Yesterday I went out on the river it was cool in the am. I got 5000 rpm's out of it normally I only get 4000 - 4200 but no more speed. I only have a 307 Oldsmobile engine. I am guessing that the Impeller is the original A cut. I am guessing that I am out of the torque range with that little motor but still should of got a few more MPH. There was a little chop on the river so I should have been getting the best speed out of it. No more going down the river than up it either. Just curious on everyones thoughts on this. Thanks
Did it seem to operate at normal speeds at lower RPM's?
I am wondering if you have something stuck in the impeller, like a ski rope or something. That would explain the higher RPM's also - if the impeller is moving less water per rotation there is less load on the motor.
I did check the impeller, there wasn't anything in there. Yes I had the same speed at 4000 Rpm's as I did at 5000. Maybe it is the spedo, I Probably should break down and get GPS, Seems to me that it should be a noticable 4 or 5 Mph more at 1000 Rpm's difference, I am fairly new to the whole boating world and especially Jet boats so I very well could be wrong.
You'd better have a chat with that Tachometer, I think it's lying to you. Check it out to a good test tool.
There is a MAJOR difference between 4 grand and 5 grand spinning an A, in my Taylor about 15 to 20 MPH, not 4 or 5.
Add to that, spinning a Berk A 5,000RPM takes 400HP, at 5000 RPM. Easy for my lightly modified 454, a lot harder for a 307 (not smacking your 307 but I would be impressed).
4,000RPM is 202 HP
4,200RPM is 234 HP
5,000RPM is 394 HP
Any sudden RPM increase on a jet pump signals either a reduction in water flow thru the pump from cavitation, or a bad tachometer, or some sudden HUGE increase in HP that just magically appeared (tippin the nitro can again eh?).
You CAN get 50 to 100 RPM on a cool morning run, I have done it, just the first run of the day before the engine heats up everything under my hood is good for 50 RPM. You will not get 800 to 1,000 RPM.
Yes, I see what you are saying SmokinLowriderSS, I would be impressed also to see 400 HP out of that 307. The Tach seems stable but I will need to check it. Through out the day I got the same old 4000 - 4200 out of it after the air temp warmed up.
How is the bottom?
1000 sounds like a lot to not see a little more speed. You are probably on the right track with the speedo being bad. Just borrow a buddies hiking GPS unit, if you don't have one.
Or...
If the tach is correct, you could be cavitating.
The bottom looks good as far as I can tell, I have never had it off the trailer out of the water. having to much fun with it in the water.
As far as cavitating I am pretty sure thats not the case. I can hear it doing it when I got the nozzle up and go for a sharp turn, but not when I am going straight.
I will have to see about borrowing a handheld gps, and also like SmokinLowriderSS said a good tach meter to make sure that mine is on the right track.
Has your holeshot changed?
[QUOTE=SmokinLowriderSS;2659827]... Add to that, spinning a Berk A 5,000RPM takes 400HP, at 5000 RPM. Easy for my lightly modified 454, a lot harder for a 307 (not smacking your 307 but I would be impressed).
4,000RPM is 202 HP
4,200RPM is 234 HP
5,000RPM is 394 HP
...QUOTE]
I'm new to the boat world, jet or not. Are those numbers generalizations or pretty close to accurate? Does it change with hull type/displacement? Thanks...
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