Converting Tohatsu 5hp to 6hp (4 Stroke) returns disappointing results.

[fwman1]

I broke in my 2021 Tohatsu MFS 5D and once done installed a replacement 6HP carb from Ready,Set,Boat. The boat is a 1436 Alumacraft Jon boat. It is lightly modified with an emphasis on light weight. Prior to the conversion to the new carb, the engine was running 5300 rpm WOT (Tiny Tach), and about 11.75-12.15 mph according to my Garmin fishfinder gps.

After the conversion, my rpms are up to 5380-5400 rpm, and speed to 12.25-12.5 mph. The general operating characteristics of the new carb are good, with normal operation across the board.

I expected a better increase in rpm and mph given no other changes were made. Does anyone have and suggestions/ideas/experience that might help out? Is there something about the D model of the engine that limits the carb swap improvements

Ready,Set,Boat advertises this carb as being the 'true 6HP carb'. I even sent my model number and serial number before purchase to confirm. Here is a link to the carb I purchased.

Tohatsu 6HP MFS6 4-Stroke Outboard Carburetor | eBay

(This carburetor fits the Tohatsu MFS6 6HP 4-stroke outboard engine. Includes a pair of intake gaskets. It's not a 6hp carburetor and you will lose power). Need it faster?.

www.ebay.com

 

[fwman1]

I went to my boat and removed the 'new' 6 HP carb I find that it has a 19mm venturi opening that is identical to my OEM Tohatsu 5HP Keihin carburetor 19mm venturi. It would appear that this is a 5HP carb, although there are absolutely no marking on the carb. The gaskets, and carburetor isolator block both have 21mm diameters from the factory, and from Ready,Set,Boat.

Can anyone confirm that the 3JE032000M carburetor has a 21mm venturi diameter

 

[matt167]

Personally I don't trust aftermarket clone carburators to be exact. I buy them for mowers and stuff to get them running but I pay $50 or way less usually.

If you look at the description it's jetted as a 6hp. So the venturi is the same or it's not actually what they say it is

 

[fwman1]

Personally I don't trust aftermarket clone carburators to be exact. I buy them for mowers and stuff to get them running but I pay $50 or way less usually.

If you look at the description it's jetted as a 6hp. So the venturi is the same or it's not actually what they say it is

Thanks for the response. Yes, I believe they sent me a 5HP carb, and am reaching back out to them. Their customer support is quite underwhelming so far. I would not purchase from them again just on this account.

Additional fuel without addition air can only provide marginal additional power. It is unlikely they'd go to the trouble to manufacture a carb with just larger jets and not the appropriate air inlet capacity. They offer a 5HP upgrade on their site, and I think this is what I received. I'll update this thread with their response.

In the meantime, I am still searching for an answer that I can use to prove that a 6HP carb has a 21mm throat diameter.

 

[fwman1]

Ready,Set,Boat responded to me and explained the difference in measuring from the rear of the carburetor and the venturi (narrowest point inside the throat of the carburetor). I compared the venturi size on the replacement carburetor to the 5HP carburetor that came on my engine, and it is in fact larger. So, it is time to eat some crow.

While the improvements were slight (3%rpm, 4% speed), I did receive a carburetor designed as a 6HP unit. To their credit, Ready,Set,Boat offered to allow me to return the unit, but once the difference was made clear I declined their offer. At some point you just have to say let it go or buy a larger engine.

 

[jbcurt00]

And this is why most carb swap hp 'increase' mods aren't recommended.

 

[matt167]

If using an OEM carb the swap could possibly be worth it. But with an aftermarket. It's unknown if the carb would perform as well..

In the case of a lawnmower or something. It will make a non running engine run for cheap. But it may or may not be optimal. 1hp isn't a lot though

 

[pvanv]

I always use the OEM carbs and have had good results.

 

[km1125]

Here's the thing. The "6HP" carb doesn't do ANYTHING to help the power of the engine over the vast range of RPM, EXCEPT when nearing the top.

