hp. It had been sitting for a while so I put in a new impeller, but when I put it in the river it stops spraying water at about 1900 rpm's. At idle it has a good spray coming from the exhaust port and it does good up to 1800 rpm's, at 1900 rpm's it gets a little faint and at 2000 rpm's it stops. Any ideas folks?<br /> Ralph

Re: Force won't pump water after 2000 rpm's

Welcome to iboats.<br />I would suspect a blockage in the coooling circuit somewhere. Salt water tends to crystalize inside, and corrosion can keep the thermostat from opening.<br />Now saying all that, it's important to know how the impeller werks. It is made of flexable neoprene, and the housing is not perfectly round. As it turns at low RPMs, the vanes of the impeller are in contact with the walls of the housing, which means the pump is acting like a 'positive-displacement' type of pump. As the RPMs go up, the vanes tuck-in, and the pump is now a "centrifugal" type. Posative displacement allows for higher pressure while maintaining a high volume (what goes in must come out).<br />But a centrifugal pump supplies the engine with a high volume STEADY pressure. If there is an obstruction in the block (IE, Pieces of old impeller vane caught in the exaust port or a stuck tstat), it can't supply suifficiant pressure to get past the ristriction.<br />First check the thermostat. Hell, I would just replace the puppy, as they are only about 9buks.<br />But by all means, do not operate this engine very long without it's thermostat, as it can warp the exaust port plates or head, causing major problems.

Re: Force won't pump water after 2000 rpm's

Thanks for the reply 12Footer, I tested the thermostat today. I hung it in a pot of water on the stove with a thermometer. It started to open at about 135 degrees. After that I put the garden hose to the port the thermostat came out of to try to back flush anything that might be in it. After that I hooked an extention pipe to the thermostat port and started the engine for a few seconds. It pumped water out the thermostat port so I put it all back together. I noticed there is a threaded and pluged hole on top of the power head leading into the water jacket right in front of the thermostat. If I can find a preasure gauge to fit this fitting, what kind of preasure should I be getting? I'll try to get to the river this week to see if there is any improvement. 2000 rpm's doesn't work in a barrel. I also did a compression test today. 142 psi in the lower cyl,151psi in the upper.<br /> Ralph