RPM's

[air0msw@yahoo.com]

See anyone can help me out with this. I have a 2001 Glastron GS249 (25') with a Volvo SX outdrive with a 5.7 GS motor with a carb. I can't get it above 4000 RPM. Verified it is correct reading. Boat runs fine, no bogging down, hesitation, misses. Had a 14 X 17P aluminum prop. Went to a 17P S/S but nothing changed. After further reading, found I guess I should have went down to a 15P. Had the S/S re-pitched to a 15P to see if it would do anything before buying a new prop. With it on, didn't see any difference in speed or RPM. Couple of people I have talked to with the same boat, claims they get 47 or 48 MPH. Best I can do is 37 MPH. I do have bottom paint on it but I didn't think it would make a 10 MPH difference, I figured about a 3 to 4 MPH loss. Anyone have any ideas? Any chance the cable is out of adjustment not letting it open up to the normal WOT RPM which I think should be 4600 - 4700 RPM? Or is it something else? It revs up just fine. This one has me scratching my head. Appreciate any input on this.

 

[Blk-n-Blu]

Re: RPM's

Check this, from the" Adults Only Stickey" Thanks to Don S:
http://forums.iboats.com/showthread.php?t=295070

 

[LAC_STS]

Re: RPM's

I have the same problem and have ran through most of the list linked above.


To see if the throttle is working right, with the engine off take the flame arrestor off. Put the throttle to WOT and look into the carb. The it should be all the way open. You may have to use your finger to push open the air vents on the secondaries to be able to see if they are open.

The air butterflies on some carbs dont open unless there is a vacuum (under load).


You can check this and if they are not opening right try adjusting your throttle cable.

 

[John_S]

Re: RPM's

See anyone can help me out with this. I have a 2001 Glastron GS249 (25') with a Volvo SX outdrive with a 5.7 GS motor with a carb. I can't get it above 4000 RPM. Verified it is correct reading. Boat runs fine, no bogging down, hesitation, misses. Had a 14 X 17P aluminum prop. Went to a 17P S/S but nothing changed. After further reading, found I guess I should have went down to a 15P. Had the S/S re-pitched to a 15P to see if it would do anything before buying a new prop. With it on, didn't see any difference in speed or RPM. Couple of people I have talked to with the same boat, claims they get 47 or 48 MPH. Best I can do is 37 MPH. I do have bottom paint on it but I didn't think it would make a 10 MPH difference, I figured about a 3 to 4 MPH loss. Anyone have any ideas? Any chance the cable is out of adjustment not letting it open up to the normal WOT RPM which I think should be 4600 - 4700 RPM? Or is it something else? It revs up just fine. This one has me scratching my head. Appreciate any input on this.

I'd talk to the people that are getting much better performance, and see what exact props they are running, drive ratio, trim adjustment, load, and suspect tabs for that size boat. The prop diameter seems small for a typical boat that size. Bow lift, may play an important factor.

Outside of dragging your tabs, still think that is a large perf difference.

 

[air0msw@yahoo.com]

Re: RPM's

Thanks for the link. I have checked or done everything on Don's list with the exception of #3,11 and 15. So I will try this next weekend when I am down at the lake. I did check the other boats like mine and they are running 17P. I thought for sure re-pitching the prop would bring it up some before I went out and bought a new 15P S/S prop.