6hp 4 stroke tohatsu (mercury) won’t reach WOT under load

[RRDave]

I have an early (late 90’s/early 2000’s) mercury 6hp (rebranded tohatsu). It has a new carb, new plug. It starts and runs excellent. In neutral, it can rev to the limiter, however, when in gear, underl load can not reach anywhere near WOT. It does however, run smoothly with the throttle wide open, just a lower than expected rpm. It also has a new prop. In a bucket, with the prop removed, in gear, it will also achieve the expected max rpm.

When pushing my 1236 Jon boat, it will troll at idle all day long, however will only reach a max spread approx 5mph…others with similar set ups have reported 15mph+.

Any ideas? My only thoughts are a fuel delivery issue, but like I said, it runs smoothly and doesn’t appear to be starved of fuel (no sputtering/bogging) it just won’t rev as high as expected. I’ve also confirm three throttle does open all the way on the carb.

Compression is over 120psi.

Any help is greatly appreciated

 

[km1125]

Well, you could be starting it for fuel but that wouldn't be my first guess.

How high of an RPM does it get under load? Has the carb been cleaned? What prop are you actually running? How are you loading the boat?

With the size boat you're running, I'd recommend the "high thrust" prop that you find on the SailPro boats. It's nicknamed the "mouse ears" prop, but it's a low pitch (6") prop with big blades. The engine doesn't develop 6hp until it's well into the high RPM band, so if you can't get there with the stock prop you're not really using a 6HP motor.

 

[racerone]

He states that it has a new carburetor.----Is this a new " aquasition " and what motor did you have on the boat before this one?----Was the new carburetor added to upgrade power on the motor ?

 

[RRDave]

Thanks for the replies. The carb was replaced because I thought it was the source of the problem, which I now know it’s not.

I’ve used both a 7.8x7” and 7.8x8” prop with no noticeable different in speed (~5mph). The motor is on a 1236 aluminum jon boat (loaded with gear, battery, trolling motor and 200lb passenger). For reference, I have the same motor (except long shaft) and prop on my 18.5’ fiberglass hull as a kicker and it reaches wot, so doesn’t feel like it’s a motor/prop issue.

Carb new, plug new, thinking its fuel delivery (fuel line, or pump - maybe air leak) or ignition..but again, since it runs smooth, I can’t rationalize one over the other.

 

[MattFL]

How big and heavy is the boat? What is the approximate weight? Maybe you've just got too much boat for 6HP to get on plane.

 

[RRDave]

I did start to think my expectations were too high, but have since seen multiple examples of the same boat (12’ aluminum Jon boat) doing 15mph+ with similar loads. Any difference in weight and/or distribution doesn’t justify the underperformance I’m experiencing (6mph max)

 

[km1125]

Have you adjusted the valves or checked to see that they are correctly adjusted?

Also, just curious, but what speed can you get in the 18.5’ fiberglass hull when that motor is at WOT?

 

[pvanv]

I have checked valve lash on many of these; none needed adjustment. It does sound over propped.

 

[RRDave]

I hate to admit it, but I think the comments about engine too small and/or over propped are right. I took my other motor, which is identical, except long shaft and compared speeds. I also ordered a tach and did some measurements. Both idle about the same, around 1000 rpm and both reach a max rpm of approx 5000 rpm when pushing the boat at wide open throttle. Unfortunately, both reach a max speed of approx 6mph. Like I said, this is drastically different than the 15ish mph I’ve seen on YouTube with similar boats. I may have to resort to upgrading to a 9.9 to achieve higher speeds.

Thanks to everyone for the feedback…anyone want to trade??

 

[MattFL]

5000rpm at 5mph sounds like the motor is indeed likely putting out it's rated power but that's just not enough to make that boat go. If you've got a lot of weight in the back, try moving some weight forward. Small boats with small motors are very sensitive to weight bias.

If it helps, here's mine on a little jon boat, it tops probably around 15mph with the load as shown (not full throttle here):

 

[km1125]

The issue is getting the hull up on plane. At the lower RPMS (even 5,000) the engine isn't developing the full 6HP. I'm guessing MattFL's boat has a lot less beam than yours and that's why he can get on plane.

If the motor works for you in all other ways, (light weight, fuel economy, etc), then I'd seriously consider tracking down one of the high-thrust props. I used these motors on several inflatable dingys and that prop was a world of difference in being able to plane the loaded dingys.

If weight and size is not a problem, then jumping up to an 8 or 9.9 would make sense.