Need suggestions to tweak performance

[alden135]

OK, I am finally (hopefully) past all the minor repair issues with my new used boat . I'd now like to start working on performance issues. The boat is a 1990 sunbird cuddy 204 w/ a 120 johnson (j120tles). I seem to only be getting 25 mph and 3000rpm wot with the current 19 pitch prop.<br />The engine starts by merely bumping the key, idles great powers up smoothly, and gets on plane as fast as I would expect a 120 to do so. The throttle plates are opening fully. I have replaced the following:<br /><br />gearcase,timer base,plugs,coils,power pack,plug wires,trim motor also did link/sync.<br />I suspect that because it's only a 120 that I'll only go so fast but am concerned about the rpm's only being 3k. I'm pleased overall regarding the smoothness of operation but would like to get the rpm's up somehow so I don't ruin the motor. When at wot it seems like it wants to go but can't for some reason. I suspect this may be some sort of prop or set up issue and could use some help.<br />Thanks,<br />John

 

[JB]

Re: Need suggestions to tweak performance

Hi, Alden.<br /><br />Time to look at rigging and prop.<br /><br />19" sounds like too much pitch for your boat. You are waaaay below the power band in rpm. Check out the FAQ on prop selection. My guess is that you will end up with a 15" SS.<br /><br />Just guessing your boat at about 2400# on the water, that engine should be able to put you at around 44mph at 6,000rpm ideally propped and trimmed.<br /><br />Good luck.

 

[alden135]

Re: Need suggestions to tweak performance

Thank you JB. Would it matter if I bought an aluminum prop rather than a ss prop so long as it was a 15 pitch? The ss are very nice but I'm a little concerned with damaging my gearcase should I hit something. I did read the FAQ.

 

[JB]

Re: Need suggestions to tweak performance

SS props have rubber hubs, Alden. They present no more danger to your drive train than an AL prop (unless you are talking to an AL prop salesman).<br /><br />SS will give you better performance. . . less drag and often cupped for slightly increased effective pitch at high rpm.<br /><br />Follow the advice from the FAQ in selecting the ideal pitch and rigging your engine to use it best.<br /><br />If Dhadley comes along he will probably have even better advice than I can give you.<br /><br />Good luck.

 

[alden135]

Re: Need suggestions to tweak performance

Thanks for the info JB.

 

[Solittle]

Re: Need suggestions to tweak performance

Where is the cavitation plate relative to the bottom of the hull?<br /><br />Do you have assess to another tach to try? 3K seems way to low.<br /><br />As to the prop I'm with JB on the SS. See if you can find a prop shop that will let you "try before you buy."

 

[Solittle]

Re: Need suggestions to tweak performance

Where is the cavitation plate relative to the bottom of the hull?<br /><br />Do you have assess to another tach to try? 3K seems way to low.<br /><br />As to the prop I'm with JB on the SS. See if you can find a prop shop that will let you "try before you buy."

 

[alden135]

Re: Need suggestions to tweak performance

I did check the cav plate when I bought the boat per a post I had read. I don't have access to another tach. I think the 3k jives with the speed I'm getting.

 

[alden135]

Re: Need suggestions to tweak performance

]web page[/URL] <br /><br />This is the boat before all the work. The gearcase has been replaced and no longer has a fin