Prop QUestion for a Mercury Thunderbolt 800 80 HP

[krizm200]

Hi everyone,

I have been reading through posts on props, and coming up with more questions than answers. I have a 1972 Mercury Thunderbolt 800 80 hp, 4 cylinder outboard on a Mako 15 (total weight about 1200 LB's). My prop is a 3 blade aluminum 13x19 that I replaced last year (same prop size that was originally on the outboard). The highest rpm I get according to my new tach (Faria Euro) is about 4300 at WOT. At 3000 rpm's my speedometer reads about 23 mph (this is a Faria GPS speedo). I have 2 questions:

Should I be getting higher rpms? I keep reading the outboard should get 4800 to 5300 rpms. If I should, would a 13.25x17 prop make more sense?

I usually run the engine at 3000 rpms, Mako is on plane, and the engine is running smoothly. Will this cause damage to the engine? I read a post online that mentioned this could destroy a piston? They wanted WOT for running the engine. I don't understand why running under WOT would cause damage to the outboard.

I will eventually try to pull up a water skier (I need to install the pylon yet). I know I can hit 30 mph, so I assume I should be able to pull up my 13 year old daughter on 2 skiis. I understand 25 mph should do it.

Any help would be appreciated. Still learning the ins and outs since I bought the boat last Spring.

Thanks.

 

[Chris1956]

Are you sure about the weight of that Mako? Seems pretty heavy to me.

Anyway, a 17P prop should give you about 4700RPM, and perhaps a bit more, as you will get closer to the WOT power band.

Your daughter should probably initially ski at 26MPH, Plus/Minus the speed she likes.

Your motor is overpropped IMO, running it causes the motor to need to strain at all throttle settings, except idle. Also, most of those older Mercs do not have thermostats. They rely on engine RPM to pump enough water to keep them cool. If you lug your motor with a too large pitch prop, you will cause more heat, and less cooling.

 

[Faztbullet]

Do you have power trim???

 

[jimmbo]

Before condemning the prop, is the engine in good mechanical condition? Are the carbs opening fully? Is the timing advancing to the correct max advance? You are correct about the WOT range being 4800 - 5300. Usually a 15 ft with that engine would go about 34 - 36 mph
You say you replaced the prop last year with a prop that was the same pitch as what was on the engine. If the prop was indeed an original prop from the 1972 era, you would have needed a different thrust washer, but also the pre 79 props were a different blade design, they didn't have progressive pitch blades, or cupping, and very little rake. Those differences would certain have the changed the load on the engine thus affecting its rpm. I am referring to Mercury props

 

[krizm200]

Thanks for all the replies everyone. Really appreciate it! To answer everyone's questions:

Are you sure about the weight of that Mako? Seems pretty heavy to me.
I checked online on the Mako site, the Mako 15 1974 weighs 850lb itself. I assume the engine is about 250lb., then with fuel and fishing gear, anchor,, etc. I am guessing at 1200lb.

Also, most of those older Mercs do not have thermostats.
Absolutely true, no thermostat in the engine.

Do you have power trim???
No power trim. I had been looking for a used system, but was not impressed with the "aged" look of what I found online, so the trim is manual only.

Before condemning the prop, is the engine in good mechanical condition? Are the carbs opening fully? Is the timing advancing to the correct max advance? You are correct about the WOT range being 4800 - 5300. Usually a 15 ft with that engine would go about 34 - 36 mph
I have completely rebuilt the engine, fuel pump, fuel lines and delivery, carbs, floats, starter motor, readjusted the throttle so reverse and forward gears work, new water pump (including the base), impeller, lower unit seals, new prop, new fuel tanks. Carbs are opening fully (checked it a number of times during the carb rebuild). New spark plugs and lower unit lub. The only thing I have not changed was the pistons and head, the engine has 155lb pressure (between 155 and 160) on all 4 cylinders and the pin clip on the head has not been removed, so I think it is all original. The engine starts easy and runs like it is new. (Still want to repaint it this summer). I'm not sure how to check the timing advance. I will check WOT on Saturday, check the top speed. I usually don't go full out, too fast for me.

You say you replaced the prop last year with a prop that was the same pitch as what was on the engine. If the prop was indeed an original prop from the 1972 era, you would have needed a different thrust washer, but also the pre 79 props were a different blade design, they didn't have progressive pitch blades, or cupping, and very little rake. Those differences would certain have the changed the load on the engine thus affecting its rpm.
Now there is something I didn't know! I think the prop is a Solas that I bought online in the Spring, the old prop was a bit beat up. The original was a Mercury, and it could very well be the original. Thrust washer was changed out.

So what's the consensus? Too much prop? I'll get back on Saturday afternoon after I'm done fishing! I'll have the actual WOT rpm and speed to, but I have never been able to go more than 4200 before.

Thanks everyone!

 

[krizm200]

Correction on the old prop. Took a look at it in the garage. It is an older Michigan Wheel, 13x19. Not nearly as old as I thought it was. Both the new prop and the old look the same for pitch, cup, etc.

