1975 mercury 650 not enough power

[Pgarri03]

I have a 16ft tri hull with a merc 65 on the back. It's a 1975 650. I cleaned the carbs, rebuilt the fuel pump, replaced the plugs, checked to see if they would jump a 7/16" gap, cleaned the cap, replaced all the fuel lines, took the cap off the tank to make sure it's vented, the compression is 150 on all cylinders, and did a link and sync. There is a 14deg prop on it. My problem is that it will only get on plane when we have two people or less in the boat. Shouldn't this motor be able to push this boat better? I went through everything and it's not made a difference. Also I adjusted the throttle to make sure they're opening all the way. The timing is 23deg at full throttle. Please help!!!

 

[schematic]

Re: 1975 mercury 650 not enough power

sounds over-propped. What is the max rpm when you are flat out with one person in boat?

 

[emckelvy]

Re: 1975 mercury 650 not enough power

Well, it's less than 50 cubic inches and has to rev to make power. You need a smaller prop to improve your hole shot and get the engine up into its recommended rpm range. If you continue to lug the motor it'll melt pistons from detonation.

You'll probably need to pitch-down to a 12" prop, at a minimum. Maybe even 11". Just depends on what your WOT rpm is with a light load (only the driver).

HTH.......ed

 

[Yesterday]

Re: 1975 mercury 650 not enough power

I have a 16ft tri hull with a merc 65 on the back. It's a 1975 650. I cleaned the carbs, rebuilt the fuel pump, replaced the plugs, checked to see if they would jump a 7/16" gap, cleaned the cap, replaced all the fuel lines, took the cap off the tank to make sure it's vented, the compression is 150 on all cylinders, and did a link and sync. There is a 14deg prop on it. My problem is that it will only get on plane when we have two people or less in the boat. Shouldn't this motor be able to push this boat better? I went through everything and it's not made a difference. Also I adjusted the throttle to make sure they're opening all the way. The timing is 23deg at full throttle. Please help!!!

Something seems a little off here. I also have a 15.5ft tri-hull and my little 40HP Mariner brings her onto plane with 4 people in it (combined weight of 600lbs+-) running a 13" prop, and do 30MPH with 2 people in it (combined @ 300lbs). With the 850 on the back of it now will do 40 MPH, and it's under prop'd with a 17" pitch. My initial thoughts: Is your flotation foam saturated with the water weighing your boat down?? Do you have a pile of other gear in the boat??

 

[Pgarri03]

Re: 1975 mercury 650 not enough power

We don't know about the foam. That was one of our concerns. Is there a way to check with out pulling the floor? And there is no tach on it. Our next step was to install a tach and see what it's actually doing for rpms. Any recommendations on getting an inexpensive one? And we know we have to fix this before we have a melt down.

 

[mercurymang]

Re: 1975 mercury 650 not enough power

I have the same motor and the same boat. I run a 12" pitch and I would say that's probably what you need.

 

[Yesterday]

Re: 1975 mercury 650 not enough power

And you're not over reving MM? I just double checked my pitches for my 40HP and the 13" runs WOT @ around 5720RPM and the 15" @ 5280 +-. I realize that they're significantly smaller diameter, but my understanding is that the diameter is a fairly nominal concern. From those numbers I was worried that the 17" was going to be under proping the 850 and well it's reving over 6000 RPM WOT.

My hull original weight is only around 600 lbs, so perhaps yours is a much heavier boat?

Added:
I looked into it...
" The weight of the boat is 1925 lbs. which does not include passengers, aftermarket boating ..."
Umm that explains it! That's a heavy boat compared! Perhaps Pgarri, you might tell us which boat specifically you have?

 

[mercurymang]

Re: 1975 mercury 650 not enough power

And you're not over reving MM? I just double checked my pitches for my 40HP and the 13" runs WOT @ around 5720RPM and the 15" @ 5280 +-. I realize that they're significantly smaller diameter, but my understanding is that the diameter is a fairly nominal concern. From those numbers I was worried that the 17" was going to be under proping the 850 and well it's reving over 6000 RPM WOT.

