Right size impeller

[jetjon]

Hello everyone. It's been a long time since I posted anything. I installed a 717 motor from a 96 spx in a 16 ft jon boat. Which added quite a bit more weight than what is was designed for. I've been running on the stock impeller. It seems pretty strong to me but I'm gonna have to replace wear ring. (Sucked up some gravel). So I thought about rebuilding pump and possibly changing impeller size. Wondering what you guys would recommend on impeller size? And I know nothing about how their rated or what those numbers mean. I get about 36 mph right now. And it runs just as hard whether fully loaded or almost empty. My impeller doesn't look bad or bent, but it does have some dings in it. Let me know what you think

 

[Sportster-2001-951C-Stock]

I'd judge based on the WOT RPM. If the engine wasn't bouncing off the rev limiter and was revving to normal RPM then it's a good match.

Assuming the pump is loading nearly the same as in the ski, is overall boat weight nearly the same as the ski it came from, or heavier? The engine still weighs the same but the boat might be lighter? The wetted surface is probably more but weight distribution will effect that quite a bit so if someone's been riding up front or will be, consider the wetted surface will reflect that. If the boat is appreciably heavier than the ski was but the pump loads well without cavitation or other obnoxious effects, a SLIGHTLY smaller wheel will get some lost revs back.

A little change can bring a big difference in WOT RPM, you don't want to be hitting the rev limiter.

 

[Sportster-2001-951C-Stock]

Oh, sounds like your impeller is in reasonable condition so a little work on the edges to smooth out the dings might be the ticket. I'd go with an angle grinder.

 

[jetjon]

I can honesty say I have no idea what my rpm's are. I've never checked them. It sounds reved up. The boat alone is probably 350 to 400lbs. That's without motor and gear. When I first started running boat its cavitated and porpoised alot so I had to switch to a top loader grate. Slowed boat down 2 or 3 rpm's but helped in those other areas.

 

[Sportster-2001-951C-Stock]

You really can't judge without knowing the engine RPM, but since you had trouble loading the pump and the grate fixed that, I'm not convinced spending money on a smaller wheel will help anything.

Maybe you can find a "tiny tach" first and have a look, they're pretty cheap.

 

[jetjon]

Just ordered one. Guess I'll rebuild pump, replace wear ring, and grind impeller smooth and check rpm's. Thanks for your help

 

[Sportster-2001-951C-Stock]

The grate is interesting though, I know some of small aluminum jet boats running ski motors run very shallow gravel and rock river beds, with gravel shields covering the aluminum bottoms, and intake grates with closely spaced tangs to keep from scooping up gravel.

 

[jetjon]

Got an update. Replaced wearing and bearings in pump. Bought tiny tach. set tiny tach to 180. Wot is 6650

 

[Sportster-2001-951C-Stock]

That's pretty dang good IMO. Dual or single carbs? Watch for broken pipe bolts or RV cover cracks, there may be some resonance occurring at certain RPMs a re-pitch could correct for.

Is compression about 150psi?

Bill O'Neal was a wiz with tuning impellers, I found one of his interesting posts here:

720 motor, only getting 44 mph max speed

 

[jetjon]

Thanks sportster. I might be to old but I'm not sure,what IMO or RV means. But I'll check out thread. Thanks again.

 

[Sportster-2001-951C-Stock]

RV is the rotary intake valve, just behind the carburetor(s) The cover can crack if the engine vibrates at just the right frequency, things will begin to crack and break. IMO = In My Opinion.