OMC 3 Liter Engine Knocking sound coming amp Fuel issue

Guys my original 1987 OMC engi

Guys my original 1987 OMC engine block got cracked.I got older 1980 OMC 3 liter.Got engine block swapped.Engine is running good.When i am running at slow speed it is ok.But when i am trying to run faster like 10MPH+ i can hear some knocking or clicking sound coming from engine.I dont know if it is from piston or carburator or from somewhere else.Does any1 have any idea.
Also engine for running fine for few hour.But after that it suddenly started slowing .I dont really dont know how to explain.Like when ur about to to finish gas in car it acts crazy.Same things started in my boat and it stopped some times.When i give some choke and start it starts and after some time it runns wiredly and stops.My boat fuel tank was less then quarter left.Is it possible some dirt went in carburator and that is why it is doing like thing.Also i opened fuel pump uper cover for fuel filter and i dont see any fuel fiter installed.so is fuel filter necessary.Please let me know.thank you

Also can any1 tell me what type of engine oil it need.The manual for 1987 OMC says SAE-30.But mechanic told me SAE 30 dosent have detergent & gave me Merucry SAE 25W40 oil.So is it ok.let me know.thx

"My motor runs on straight 30

"My motor runs on straight 30 weight, and that is what is called for in these older boats.

Do a compression test on a warm motor.

Then do a vacuum test.

Heres the link.

http://www.secondchancegarage.com/public/186.cfm"

"Guys the koncking sound comin

"Guys the koncking sound coming is very minimum.What i did is changed oil,filled with premium has and set the timing to 4degree according to manual.
Now engine is running good on slower speed.when i try to go faster like more then 15mph it goes but after sometime suddenlt it loses power and boat slows down.when i full throttle again it goes and does same thing after some time.so can any1 tell me what could be the problem.thank you
Edit/Delete Message Reply With Quote"

"For the slowing down problem

"For the slowing down problem if it feels like fuel starvation then check your fuel tank vent line and the end for being plugged, this can cause a vacuum in the tank at higher speeds, a quick way to check is when you have the problem crack open the fuel cap and listen for a whoosh!
You may or may not have a water/fuel separator (spin on looks like an oil filter) but you should have a filter where the fuel line attaches to the card."