Mercury 9.9 4 Stroke - Tachometer / Prop Selection

[Tbarnby]

Hello everyone! I have a 2003 Mercury 9.9 4 stroke motor on my 1648 Lowe Big-Jon. I am trying to get everything set up correctly, and am using a Searon tach. I set it to the pattern that "looked" right, at idle. So, at idle, the boat moves a little too fast to troll (it has the stock 9" pitch propeller). And at WOT it's showing me 2200 RPMs. The boat is going about 14MPH at that point. I'm guessing the tach is wrong, but the engine is certainly not anywhere near the 5500 RMPs it should be running at. Does anyone know what the proper setting is for this Merc on that tach? It's basically the same tach/hour meter that everyone else sells. Also, has anyone used a Merc 4 Stroke 9.9 on a Big-Jon, who could assist me in selecting a starting point for the propeller? The calculators I have used don't give me pitch recommendations. I realize the motor is too small for this boat, and I'm not going to be cruising at 35MPH! But I still want it to be right.

Thanks in advance!
Tim

 

[RaisedByWolves]

If its anything like the tiny tach I used its most likely reading 1/2 what it should be.

I just kept going through the different settings until I hit on the one that made the most sense.

 

[Tbarnby]

Thanks for the reply. I went through all of the settings. Nothing really looked right. Finally, I removed a couple of the wraps around the plug wire, and it worked correctly. The instructions called for 6 wraps. It needed to be 4. Now, onto propeller selection. I figure a 7" pitch prop should move it into the WOT range I'm looking for. The motor calls for 4500-5500 at WOT. It's at 4400 now, with the 9".

 

[Tbarnby]

Ok, so today, I took the boat out alone. The engine ran 4750 at WOT, which translates to about 16.5mph, on the gps. I’m still going to change the prop, but may also buy the 15hp carb and exhaust port, since in 2003, the 8, 9.9 and 15 Mercs were all 323 cc engines. I’ll probably change out the carb first, so I’m not wasting cash on props.

 

[Tbarnby]

2002 Mercury 9.9HP 4 Stroke to 15HP Carb Conversion

I'm posting this, because I've never seen an adequate response on the Internet, and figure people may find it in a search.

OK, so, I replaced the carburetor with a 15 HP. I ordered it from readysetboat, and received it quickly. I originally ordered it from iboats, and after 2 weeks, they still had no ETA on shipment to their warehouse...so I canceled that order.

I replaced the carburetor (had to do some minor sanding on a brass fitting, for the choke to work). I took the boat out, loaded as before, and got 21.1 MPH. It easily gets to 5500 RPMs with the Lowe 1648 Big Jon, and 9" pitch prop. Noticeable difference in the size of the Venturi between the 9.9HP and 15HP carburetors. As the timing is the same on both engines, the exhaust parts are the same, and the displacement is the same, I Figured this was all I had to do. And that seems correct. The performance increase is very noticeable, and the conversion is very easy, and made the boat much more functional.

Ok, so today, I took the boat out alone. The engine ran 4750 at WOT, which translates to about 16.5mph, on the gps. I’m still going to change the prop, but may also buy the 15hp carb and exhaust port, since in 2003, the 8, 9.9 and 15 Mercs were all 323 cc engines. I’ll probably change out the carb first, so I’m not wasting cash on props.

 

[DaleH]

, … so I’m not wasting cash on props.

Maybe, but you could be DAMAGING your motor by lugging it and having your RPMs be way too low! You are seriously over-propped.

Did you TRY this prop calculator yet? https://forum.tinboats.net/viewtopic.php?f=4&t=37433

Personally, I’d rather waste a few hundred dollars on propellers, than blow a motor worth thousands. Use the Calculator, and if the prop is not right, Turning Point will swap it out for something like $35, until YOU get the right one YOUR motor needs.

 

[Tbarnby]

Agreed. The carburetor upgrade corrected the issue, completely. I may have to up the pitch of the prop at this point by 1/2" or so. It over-revs slightly now, so I just throttle down a bit.

, … so I’m not wasting cash on props.

Maybe, but you could be DAMAGING your motor by lugging it and having your RPMs be way too low! You are seriously over-propped.

Did you TRY this prop calculator yet? https://forum.tinboats.net/viewtopic.php?f=4&t=37433

Personally, I’d rather waste a few hundred dollars on propellers, than blow a motor worth thousands. Use the Calculator, and if the prop is not right, Turning Point will swap it out for something like $35, until YOU get the right one YOUR motor needs.

