73 Mercury 65HP Runs Good But Has No Speed

[bohemia_boater]

Boat is a 1970 StarCraft<br />Motor is a 1973 Mercury 65HP<br /><br />Bought the boat at the end of last summer. Had a local marine shop do a tuneup on the motor and the lower unit needed a bit of work.<br /><br />Now the boat is in the water and the motor runs good, but I don't get much speed. The motor is topping out around only 2900 rpm and is barely making a wake. Because I am running it WOT, it is really eating up gas.<br /><br />The boat has the older style Mercury controller with a warmup lever. At slow speed, I have to lift this lever back up to prevent it from stalling out.<br /><br />Any ideas on what the problems with speed and the stalling could be?

 

[rickdb1boat]

Re: 73 Mercury 65HP Runs Good But Has No Speed

Check to make sure you are getting fire to all the cylinders. Sounds like your running with a dropped cylinder. Check spark plugs after running. What do they look like? Compression OK? Could also need Carb rebuild! Let us Know!

 

[aeronutt29]

Re: 73 Mercury 65HP Runs Good But Has No Speed

My 100 HP Merc did the same thing. Turned out to be a broken reed valve. Put your hand in front of the carburetors while the engine is idling. If fuel spits out of one of them onto your hand, it means you have a broken reed valve. <br /><br />Assuming that you don't have a broken reed valve, run a compression test and spark test. Without even compression and good spark nothing else matters! If it's been a while since the carburetors were off, it can't hurt to pull them apart and give them a good cleaning. Use an entire can of Gumout and blow it all dry with compressed air if you have a compressor. Once these easy things are done, post a reply with your results.

 

[bohemia_boater]

Re: 73 Mercury 65HP Runs Good But Has No Speed

I spoke with the previous owner and think I have narrowed the problem down to the prop. While we were test driving it, we lost the prop. He put his 'spare' on for me but doesn't think it was the same prop as was on it.<br /><br />The prop is 10 1/4 diameter with a 14 pitch. If this is too small in size and pitch, that could be part of the reason for the lack of speed and the high rpms and consumption of gas.<br /><br />My new question is, what size prop should I go with? A guy in a local marine store tried to tell me to go down in pitch, but I think that is wrong. Don't I want to go up in pitch? And what about diameter? Do I want to go up in that size also? Lastly, does someone know of a good website to buy props inexpensively?<br /><br />Thanks to all.

 

[mrc]

Re: 73 Mercury 65HP Runs Good But Has No Speed

While it depends on the size of the boat, that sounds<br />like an appropriate prop for that size motor.<br /><br />BTW, smaller prop or less pitch = more RPM, more<br /> power (hole shot), less speed.<br /><br />I agree with earlier posting, sounds like you've<br />lost a cyclinder, try looking around under the<br />transom <br /><br />Seriously though, check spark on all cylinders.<br />Try disconnecting spark plugs one at a time and<br />see if motor doesn't seem to care if a particular<br />one is disconnected (that would be the cylinder<br />that isn't doing its fair share). <br /><br />Check for indications that fuel is getting to<br />the bad cyclinder (if one was indicated by earlier<br />test), by looking at spark plug, should be "wet".<br />If not it is probably time for a carberator rebuild.<br /><br />Check compression on bad cylinder, should be the<br />same (within say 10%) as the others.<br /><br />Inside the carb, there is two valves between the<br />air intake, and the block. The first one is<br />the choke, which by restricting air flow before<br />the venturi richens the mixture for starting.<br />The second one is connected to the throttle, which<br />regulates air flow through the venture thus controlling how much fuel/air gets to the cylinders. With the engine off, verify that at WOT, the second set valves presents as little restriction as possible. Could be a throttle <br />linkage has slipped, not allowing full throttle.<br /><br />Anyway, keep us posted.<br /><br />Mike

 

[ledgefinder]

Re: 73 Mercury 65HP Runs Good But Has No Speed

I also have a 1973 Merc 65hp. Be aware that the lower unit is a 1.64/1 ratio (at least the one on mine is, & I may have the last digit off), which is rather low. For example, Merc 50hps of the same era, and most OMC V4s, run 2/1 ratio. So a 14" pitch on your lower end is more like a 17" pitch on a lot of other motors. The 65hp triple is only 50 cubic inches, so to make horsepower it's tuned to rev high & doesn't make much power down low. For example, I also have a 1979 OMC 85hp that is 99 cubic inches. The 85, running a 17" pitch prop, will pull skiers up with authority (to put it mildly); on the 65, I had to go with a 10" prop. <br /><br />The prop's pitch is the distance the prop advances through the water with each revolution of the propeller (assuming no slip). So if you go to a lower pitch prop, it's like downshifting to get more torque to go up a hill. It helps two ways 1) it gives the engine better leverage and 2) it helps the engine get up into the rpm range where it makes more power. <br /><br />So, if you're only getting 2900 rpm WOT, a lower pitch prop will help a lot on one of those high-revving 65hps. That being said, it sounds like the motor may be running weak (maybe before it could pull a 14" prop). First thing I'd check is the compression. It's easy to do on these triples; the lower plug isn't buried like it is on the older Merc 4cylinders. Ideally, the compression should be same on all three within 5psi. I'd expect around 135psi, but it depends on your gauge, battery, starter motor, throttle position, etc. - evenness is more important. If one or more cylinders is low (probably the top cylinder), that's the problem. Other easily checked/fixed stuff - try squeezing the fuel line bulb while it's running (checking fuel flow/pump), run a little carb cleaner in your fuel, and check the timing. <br /><br />I'd check the compression before I spent $ on a prop, or at least see if you can find out if the boat/motor setup successfully pulled a 14" prop before. My motor had a 13" when I got it, but it was on a pretty light 14' Larson.

 

[ledgefinder]

Re: 73 Mercury 65HP Runs Good But Has No Speed

Just re-read your post & saw the part about the 14" being a replacement prop. That makes it a lot more likely that the prop is the problem, particularly with that 'high-end' motor. The boat want's to be propped so that it's at, or slightly over, the redline with it's lightest load. Your redline is at 5500, which with a 14" prop and 1.64/1 gears translates to 45 mph (assuming no slip). That is really hauling *** for a 65hp, and wouldn't happen unless the boat is real light. A typical figure for slip is 3-5% (at 45mph the water's pretty hard, more slip at slower speed). How big's the Starcraft.