At 5400 RPM, with even a 6HP carb the engine is only going to develop about 5.3HP, so there's not much of an improvement over a 5HP carb. Getting the engine to 6000 RPM is what gets you to 6HP and the "6HP" carb will provide the airflow to get there. The 5HP carb won't.

So, that suggests you should be considering a PROP change. What are you currently running? The stock 8" pitch? I'm a bit surprised at the low top speed. How much does that boat actually weigh? I would be expecting at least 15mph even without topping off the RPM.

 

[fwman1]

km1125,
Sorry, I missed your reply.
The boat is an Alumacraft 1436 jon boat. It weighs 130 pounds bare. My engine is about 57 lbs, myself 180 lbs, battery and trolling motor about 70 lbs, and tackle/rods around 15 lbs. So figure around 450-475 lbs ready to go. I have arranged the load for best balance, and have the engine thrust rod on the fourth position from the transom. I believe that drag, as much as weight is my enemy on this application.

I had originally purchased the 5hp for a 10 foot jon boat. It would have been generously powered for that boat. My wife saw me out in the boat during engine break-in, and insisted on a larger boat. If I'd bought a new engine for the 1436, a ten HP would have been my minimum. As it is, I have an almost new 5/6 HP, and am trying to make it work.

Per Paul's recommendation, I did go to a 7" pitch propeller (Solas). The prop change allowed me to get RPMs to 6k. Speed at WOT has increased. At one point for a second I saw 13.9, but it will run 13.5-13.8 consistently depending on the water state (smooth, rippled, choppy).

I've read the Tohatsu props that ship with the engines are typically a bit too coarse, or high load. Since my change of an inch provided a 500+ RPM change, I'm apt to agree. I'd love to try a Solas 8" just to see, but believe that the 7" is likely the best choice for my setup. At some point you just have to shut up and fish.

Thanks to all for your help and guidance on this issue.

 

[MattFL]

The carburetor quality can make a big difference in performance, my guess is the aftermarket won't be as good as OEM.

Just to give you a data point, I've got a 6HP Tohatsu on an old 13 or 14' Duracraft flat bottom jon boat. The boat probably weighs 100#. With just me (165#) I believe it does about 17mph and hits the rev limiter, add in my son and some fishing gear and top speed is 13-ish mph. If you're getting on plane fine, is it really worth all the hassle for maybe 1mph more top speed? For reference, here's our covid escape day. The boat launches were closed so instead of our larger whaler, we dropped the jon boat into the intracoastal off the side of the road and did a little fishing. Maybe this will give you a useful comparison to your boat.

 

[fwman1]

MattFL,
Thanks for taking the time to post the video, and your experiences. I could only dream of 17 mph on my boat, lol. Here is some more info garnered since my last post. BTW, I'm not sure if I'm quite fisherman enough to handle my nice catch being beheaded by another fish! Cool video!

After installing the Solas 7.8X7, the performance really improved. That being said, my Runleader tach was showing an even 6,000 rpm. As the recommended max RPM for this engine is 6,000, I thought I'd hit the nail on the head. After some review that turns out not to be the case. I had the tach set up for a 6,000 MAX range. After adjusting the tach to a max upper limit of 12,000, I found that I'm actually turning 6,200 rpm on plane at WOT. This is only 100 rpm under the rev limiter! While I can adjust the rpms via the 'nut' on the tiller, I think it is best to make one more swing at another prop.

When I went from the Tohatsu 8" to the Solas 7" I gained over 700 rpm.
I called Solas and explained the application and behaviors, and they recommended trying their 7.8X8" prop. Going from Solas to Solas they expect that there should be a drop of approximately 200 rpm. Assuming prop slip doesn't go through the roof again, that should put me right at 6,000 rpm. I'm buying it from a supplier that will allow a return if it doesn't work out.

Stay tuned...