 

[RaisedByWolves]

Sounds like you’re on the right track.

I would check your plugs for color at WOT and see what they say with the new carb.

 

[Tbarnby]

I actually did that as soon as I got off the lake. They look pretty good. No flooding, no lean condition. Just a nice tan patina. I put new plugs in before I changed the carb, just to see if there was a problem. I'll probably head out tonight and do some Walleye trolling. I live between Lake of the Ozarks and Truman Resevoir, so weekends, during the summer, are a no-go for me. Too many drunks, and too much noise for my taste. also, a 1648 Big Jon, sharing company with cigarette boats? Not a good time. If it's not tonight, it's gonna wait til Monday.

Sounds like you’re on the right track.

I would check your plugs for color at WOT and see what they say with the new carb.

 

[wmk0002]

2002 Mercury 9.9HP 4 Stroke to 15HP Carb Conversion

I'm posting this, because I've never seen an adequate response on the Internet, and figure people may find it in a search.

OK, so, I replaced the carburetor with a 15 HP. I ordered it from readysetboat, and received it quickly. I originally ordered it from iboats, and after 2 weeks, they still had no ETA on shipment to their warehouse...so I canceled that order.

I replaced the carburetor (had to do some minor sanding on a brass fitting, for the choke to work). I took the boat out, loaded as before, and got 21.1 MPH. It easily gets to 5500 RPMs with the Lowe 1648 Big Jon, and 9" pitch prop. Noticeable difference in the size of the Venturi between the 9.9HP and 15HP carburetors. As the timing is the same on both engines, the exhaust parts are the same, and the displacement is the same, I Figured this was all I had to do. And that seems correct. The performance increase is very noticeable, and the conversion is very easy, and made the boat much more functional.

Those speed numbers match up well with what my 1648 Alumacraft ran with a 9.9hp and 15hp Johnson 2 stroke. I did find that a 4 blade aluminum prop down one pitch was a better overall fit than the factory 10" pitch 3 blade. Sacrificed maybe 1 mph on top speed but it came out of the hole a lot better and did better with a variety of loads - basically was a more universal prop for my setup. I also had a 3 blade that was an 8" that had the best overall holeshot and was the best prop for when I had 3 adults in the boat but was pegging the motor out too hard when I was solo which is more often than not.

I now have the 15hp on a 1440 flatbottom and I probably need to swap back to the factory 3 blade for that setup.

 

[Tbarnby]

Thanks for the advice on the prop. While my setup is fine for myself, it kinda stinks when I have my daughters in the boat, which is almost every time. I am almost never alone when fishing. Why have kids if you don't want built in fishing buddies?

Those speed numbers match up well with what my 1648 Alumacraft ran with a 9.9hp and 15hp Johnson 2 stroke. I did find that a 4 blade aluminum prop down one pitch was a better overall fit than the factory 10" pitch 3 blade. Sacrificed maybe 1 mph on top speed but it came out of the hole a lot better and did better with a variety of loads - basically was a more universal prop for my setup. I also had a 3 blade that was an 8" that had the best overall holeshot and was the best prop for when I had 3 adults in the boat but was pegging the motor out too hard when I was solo which is more often than not.

I now have the 15hp on a 1440 flatbottom and I probably need to swap back to the factory 3 blade for that setup.

 

[DaleH]

Check out that prop calculator by Turning Point and see if they offer one of their 'Hustler' brand 4-blade props. These props are aluminum,but blades almost as thin as SS props at 1/4qtr to 1/3rd the price. Different boat, but I used their calculator to prop my 16' Starcraft with a 60hp OMC using it (typical load was 2 people) and that 4-blade carried a load of 6 people only 100-200 RPMs lower @ WOT.

4-blade props can give amazing load capability, if setup correctly!

 

[ktoelke54]

If a 9 pitch is getting you where you want empty but struggling with a load, having an extra 8 around ought to work. I have a 25 hp on my 1648 and switch between a 13 and 11 Turning Point often. (I would use a 12 pitch but Turning Point doesn’t make one for my motor.) The 13 struggles with a heavy boat whereas the 11 handles it with ease. Well worth a 5 minute job and the extra $50 or so.


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[eeshaw]

Dale, this might be an ignorant question on my part but is the statement you made why Mercury 9.9 Bigfoot motors come stock with a four blade on them?