1994 75hp 2 stroke 3 cyl Mariner motors lacking speed

Hi I have 2 x 1994 75hp 2 stroke 3 cyl Mariner motors on a 23ft double hull boat ( cat) which was purchased a year and a half ago. Props are 3 blade 17 inch props. When tested upon purchase, I was then getting a top speed of 48km/hr with 2 people on board. This past year an a half I have changed the stators and rectifiers on both motors . Serviced both motors last month and the compression tests were normal. Thereafter the boat was sluggish and would not get on plane. A trigger on 1 motor was diagnosed as the problem. Yesterday, the trigger as well as all control cables were replaced with new ones. I took the boat out for a test this evening with 5 people on board and it ran very well however, I am now only getting a top speed of 27km/h. I use to get better top end speed before even with 10 people on board ( If I recall correctly , it use to be just over 30 km/h). As rev counters are just being installed , I cannot say what RPM the motors were running at Please bear in mind that I am not technically minded at all and totally dependent on reputable boat technicians for advice , service and repairs. Please overlook any shortcomings in my attempt to explain. I would appreciate any advice that can help diagnose any possible problems or assist in getting these motors to perform as before. Thanking you in advance. Regards

Compression test again.--------------Still running with " automatic " oil injection ??--------------Remove intake cover and observe fuel coming up main fuel nozzle ( strong flashlight required ).-------------Put timing light on each sparkplug wire and check verify proper spark on all 6

Racerone I appreciate and thank you for your prompt reply. I've voluntarily removed the " automatic" oil injection on both motors. I add oil to petrol by hand on a 1:50 ratio. I will request the mechanic to do as per your suggestion and post at the soonest. Regards

If the boat sits in the water, you could have growth on the hull and props. That'll kill the speed a bunch.

Jeff

Thanks Jeff The boat does sit in a mooring and the hull does have some growth , but not much. Props are clean. However , I will have the hull cleaned out this week and then test the boat once again. Racerone Met and informed the mechanic of your suggestions. The mechanic is a technician at one of South Africa's most reputable boat building companies and he is responsible for all installations and servicing of new as well as older motors for this company. So I trust his judgment and have found him to be honest. According to him, he's done the compression test twice , both showing normal results. He also says that there is proper sparks on all 6 cylinders and there is no problem with fuel coming up main nozzle. He's taken the boat on a test run and has run both the motors together as well as individually and is happy that both perform equally well but is at a loss as to why the boat has lost so much of speed. He even inspected the propellers thinking that they may have been damaged but they turned out to be fine. Basically , he feels that there is nothing further that he can do with the motors to increase speeds to previous levels. I'm now left with Jeffs' suggestion of cleaning the hull and hope that , in doing so , it would rectify the problem. Once again , I thank you and Jeff for your kind assistance. Regards

Maybe check your hull see if there are any leaks or water in the buoyancy tanks