My hull original weight is only around 600 lbs, so perhaps yours is a much heavier boat?

Added:
I looked into it...
" The weight of the boat is 1925 lbs. which does not include passengers, aftermarket boating ..."
Umm that explains it! That's a heavy boat compared! Perhaps Pgarri, you might tell us which boat specifically you have?

No, I'm running around 5000 rpm at WOT. I would think that a smaller diameter would allow for more slippage. What kind of speed do you run at WOT?

 

[Yesterday]

Re: 1975 mercury 650 not enough power

Are you asking about me or the OP? if me, then both the 13" and 15" are topping out at 30MPH. Im etting 40MPH out of the 85 and would probably be higher if prop'd right.

 

[schematic]

Re: 1975 mercury 650 not enough power

Until you get a rpm reading at WOT, its all mumbling....

 

[mercurymang]

Re: 1975 mercury 650 not enough power

Are you asking about me or the OP? if me, then both the 13" and 15" are topping out at 30MPH. Im etting 40MPH out of the 85 and would probably be higher if prop'd right.

I was asking you. to the OP though, schematic is correct. You need to find out what your WOT rpm range is and then get a tach and find out what you are spinning.

 

[Pgarri03]

Re: 1975 mercury 650 not enough power

It's a reinell. Is it rreally that heavy???

 

[Yesterday]

Re: 1975 mercury 650 not enough power

It's a reinell. Is it rreally that heavy???

No, that number is for mercurymang's boat. I can't easily find the weight of yours. As for a cheap tach, the only "cheap" options you really have are either to find a used one, or get a "tiny tach" knockoff from ebay for about $20.00. They're actually pretty accurate as long as you can wrap the trigger lead around a plug wire and keep it isolated from the rest of the plug wires.

Schematic is right, without that we're really (pardon the pun) dead in the water.

 

[Pgarri03]

Re: 1975 mercury 650 not enough power

Thanks guys. As soon as I get a tach reading, I'll post it. I'm a car mechanic and I'm doing my best with this two stroke. Lol

 

[Pgarri03]

Re: 1975 mercury 650 not enough power

On a side note, if you can't find the information you need in this forum, you're not looking hard enough! This is a great community.

 

[Yesterday]

Re: 1975 mercury 650 not enough power

On a side note, if you can't find the information you need in this forum, you're not looking hard enough! This is a great community.

You're right there. It's an almost endless resource for those with something to learn. Hard to go wrong, really.

 

[Yesterday]

Re: 1975 mercury 650 not enough power

Hmm you might PM the OP of this thread and ask him how his 50HP pushes his tri-hull, and what pitch prop he uses. It could be very similar to yours??
http://forums.iboats.com/boat-resto...15-reinell-tri-hull-restoration-450405-6.html

 

[Andy_1602]

Re: 1975 mercury 650 not enough power

I am running a 1982 70hp Merc on the back of a 15 foot Skeeter bass boat. The boat is aft heavy. It also has a 6 inch set back with a jack plate. Cavitation plate 1-1/2 inch above the bottom of boat setup. I am running a water pressure gauge. It was like this when I bought it. I am running a 15px10-1/8 cupped SST prop. My boat would take someone moving the weight to the front with two people in the boat plane out or took a very long time. I put a hydrofoil on the engine now plane in seconds with three in the boat. I did buy a 13p" prop that did help but nowhere as well as the hydrofoil. I did lose about 3mph but is worth it. Mine runs 39mph 5300 rpms two people. 41mph 5600rpms one person no gear. 36 three 5000rpms. All gps checked speeds. Since my speedo in my boat in hooked to the engine pickup it is not right. Depends on the size of people! LOL!
My 2 cents, Andy

Added: my boat weight is 875lbs hull weight factory spec sheet
Check if foam is water soaked drill a small hole down though the floor not to go though hull. Water will be in bottom of floor and on drill bit. Seal with waterproof silicone.