1966 duo porblems

[Hites87]

Hi I'm new to the forum and new at boating ... I have a 1966 duo fiberglass boat with a 1966 merc 500 on the back the issue I'm having is that I'm topping out at 21 mph (gps) I do not have a tach but I can tell u that's it's screaming at that speed I have a 3 blade prop on it now and it came with a 2 blade but I don't want the vibration at slower speed so I haven't changed the prop but I thought it would move a little quicker than wat I'm doing if n e one knows n e thing I could change or n e ideas please let me know thanks
Alex

 

[scoutabout]

Re: 1966 duo porblems

Welcome Alex!

Sounds like a cool boat. Got any pics?

Your problem could be coming from a couple of areas. A boat that age could have hundreds of pounds of wet substructure weighing it down under the floor. Or a deformed hull that isn't riding right. An engine that age could have issues from a slipping prop to dirty carbs to iffy ignition. Although it sounds like it's screaming along you should get a tach hooked up and research its WOT rpm range to see if you are making max power. Could be an incorrectly pitched prop too. What pitch are you running now? (Should be a two digit number stamped on the hubs somewhere of both props you have).

Can you tell us a bit more about the problem - like if it's a new problem or has it always run like this as long as you've owned it? Also, how big is this boat that you are pushing with fifty potentially tired horses?

Don't worry - there's enough talent around here that someone will help figure it out eventually.

 

[Hites87]

Re: 1966 duo porblems

Ok I'll get a tach tmw hopefully but I can tell u that the boat has done it since I got it and I can tell u that every cylinder has 128-130 lbs of compression and the 2 blade rotor has these # on it 48 32180 11

 

[Hites87]

Re: 1966 duo porblems

Sorry it's 48 32190 11 and it's a 14 ft boat

 

[scoutabout]

Re: 1966 duo porblems

K good start. What's the pitch of the prop that's on it?

 

[Hites87]

Re: 1966 duo porblems

The one I'm currently running is a 48 38098 11

 

[Hites87]

Re: 1966 duo porblems

I was told by my local marina to check and see if I'm firing on all 4 cylinders but I know I am I can tell I am a motorcycle mechanic I know when an engine is running like crap and this one is not ... Not saying I am god lol by n e means but I have a very good knowledge of engines ... So I went thru and pulled one spark plug wire at a time and they all had the same reaction so I would assume I'm running on all four