14' Sea Radyer w/120hp only goes 28mph?

[bowens01]

I have a 95 14' Sea rayder with a 120hp mecury sportjet installed and the boat only goes 28mph. The engine is a 96 model. The old engine was a 90hp sportjet and I was able to hit 33mph. The motor seems to be running great. Why would the boat go slower with a larger motor that only weighs 30 more pounds? What can I do to get it to go faster? How fast should it go?

 

[ProbeGT]

Re: 14' Sea Radyer w/120hp only goes 28mph?

first of all, you can't use the 120HP engine with the 90HP pump. Yes it will fit but the gearing is different. Your engine should run in the rev limiter @ WOT.

second, inspect your impeller and wear ring. You can do this by removing the intake grate under the boat, don't forget to put it back with blue loctite on the screws.

Check the cleareance between the impeller and wear ring. If your boat runs only 28MPH, it must be a big clearance, like more than .045" If the wear ring has big scores on it and the impeller is damaged, you will need to change them or rebuilt them.

If the impeller and wear ring are correct, the other problem could be cavitation.

cavitation will make your boat very slow to plane, poor top speed and on takeoff the engine should rev like it's attached to nothing.

Cavitation may be due to :

Bent ride plate (this happens when engine is runned out of water and then makes a backfire)
Bad ride plate gasket
exausth leaks from the engine to pump gasket.

there may be other reasons but that's all i'm thinking of for the moment.

 

[bowens01]

Re: 14' Sea Radyer w/120hp only goes 28mph?

Thanks for your response. Much apprectiated. If the impeller and wear ring end up being bad will this increase the top end speed a lot or would I be better off to install a 120hp pump? Is there anyway to use the 90hp pump without replacing the gears? I have spent so much money on this thing and I wasn't looking forward to having to install a new pump.

 

[ProbeGT]

Re: 14' Sea Radyer w/120hp only goes 28mph?

both pumps are the same, with the same impeller and wear ring.

the 90HP pump ratio is 1.14:1 and the 120HP pump is 1.1

it'll work, but be sure that your engine does not rev more than 5300RPM WOT.

you might have more top speed with the 1.1 ratio since the engine would be able to spin it but in a small 14' boat, you should be able to hit about 40MPH even with the 90HP ratio.

If the impeller is bad, you must replace it or rebuild it.

if the wear ring is bad, most people rebuild it with JB well, or another solution is to build a Stainless Steel sleeve to insert in the wear ring. But the JB weld solution is the easiest and cheapest.

I'm still trying to figure why my 120HP F16 can't go more than 38MPH and i have a completely rebuilded jet pump with OEM parts and Solas impeller so...

When these things go bad, you spend a LOT of time just to fix it.

 

[H20Rat]

Re: 14' Sea Radyer w/120hp only goes 28mph?

both pumps are the same, with the same impeller and wear ring.

the 90HP pump ratio is 1.14:1 and the 120HP pump is 1.1

it'll work, but be sure that your engine does not rev more than 5300RPM WOT.

you might have more top speed with the 1.1 ratio since the engine would be able to spin it but in a small 14' boat, you should be able to hit about 40MPH even with the 90HP ratio.

putting the 90 pump onto a 120 engine basically turns your boat into a heavy 90 hp. It is tough to hit 40 mph with the 90 motor by itself, and since you are limited by rpm you can't put any more power to the pump than a 90 can.