volvo 280 knocks

i have a volvo 280 outdrive behind a 305 v8 gm engine. this boat sat with little use for 2 years. the engine runs ok, poor idle, very ruff. the outdrive has a knock in the very top or the input to the top, cant tell which at idle. when the rpm is increased to 800 or 900 rpm the knock is not there. the boat seems to run fine forward and reverse. i get 26 mph at 3400 rpm, and the boat max is 4400 rpm when running and trimmed. what is this knock at idle. is it because of the ruff idle, or do i have a problem. is there a way for a marginal mechanic to check this out. last, is it safe to use the boat.

 

This would get more answers in the powerboat threads. A rough idle will make the gear knock. That is because they get loaded and unloaded as the RPMs change rapidly. Worn u-joints can do that too. A mechanic's stethoscope can help. Also, you can use a 1" hose. Put one end in your ear and move the other around until the noise gets loudest.

 

thanks for the reply, i have purchased a service manual for the outdrive. it seem i just have to pull the upper gear case to look at the u joints. do u know if there is a better way or???? i think i need to find the knock, we travel by boat sometimes 50 mi from anything.

 

Taking them out is the best way. They need to be removed and lubed anyway. If it hasn't been done in a long time, grease may be enough to quiet them down.

 

after removal, and inspection the ujoints are tight. ther is some play between the u joint and the output of the gear box, somewhere inside the gears. as i rotate the ujoint back and forth , i can move the ujoint about 1/16 inch before the gear out travels. in both directions. is this normal? also there was about 1/2 pint oil in the bellows. probably a bad yoke out seal. seals are easy. the key is how much play is allowable in the gear asssy. thanks again for your time. jim h

 

All gears have some backlash. To set them you need a set of fixtures that is quite expensive. Excesive gear clearance wouldn't cause knocking. The bellows shouldn't have any gear oil, the seal is leaking. Now that it is out, also check the splines for wear and the coupler. A worn input shaft bearing can also make noise at idle.