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  #16  
Old 09-05-2004, 09:38 PM
jfc52 jfc52 is offline
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Thanks Firestorm and others for all the help and suggestions. Will check the compression and carb to make sure the other two barrels are opening. Sounds like the prop shop recommendation is about right for the prop. Need to find out why the rpm's are low.
Thanks again.
Jeff
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  #17  
Old 09-07-2004, 08:08 PM
jfc52 jfc52 is offline
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Thanks Firestorm and all others for all the info and suggestions. Will check compression and carb to see if other two barrels are opening. Sounds like the prop shop recommendation is about right and maybe now it is the motor not getting the rpm's. Will keep you posted what happens.
Thanks
Jeff
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  #18  
Old 09-15-2004, 05:13 PM
jfc52 jfc52 is offline
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Checked the motor this past weekend. Compression is 130-135 on all cylinders. The four barrel is opening up properly. The motor revs fine in neutral. Am I overpropped? Should I go to a 14" x something to get the rpm's up?
Jeff
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  #19  
Old 09-16-2004, 12:33 AM
firestorm firestorm is offline
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Glad to hear compression and carb are ok You still are not making full rated power if you were your rpms would be right I still wonder about the cam wear and also if it is a true marine engine not a car engine with marine parts on it. I am shooting from the hip here but a car engine would give you about 165 to 180 hp which I think would land you about where you are at on rpms with the 15x14 prop other possibability is a marine engine rebuilt with automotive parts cam ect... running the wrong cam would drop your power quite a bit and kill your rpm as the power curve for a chevy is not where near where you want a marine engine. THe average 4 door gm is planned to run about 160 to 200 hp at about 2500 - 3500 rpms for a 100,000 miles and seldom ever goes above 4000 rpm many were geared to run even lower rpms in high gear so the cam is not ground to give high lift or duration but is set to give best pulling power at the 2500 - 3500 rpm range. one other thing to think about is if the carb is jetted to run the rpms you need you may check with the people who built the carb and see what it is set for it should be jetted for at least 600 cfm. A simple carb formula is cubic inch displacement x desired rpm / 3450 plus 10% ie 350cuin x 5200 rpm = 1820000/3450=527.536 cfm +10%= 580 cfm carb. Also Mr Gasket offers a program called desktop dyno that is suprizingly accurate and it allows you to build mock engines and dyno test them we ran a mock up of my brother in laws 383 small block before he took it to the dyno and came up with 807 hp it dynoed at 812 so the program is pretty close to real numbers. However it cannot take wrong parts info and poor tunning into consideration it uses ideal correct settings. Hope you get it figured out good luck.
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  #20  
Old 09-17-2004, 07:18 PM
jfc52 jfc52 is offline
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Thanks again for the info and advice. I am convinced the motor is a marine engine and is running as well as it can. Would going to a smaller prop (14") give me higher rpm's? Would I go to a 14x15 or higher pitch? Would a higher pitch(17) negate the advantage of smaller diameter? What I really need is a try it before you buy it program.
Jeff
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  #21  
Old 09-17-2004, 09:09 PM
firestorm firestorm is offline
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OK I still beleive power can be improved but that can come later try this to get a prop recomendation. http://www.ballisticprops.com/MIWheel/propit/main.htm
This is a online prop calculator and if you give them the correct info they will give you the right prop for the job.
It is not the fastest service in the world may take them a week or so to get back with a answer but you will get one. Most of my posts have centered on the power issue as a whole it takes a certain amount of power to move a boat thru water at speed. In rereading your replys to me and others I noticed that in the 2 props you have tried the rpms stayed the same only the speed changed this again makes me wonder about the engine cam and possably the igniction system as a curiousity what does the engine free rev to in neutral? If by some strange deal the engine has been limited in rpm due to a limiter or by malfunction the you may be able to go to a larger pitch prop and gain more speed. I say this because usually a minor prop change has more effect on the full throttle rpm than the boat speed and you adjust prop size to get enough load on the engine to keep rpms to a reasonable level. Simply put a smaller prop on most setups raises engine rpm but boat speed stays about the same just has faster acceleration and runs higher rpm at top speed. A higher pitch prop normally does the oppisite boat is slower to accellerate and engine labors at a lower rpm to make speed. A friend of mine had a master craft ski boat that had a rev limiter to allow them to use a lower pitch prop to get out of the hole and limited engine rpm to prevent over reving. I wish I had kept the 14.5x19 prop I had cause I would loan it to you to try but I threw it away a few years ago when I decided I was not very interested in boating at the time. go try the calculator I gave you and see what they say.
Good Luck
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