 

[MattFL]

Around here you have to reel fast or you'll likely get cut off, or stolen by a shark. With such small motors and such big swings in how the boat is loaded with a small jon boat, IMHO it's going to be difficult to split hairs on the prop. For example, say you're turning 6200rpm just you; add one fishing buddy and that's a significant increase in load that can bring you down several hundred RPM. For example I keep mine with the stock prop which will hit the rev limiter with just me, but not when I have my son with me, and with three total in the boat we can just get on plane. Unless you've always got the same load, then I guess it makes more sense to fine tune the prop selection.

 

[fwman1]

Yep, just me. Wife won't ride, so I'm trying to do the best with what I have.
It is the largest lake in Alabama, and I'm the slowest boat on it. ;>).

 

[km1125]

I'm wondering if engine height would make any difference in your application. Where is the ventilation plate ride relative to the bottom of the hull?

Also, since that hull is a flat-bottom (right?) then that's a lot of surface area for drag. You might experiment trying to introduce some air into the flow of water underneath the boat, which will reduce drag considerably. The Navy actually did a bunch of tests on this decades ago and they even drilled hundreds of small holes in the bottom of boats and pumped compressed air in there and it made a huge difference.

You could do something much simpler. If you know (or can figure out) exactly where the hull is hitting the water under the boat when on plane, you could simply drag something just ahead of this to disturb the water and 'trap' air in the water. You could also glue very small items onto the hull at the same point to do the same thing.

 

[fwman1]

When I set up the engine on the boat, the short shaft and transom matched up perfectly according to manual. I agree that drag is likely my biggest issue. I'm considering that since my boat does not come out of the water, the bottom needs a good cleaning.

I received the Solas 7.8 x 8" prop and took it out for a spin. The Solas rep I spoke with expected around a 200 rpm drop. That was a low estimate. I lost about 400-450 rpm. On the new prop, rpms are 5760. Top speed is between 14.3-14.5 mph. The engine sounds happy enough. It doesn't seem bogged down, although it is no longer singing at those 6200+ rpms. My Tohatsu manual states 5500-6000 rpm in the specs. My engine has only about 17 hours now. Hopefully I'll gain a bit more as the engine completes its break in.

I will continue with the 7.8 x 8" Solas prop. I'll save the Solas 7.8 x 7" for trips with another angler. With another person, the 7" pitch would likely be about right.

For those who come behind me.
2021 Tohatsu 5hp
1436 Alumacraft flat bottom Jon Boat. All up weight about 500 lbs.
Stock Tohatsu 7.8 x 8" prop: 5300 rpm WOT and 11.75-12.15 mph

After 6 HP aftermarket carburetor swap
Stock Tohatsu 7.8 x 8" prop: 5380-5400 rpm and 12.25-12.5 mph
Solas 7.8 x 7" prop : 6200 rpm and 13.8-13.9 mph
Solas 7.8 x 8" prop : 5760 rpm and 14.3-14.5 mph

 

[MattFL]

Sounds like you found the happy spot, great job. How much did the Solas prop cost if you don't mind saying? Also just for one more datapoint to add to my info above; my motor is too long for my boat by a couple of inches.

 

[fwman1]

Thanks MattFL, I found my best price for the 8" on Amazon. It was about $60 shipped after tax with Prime. Boats.net has a good price as well, but they were not able to ship in a reasonable length of time. I got the first Solas (7") from Propeller Depot. They were nice to work with and shipped the same day. They are in Florida, so I got it quickly.

That is interesting about your motor length. It looked to me that your boat was a bit narrower than mine, but it is hard to tell for sure. I wish I'd have purchased the factory carburetor, but you live and learn...in theory!

 

[battle1]

Any new updates on your outboard? I have it on a 12ft v gamefisher. Mine is a 5hp but I've added a floor and deck. I have the 7.8 by 8 now but I'm thinking I may have to go 7.8 by 7