HI Jeff
Just had a further look at the VIPER 15 specs on the Mtronik web site.
They mention that some 2.4Ghz sets are designed for aero use and have a failsafe setting on the motor channel relating to full down on the throttle stick. it's possible that the Fwd/rev setting on the Tx also need changing the original setting would have adjusted to the settings on the 40Mhz set.
You should be able to reset this but you will need the instruction manual for your TX. A quick work round would be to use another control stick on the TX and the appropriate receiver channel ( use the rudder servo to identify the correct Rx channel) and see if the problem resolves.
Hope this helps.

HI Jeff
The Viper will be controlling a brushed motor with two wires from the ESC to the motor. Brushless have three wires, sorry should have mentioned that, sorry.
The Viper usually sets up automatically but the settings ca be adjusted to taste by use of a small button on the ESC. When you switch on the small switch attached to the ESC the red and green lights flash and if you press the button whilst flashing you can then set the max forward speed by moving the transmitter control stick to the up position , then move the stick to the central position, the red light will be solid so move the TX control stick to the down position this will set the reverse speed, finally move the stick to the neatral position and the set up is complete. The ESC will retain this info unless you choose to press the button again at switch on, so there is no need to set up more than once.
The details are at http://www.mtroniks.net/resources/VIPERInstructionsheet_2.pdf.
It's possible that you have an earlier version or possibly a forward only ESC - not sure about this but it's on their web site. Did you see the vendor running the motor both ways Ie fwd/rev?
You mentioned very fast and if you have a speed boat this may be the case or the previous owner has set up the ESC to give more forward speed by adjusting the neutral point.
I suggest you try adjusting the ESC in accordance with the Mtronic instructions in the link and see if you can get control.
Please let me know if you are still having problems and I will do my best to help further. 😀

HI Dave, I live in South Wales, the esc I have is an MTroniks Viper Marine 15 Electronic Speed Controller, as to whether it is brushed or brushless I don't know no writing on motor, bought boat with no rx or tx from ebay, seller showed me it working with his 40mhz rx and tx worked great. I bought rx and tx brand new. I can control servo from tx. motor plugged into channel 3, trims in right position still the motor spins very fast. 12 volt battery in boat. very confused

HI Jeff
Welcome to the site.
Not familiar with the radio but a quick Google suggests you have a sky602a with the sky 607 RX on 2.4Ghz. Not sure of you location as your postcode is incomplete.
You have not indicated the type of ESC you are using but it seems likely it has a built in BEC. is it a brushed or brushless set up?
I suggest you check out the radio first and make sure you are able to control a servo from the transmitter. When you switch on and plug in the battery can you control the rudder servo? if not it's possible that the TX/Rx are not bound so you need to set this first. There should be instructions with your kit explaining the process. You may need to purchase or borrow a separate 4.6v battery to power up the receiver to complete.
As you have a 6 channel receiver it's Important that you plug the servo and ESC into the correct channel corresponding to the two main sticks on the TX. The channels should be marked on the RX and normally Ch1 to Ch4 relate to the two sticks on the TX. Other channels may be controlled by switches or rotary knobs on your Tx and this may explain your lack of control if your ESC is plugged into a wrong channel. Looking at the photo on the web Ch3 is marked as throttle so this should be used. Make sure you plug in the correct way round with the neg (black?) lead connected to the neg pin. Hopefully this is marked on the side of the RX but it won't work if reversed.
Assuming you have a working radio you need to make sure that your TX sticks and trims are in the neutral (centre) position before you connect the battery. The ESC should then identify this position and set itself up correctly. Some ESC's require that you have the Tx stick set up as normal , other need reverse. I assume the switches at the lower right on the Tx perform this function.
Hopefully this will result in a working setup, however some ESC's have particular requirements which may differ from my suggestion.
If you advise the type of ESC and motor I may be able to help further.
Good luck and I hope you get your system up and running 😀