New Duromax 16 seems low on power

justinduke

Hey guys,

I'm not running a duromax on a buggie, I am actually using it to turn a small airplane style propeller that basically acts as a trolling motor for bowfishing. Couldn't find any other sites who deal with the clones as much as you guys though.

On to my problem though, I took my old motor off (9hp briggs) to swap to the duromax for more power. The duromax seems to be running ok but doesn't seem to be any more powerful than the 9hp. I put a tach on the old motor and it would turn the prop about 2700-2800 rpm which is about the right speed for this prop. I was hoping to be able to add some more pitch to the prop when I went up to the 16hp. However, the duromax can only turn the prop 2600-2700 rpm, even with it at the exact same pitch as the other motor.

For my setup I got rid of the governor connections and just ran my throttle cable straight to the carb. I double checked to make sure the choke is fully open and that the butterfly in the carb is fully open as well and both are good.

Does anyone have any ideas of what else I should check? I know the duromax is probably a little over rated on its hp rating but the duromax is 420cc vs the briggs was about 290cc so I would think there would be a big difference.

Also, the motor is brand new, less than an hour on it. All stock except I got rid of the little emission thing or whatever it is that hangs off the exhaust manifold.

BGW

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AirJunky

So are you running the prop thru some sort of belt, clutches, sprockets or transmission? Or just right off the crank? If it's right off the crank, I can imagine the problem is because of the engine RPMs of the two engines not being much different.

justinduke

No clutches or belts, just direct drive through use of a hub to mount the prop to the shaft. I'll attach some pics of a setup that looks really similar to mine. Only difference in the pic is that the fan in the pic is still using the governor I believe.

I'm not sure what you mean in difference in rpms between the motors. If one motor has more power it should turn the prop at more rpms than the other motor since I have bypassed the governor on both motors.

Attached Images

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AirJunky

With nothing to adjust the RPMs of your final drive, your not really taking advantage of the horsepower or torque. The 3000 or 4000 RPM of the engine is what your getting.... either engine. The Duromax will probably get there faster, or push a larger prop at the same speeds, but the RPMs haven't been improved with the new engine.

On our carts we use a pair of clutches with a belt between them. The clutches "shift" to higher gears as the RPMs increase. So the final drive RPM is improved dramatically. A motorcycle does something similar, but manually & thru the sprockets & chain.... increasing the wheel RPM as you shift up & want to go faster. On a bike you can "see" the effect easier, ie; 4000 RPM in 1st gear vs 4000 RPM in 2nd gear vs 4000 RPM in 3rd gear, etc. The engine's horsepower isn't changing, but the gears allow the horsepower to be used at a wider range of speeds than if it was just connected directly to the crank.

I've never seen the type of boat your playing with. Are there websites or forums like this for that sport too?

justinduke

Yes, there are sites for boats like this but not many people with this particular motor so that's why I am asking here.

All boats of this type have a single gear ratio. Most run direct drive while some run a gear reduction unit that is still single speed.

It would be like having a go cart go 20 mph with a 9hp motor and then swapping a 16hp motor on and still only being able to go 20 mph, even without changing any gear ratios.

More power should mean more rpms on a fan setup like this, however it seems to have slightly less power than the 9hp motor the way it is running.

AirJunky

Yea, I think the result in an example like this would be that both carts do 20 mph. But the one with the higher horsepower motor would do it quicker & be able to carry more weight & still do the same speed. It's possible it would go faster too, but would still be limited by the top of the RPM range.

But the speed itself is a limitation of the gear ratio & what RPM the engine's limit is at. A lower horsepower engine may need to run at lower speeds to be able lug the weight of the cart & passengers.... which is a matter of what RPM range the engine's power band is at.

There are ways to increase the Duromax's RPM range, both the top end & the power band. The guys building engines in go carts have really gotten into it more than this group.... and more so with the smaller engines, ie; 6 - 11 hp. Basically their messing with things like carburation, forged pistons & lightened flywheels.

justinduke

Just ran it for 20 min at 1400 rpm and checked the plug and it looks like it is running lean. I didn't see any adjustments on the carb except the idle screw. Is there a way to adjust fuel mixture with a hidden screw or do I just need to rejet?

fstg21

Rejet. The engine will not color the plug unless you add some octane booster. Newer fuel is too clean.

1400 is idle for that engine. They run very lean at idle to meet the new Epa standards.

The OE pilot jet is probably a 40 or smaller. It's easy to swap to a 45. OE honda part is about $14.

The main jet is probably a 100 or under. Depending on altitude a 105 or bigger would be in order for a 420cc.

justinduke

Ok so do I need to run it at full throttle with octane booster to get accurate results?

If I do still need to rejet then what honda motor would these jets be on? Is there a good online place to order jets?

Altitude is 5' above sea level or less. I live in southern Louisiana.

fstg21

So, a 110 would probably benefit that boat.

99101-ZH8-1100 110 main jet for a gx390

99204-ZA0-0450 45 pilot jet for a gx390

Any small engine shop can order them.

I order online from boats.net

To check the plug, add octane booster warm up, then run at full load for 2 minutes and shut it off while its still wide open. Then pull and check.