TOPIC: New Engine

Took off the old 1978 Merc. 70hp Problem today with porposing,it got worse over 4ooo rpms Runs ok at 3000-3500 rpms Have to watch it . But this didn't happen with the old Merc 70 hp My neighbor said in the old days, they would run a bead of silicone on the bottom edge of the boat to cause a turbulance?? Whos knows ,mabey I'll try it

don't put anything on the bottom try raising that motor up a few inches. You have plenty of room to do so and you cavitation plate is too far below the transom. You could go 3rd or even 4th hole from the looks of it.

I'm betting the new motor weighs a bit more than the old one. I'd try moving some stuff around in the boat - maybe tuck a gas tank up under the bow? Too much weight in the ass-end will make it perform erratically. Can I assume you've played around a bit with the trim angle?

Ok I looked again, the cavitation plate is only down 1 " but I can trim it so it's almost level, I don't know if raising the motor 1" will improve things I'm gonna move stuff around ..put the gas tank up front . I was going to try and put 2 50 lb. barbell plates under the bow .. to see if I can keep the bow down This new engine weighs 247 lbs

Moving existing weight is much preferable to adding more weight...

you can try shifting weight around but I do recommend moving the motor UP. It will make a diff going up 2 to 3 inches.
Don't put things like weights in the bow unless you plan on securing them down. You do not want anything coming at you should you hit a large wake or heavy seas.
You cav plate needs to be at minimum equal to the bottom of the boat not below it and you have room to go up without adding low water pick up or changing your prop.
Just my friendly thoughts is all.

I agree with raising the motor. Some boat/motor combos simply have a weight/speed "zone" that they porpoise at, and adding speed to "drive through" that zone does work. You may be on the hook for a new stainless prop about 2" more pitch though. And, the extra speed may add smile wrinkles to your face.

Thanks for all the help I raised it up 1 notch, or 1 " I was gonna go 2 notches ..but the new engines don't have the 2 big screw bolts that tighten it to the transom they have 2 little tangs hanging down,so moving it up 1 notch or 1" I didn't want to push it up too high . But the cavitation plate is level with the bottom of the boat I'll see if 1 " makes a difference.. Update today ...I called Mercury product support He said don't worry about the tabs not touching the transom Just torque the mounting bolts ,thats the mail thing I was worried about. As soon as the sun comes out we'll put it in the water !

Well ...it was a little better about 3-3600 rpm it just starts porposing again.. but not as bad My dealer said he had set it up for a 20" transom but he had the cavitation plate 2 " below the boat he said it would go ot to 5500-6000- rpm . Thats why I raised it uo 1 notch,or 1 inch, All I can get out of it is 4000 rpm Mabey I need a smaller prop ??

DEFINITELY a smaller prop!!!!!!!! Too little RPM's. What is the pitch of your prop?

Hi,
You are receiving some good advice.
Each hole yields 3/4 inch change in height. If you were 2 inch below, you are still down too much. Raising-up two more hole would yield approximately 1/4 inch above.
Although your propeller pitch may be too high (assuming that you are running wide-open and trimmed-out with the rpms you state), I would raise the motor first before changing propellers to get a better understanding of what change may be needed. The motor should have not been mounted this low at the dealer.

Best Regards,
Joe
www.fergusonpoolemarine.com
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The prop is 12-22- 1 1.83 gear ratio .. I told the dealer he mounted it way too low he said ....Well You have a 20' transom.. I mounted it for a 20" transom .. what the hell does that mean ?? So if I had a 22 inch transom.. he would have mounted it for a 22 inch transom .. I think he's mad at me cause I told him the cavatation plate was 2 " below the boat .. The I get the answer.. well I mounted it for a 20" transom!!! Doesn't anybody care about what their doing today

sounds like an a-hole to me tony. ron

Ron, I can fix almost anything from lawn mowers to airplane engines, but Marine engines is newer to me , I have to experiment with my own stuff, like on the job training ,If I have to wait for other people to help me,I'll be waiting a long time ! P.S. I TOOK MY PROP OFF THE 78 70 hp MERC AND PUT IT ON THIS MOTOR, IT'S THE NEXT SIZE DOWN .. WELL GIVE IT TRY AND SEE WHAT HAPPENS . The boat wasn't porposing with the 70 hp motor on it . I'm pretty sure its a combination of motor height and props.. seems like the dealer just wanted a quick sale .