 

[jimmbo]

Timing and sync
http://162.144.28.33/lib/mercury/manuals/mercury_1965-1989_40-115.html#/234

Timing specs(bottom of the page)
http://162.144.28.33/lib/mercury/manuals/mercury_1965-1989_40-115.html#/494

Some people will say with today's bad cologne that's called Gasoline, the timing should be set at 21, or even 19 degrees instead of 23.

 

[Scott Danforth]

weigh the boat to make sure your not water-logged

 

[Faztbullet]

Bring the tilt pin out 1 hole and retest.

 

[krizm200]

Thanks for all the advice! On the water logged comment, the person I purchased the boat from had it for 2 years. He was in a bad car accident and could not use it, the boat basically went to the mechanics at the marina, where they tuned up the engine every year. The boat sat in his garage. I keep the boat in my garage (why I bought a 15 footer, it fits!) I had the boat out 15 times since I bought it, no water comes out the drain hole after being on the lake for 4 hours. I can't feel any water with my finger either, very dry in the hull. No soft spots on the deck.

I will try the pin hole out one. When I purchased the boat, the engine was all the way to the stern on the last hole. I pulled it out one. I know I am eye balling it, but it looks pretty perpendicular on the one out hole. I'll see what happens tomorrow on the next hole out.

Thanks again! I'm still considering a 13.25x17 prop to see if it helps. I really don't want to purchase prop after prop though.

 

[krizm200]

Oh, I forgot. The fuel I am using is 93 Octane, no ethanol. I found a website somewhere online that stated it was the optimal fuel for my engine.

 

[QBhoy]

Totally agree with Jiimbo initial comments. Seems very slow for that set up. Not sure I’d be going lower than what you have until maybe tried setting up the trim manually, if no power trim.
I had a wee fletcher arrowsport 16 with a 90 Merc. She was similar size and much heavier built than yours. She would easily run 45mph trimmed out and not trying too hard. That would likely be the same gear ratio and was also a 19” ally prop (black max)

 

[jimmbo]

The inline sixes usually had a 2:1, but occasionally some models had a 1.87:1
The 4 cylinder 80, 85, and 75hp came with a 2.3:1
Regarding comparing the90hp merc(year unknown) to a 4 cylinder 80 hp, keep in mind the post 1982 inline 6s and 80hp and 75 hp were prop rated, so an Apple to Oranges comparison is what happens. To throw another curve in there, the 90hp may even be a 3 cylinder.

Edit: Most of the 3 cylinder 90s had a 2.3:1 gearcase

 

[QBhoy]

It was a 3 cylinder. I thought it would likely be the same ratio as the old 80 4 pot

 

[krizm200]

Hi everyone,

I had the boat out on the lake today. I moved the pin out one hole notch, caused the boat to porpoise after hitting 25 mph. I moved the pin back to the second hole and no more porpoising. The max WOT I had on flat water was 4200 or there about, just above 30 mph. One thing I found interesting, I used to use 5 gallons of fuel to go to the far side of the lake and back, now I am getting out and back on 3 gallons. The engine seems to be running more efficiently since I did a carb rebuild. I'm ready to try a smaller pitch prop and see if it makes a difference. The engine sounds great running, no overheating that I can tell, at least the cowl does not get hot. I assume a 13.25x17 would be the next test? Or a 13x17 if I can find one? maybe a 15 pitch?

Something I did notice, when advancing the throttle, the distributor cap rotates until it hits a stop screw on a plate. Is it possible that the screw is not adjusted right?

Oh yeah, the cutthroat trout and perch were yummy.

 

[Chris1956]

before you do anything else, I would recommend you make sure your motor is linked and synched properly. Idle pickup timing should be 4-6* BTDC. Max spark advance s/b 21*BTDC.

 

[Faztbullet]

You need trim to reduce wetted surface of hull and get you thru the porpoise. This will increase RPM and speed....I don't think you have any motor problems just set up ones.

 

[jimmbo]

That screw is the max timing stop. If it is not set correctly, the timing will be off. However, don't go turning it just to see what happens.
I posted links to setting the timing and link and syncing in post #7

Take the boat to a weigh station, then just the trailer. As mentioned earlier, you might have a waterlogged boat

 

[krizm200]

HI everyone,

Jimmbo, I was not able to open the links back in Post #7 until today. Once it opened I discovered the online manual is the same as my SELOC book. It is what I have been using to rebuild the engine. I had already gone through the Timing and Synch chapter, but I am sure I will need to read it 20 more times before I truly understand what I need to do. Good advice, by the way. Bummer is I will need to purchase another timing light; used to have 2 of them back in the day.

I won't try to readjust the Max Timing Stop. I'm not convinced that is the issue anyway.

Getting the boat weighed will be a challenge. No problem weighing the boat and the trailer, but I have no where to put the boat and then take the trailer by itself back to be weighed again. I'll have to think about a solution to that one.

As for the trim, If I set the pin on the second hole, the boat gets up on plane quickly, and no porpoising. If I go out to the third hole, I get porpoising at 25 mph and above. So Faztbullet, would your suggestion be to try the first hole? I don't understand how going the the fourth hole would help, but I am willing to try if you think it might help.

Thanks for all the help everyone!

 

[krizm200]

Question on timing lights. I see prices from $20 to $200. Any recommendations? Can I go with a basic $30 or do I need a digital timing light?

Thanks.