I just picked up a used 1989 16ft bass tracker, with A 1979 70hp long shaft merc. At wot rpms at 4700, doing 25mph. Seems real slow to me!The prop # is 48 816704 a40 13p. can you help?
Welcome to iboats.<br /><br />Sounds like you're only running on 2 cylinders. Check the sparkplugs. If one looks significantly different than the others, then we need to find out why. Could be a lot of things, but let's start there. Prop sounds appropriate.
That seems about 1000 rpm's low at wot to me. There is a wealth of knowledge here on the board, It won't be long before someone here narrows it down for ya. <br />Good luck,<br />Dave
Check compression on all cyls. How is the motor trimmed, and are you able to change the trim and gain more RPM's ? What is the hole shot performance like ? How many in the boat at that speed ? Where is the antiventilation plate relative to the hull bottom ? Just a few questions to help narrow down the problem. Good luck.
I just ran a comression check, 115 on all 3 cyls. I have no electric trim, the rod is in middle hole. I tried trim up but it started to cavitate. The boat jumps out of the hole, with four people it still jumps the same but looses 2mph. The antiventilation plate is even with keel.Thanks for any help.
The prop looks to be incorrect for that motor according to this web site <br /><br /> http://www.propline.com/Select-Boat...um-Propellers/30_40_HP_and_25_HP_4_Stroke.htm <br /><br />This site shows it as a 10 1/2" dia x 13 pitch prop. That might account for the good hole shot performance and poor top end but contradicts the low RPM's. I'm running the same prop on my 40Hp Mariner on a 14ft 600Ib boat/motor combo. Great hole shot and I top out at 33MPH with just me and a tank of fuel. You should be over reving the motor with that prop at WOT, not under reving. Are you sure your tach is correct ? Is it set for the correct number of pulses to match the motor ? Can anyone else confirm this thought ?
Check to see if you are getting spark on each plug. Pull them all out. Hold the base of each against a good engine ground and crank briefly. Be sure to wear thick leather gloves or use a thick dry rag to hold the plug. Do this in a dark place or at dusk so you can see if you have good spark on each plug. If one of the plugs is not firing, switch with one of the others. If none of the plugs fires on one wire, that is your culprit and you need to start checking coil, etc. for that cylinder. P.S. Don't forget which plug came of of which cylinder. Good Luck
