Low RPM on New 9.9 4S Bigfoot

Author Message   Tom Buchanan Posted on Monday, April 08, 2002 - 03:01 am:    I just purchased a new 16.5 V Bottom with a Merc 9.9 4S Bigfoot. I went with this motor because I mostly fish 9.9 restricted lakes and was told that the Bigfoot would give me a little better performance with a heavier boat (I can't afford to have 2 motors, one being a kicker). On average I would guess that the total weight is around 1400 lbs loaded. The problem is that I am only getting around 2200 RPM. This occurs at about 2/3 throttle. The motor runs good, no misses or hiccups and the remaining throttle doesn't seem to make a difference. I have read several places that if you can't achieve recommended operating speed at full throttle (4500 - 5500 RPM) you could damage the motor. Top speed at this RPM is aroun 8 MPH. I know that this motor will not get the boat on plane but I was hoping to get a little better performance. I would also like to see the RPM's up where they belong. Any help or suggestions would be greatly appreciated!! Thanks, Tom   Jon Posted on Monday, April 08, 2002 - 12:49 pm:    Tom, I have no idea what type of performance you should expect out of a 9.9 4-stroke on that boat, but what you are getting for a top speed does not sound right. You should definitly consult whoever sold that setup to you; 2200 RPM's is really not much more than idle. Normally, an engine that runs at a moderatly low RPM (say 4000 at WOT when the operating range is 4500-5000) just requires a lower pitched prop. The pitch is the way the engine is "geared", lower pitch is a lower "gear", higher pitch is a higher "gear". The pitch of the prop required will change when you move an engine from boat to boat. I don't know what your current situation is with warranty and the proximity of your dealer etc., but first I would at least call there. If your only choice is to deal with this yourself, first be sure that engine has the correct type and level of engine oil (if it is indeed a 4-stroke). Figure out what pitch and size that prop is, there should be some numbers on it (it should read somewhere BLADES SIZE X PITCH, ie 3 13 17 is 3 blades 13 inch 17 pitch), and then go to the michigan wheel homepage (www.miwheel.com). They have a prop calculator there; it takes your boat and engine and spits back a recomended prop. This should be somewhat close to what you've got. If it says you need a 13 pitch and you have a 23 pitch, that could be the problem. If it says you need a 13 pitch and you have a 17, it's probably not the problem. The next thing I would look into is if there is some type of shallow water drive or RPM limiter; my older merc 115 has some thing so that it won't exceed around 2000 RPM's when it is trimmed up past such a point, you could either be setting some system off, or it could be malfunctioning. Next, look into the way your throttle is working; be sure that as you throttle it, whatever linkage is under the cowl is turning in unison all the way through the motion and that the carb is also being throttled along with it. You may find a problem there. If it's none of the aboved mentioned problems my best guess would be there is some fuel problem; possibly a bad fuel/air mix caused by a problem in the fuel supply. Check to be sure that you have no restrictions in the fuel supply, filters etc. and that the tank is vented correctly. If that's OK you could be looking at a bad fuel/air adjustment and/or a clogged carb (a clogged carb would be odd though if the engine is brand new and was just recently run for the first time). Good luck, Jon   Bobz (Bobz) Posted on Wednesday, April 10, 2002 - 06:18 pm:    "I know that this motor will not get the boat on plane but I was hoping to get a little better performance." If your correct with that statement, and I believe you are, you are probably near maximum speed at 8 MPH. If you are not planing the boat then you have a 'displacement hull' condition and the HP to speed rules are totally different. Second point, you are probably overloading the motor and likely need to downsize the diameter and pitch on the prop. See if you can borrow a smaller diameter and smaller pitch prop from the dealer for a trial. If your not going to get it up on plane you at least want to get the RPMs up to the recommended range at wide open throttle.

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