115 hp low speed

[Nandy]

I moved a 1974 115 hp evinrude with a 13 1/4 17 pitch to the cobia. I used to get 30 mph with up to 4 adult in the boat with the 1975 70 hp evinrude. With the same prop. The 115 is giving me just 4 mph more and the wot rps are 5200. This is very frustrating as I was hoping at least be in the 40 mph. However, I get in plane super quickly and way lower rpms. I can feel a big different in the power, just not the speed I thought... This boat should be around 800 lbs. I have a few things to try but I am looking for any further advise.
This motor check out good for compresion and had spark in all 4 cylinders. It sat for quite a few years with no gas in the carbs and started and idle great at the first try. I need to check the following.
1. Check if the thortle is really wot. I just move the motor. I kept the control and cables from the 70. I did adjust both the thortle cabld and the shift cable but I need to double check it is open up.
2. Do a decarb.
3. Check the link and sync.
4. I can probably raise the motor an inch, maybe that will do the trick...

Anything else to check?

Thanks!!!

 

[crb478]

Re: 115 hp low speed

How high is the motor right now? Try to raise it until you get into a cavatation issue, but make sure you still have water comming to the impeller. Do you have tilt and trim? If you don't try to move the pin until you get the best position for what you want.

I have a 1975 115 johnson on a 1500 lb empty CC and it will run about 30-32 mph on flat water with a 13 pitch four bladed prop. I have it in the highest hole and I still think that I need to go up another inch or so as I have a stepped tramsom. I took the tilt and trim off right now, and could probably pick up another 3-5 mph with it on as well as another 4-500 rpm. When I can afford it I will probably try to get a jack plate or a cmc tilt trim and mount it higher. I also like you had a smaller motor on it for a while(60hp) and with the tilt and trim it would also run about 30-32, but it was working harder to do it. I really think that you need to maximise out your boats set up before you spend anything on props. If you can borrow one from someone than that is a different thing as it also free. Also you might want to add a tell tail as it is fairly easy to do and gives you a visual sign, a water pressure gauge would be even better.