Need help...40 tohatsu issue!!!

[Eric J]

I have a 40 hp 3 cylinder tohatsu on my 1440 allweld triton jon boat. I just recently had 50 carbs put on to pick up a little speed. I was running 38-40 on calm lake water before carb upgrade. I took the boat out earlier today and ran it for the first time since getting it out the shop. I topped out at the same 38-40 mph. I currently run a 15 pitch michigan wheel apollo ss prop on it, which i was told was only putting out 4700 rpms. I ordered a 13 pitch aluminum because a tach was hooked up and thats what the guy that worked on it told me where i needed to be for rpm range. Crazy thing is, my buddy hopped on the boat with me after the first test run and after about 300 yards of riding full throttle running only 34 mph, i hear the motor amp up and we take off and speed picks up to 39 mph. So by myself it runs 38 but with a loaded boat it runs 39???? Please i need some advice on whether this could be carbs or timing or prop! Thanks in advance!!!!

[willyworm]

There's lots of variables here. You've probably got a combination of issues to change to get your desired result.

Now that you've opened up the ability to get more gas in you may need to open up the exhaust to get it out. Do some Google research to see if anyone makes aftermarket exhaust tuners for em.

If top speed is priority raise the motor to increase rpm and reduce drag. But not so high you can't circulate enough water to keep the block cool.

Then there's the timing, which may need to be advanced. But be careful when adjusting timing... too much in any direction is a good way to blow up a motor.

One things for sure, you should permanently install a tiny tach.

As far as props go, pitch is not the only measurement that matters. A prop with a smaller diameter will turn at a higher rpm at WOT.

[Buckin Bronco]

I'd call cuz outboards and ask them.

[Eric J]

Thanks guys. I called cuz outboards and they gave me some valuable info. I did find out that the guy that did the carb work didnt swap the exhaust tuner. Im thinking it may have something to do with it

[mello_collins]

carbs may need different jets also.

[willyworm]

I did find out that the guy that did the carb work didnt swap the exhaust tuner. I'm thinking it may have something to do with it

It may not be your silver bullet, but if you're looking to pick up speed, it dang sure ain't helping ya.

Typically a lot more time and labor is involved in changing the tuner as compared to simply changing carbs, so unless you specifically asked him to swap and provided him with the tuner, it doesn't surprise me that it didn't get done. I had to literally disassemble my whole motor just to change the tuner.

Here's one tip. Do this work yourself. Trust me, you'll learn all about your motor and it will help you down the road as you maintain it. From what I understand those Tohatsu's are good motors, so it should last you a long time if maintained properly and you don't blow it up with the modifications.

[Eric J]

The boys at cuz outboards said the exhaust tuner was the same size on the 40 as 50. I know he had to order jets for the 50 carbs he put in, but i dont know which ones he put in there.