[wilk]

I have a 1987 Yamaha 40 3-cylinder w/aftermarket jet. I've had it out twice, both low speed outings in cool weather. I've noticed very little(if any) tell-tale stream, but the engine doesn't seem to be overheating. We keep a close watch on the tell-tale due to the jet plugging up with trash in shallow water. I'm new to jets and Yamahas(mechanically speaking). Could someone give me a brief overview of the cooling system with the jet installed(ex. where the water intake is, where water goes next, and why I might not be getting a good tell-tale stream). I did a search, but it didn't turn up much other than background info on the tell-tale. Any help you can give would be appreciated. Thanks in advance.

[rodbolt]

Hello the jet unit has a standad type insert and plate arrangement almost identical to an outboard. the jet pump allows water to the engine seawater pump. from there it travels up the tube to the power head just as any outboard. check the tell tale hose and nipples for restrictions. low water pressure could mean a removed or defective t-sta a removed or defective pressure valve or the seawater pump is melted. this happens a lot on jets. takes 30 seconds of dry running to toast the seawater pump impeller.most jet failures we see here are due to slow speed/shallow water operation. either the jet lifts up a rock or other unpassable obstruction or it sucks up so much sand the engine cooling system gets clogged. I reccomend jet engines to have the head removed periodically and the passages cleaned if run a lot in shallow water. most over heated jets are due to melted pumps or excessive wear on the jet impeller causing cavitation and feeding the seawater pump a mixture of air and water causing the seawater pump impeller to cavitate and the engine gets hot. good luck and keep posting, a jet has its own peculiar problems that props dont. be aware of them or life will be sad.

[wilk]

Thanks for the reply. I'll check the tell-tale passages, and drop the jet for a good cleaning and learning session. I'm sure I'll have more question later. Thanks again.

[pine island fred]

let me share a small bit of experience here. while you have the motor apart you might be tempted to remove the 12 or 14 bolts holding down the exhaust cooling cover to clean under there. DO NOT DO IT. although the motor is probably a fresh water, one or two of the bolts will snap, then you have more problems than you need. regards fred.