Jet Drive Question

I'm a newbie to jet drives, recently got a 22' wooldridge aluminum sled powered with a 260hp Mercruiser/SBC with a Hamilton 753 pump. Running light, just me and 40 gallons of fuel the engine will only turn 3000 rpm wot. The engine and pump have 10 hours of use since rebuilt, the engine has good compression and seems to run strong. The throttle linkage has full swing, ignition timing is on etc...
If this were a prop drive I'd re-pitch until the engine would meet specified wot rpms, which in this case are 4200-4400 rpm. Is the 3000 rpm range typical with jets or should I do something to bring up the wot rpms? Thanks in advance.

 

Hi abengaard,

Generally speaking, an engine won't be happy if it's overloaded. If the engine's supposed to run 4200-4400 at wide open, that's what the jet should let it get to. A properly set up jet drive should not be sensitive to load state like a prop drive is (it is for this reason that high-speed ferry owners love jets); they should reach roughly the same full-throttle rpm whether the boat is running heavy or light.

I'm not a jet expert, but I have heard of some modifications that can be made to the impeller to better match the drive and the engine. Perhaps some of our local jet experts can chime in with what would be in order for you.

 

Matt,
Thanks for the reply, I suspected as much. So, can the impellers be repitched or machined to ease load on the engine? Also, I've been told but haven't been able to confirm that Kodiak Jet parts may be interchangeable with the 753?
Alan Bengaard

 

abengaard

The 3000RPM max you get, is that from standing start and then opening up the throttles from min to max instantly?, in an attempt to accelerate away quickly....or is it a slow gradual increase in RPM from 0 to 3000?

 

How high will the engine rpm go when not engaged to the jet? will it rev up fast?

 

The engine will go from idle to 3000 rpm quickly, there isn't a hesitation when the throttle is firewalled, but the 3000 rpm is all it will do. The boat jumps on step in about two boat lengths and I would estimate speed at about 30 mph, haven't used a gps yet. Have had the boat on flat water and wide open throttle for a distance and 3000 rpm is all it will push.
I haven't unbolted the pump coupling yet, may give it a try this weekend, though I'm sure the engine will rev freely.

 

as already noted, my next question was to be, depending upon the arrangement, either clutch out or with 'zero' thrust setting on the jet, open up the throttle. You should go past 3000 rpm. If not, sounds like engine issues.

It could well be, since a rebuild, that the engine had a problem of sorts and has been limited to 3000 rpm owing to "some reasons" with the engine, probably cost related.

 

I agree with the above it is the engine not the jet. It should be easy to find the problem with electronic testing. Just MAYBE was a govenor installed????? or a limiting chip?? What year is the boat?

 

If the boat is intended for whitewater use it may have a rev-limiter for protection although it seems to be kicking in early.

Rick W

 

You may find that as the motor has been re built that the chip has been set to limit the revs until it has done a certain number of hours or until its taken back for its firt service...check with the re builder of the engine ...