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Daddyodaman

Engine maxing out at approx 4200rpm

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Daddyodaman

As the title suggests, I did a quick top speed test in my new to me 04 21 xti, and got to (according to the gauges) 49mph, which is about what I expect, but the tacho only read approximately 4200rpm, and despite throttle doesn't seem to be able to go higher. The engine runs fine, and I have no real concerns, but I'm just wondering if this is normal. I was expecting higher revs at that speed, but I'm sure the hull is only capable of so much speed. Of course the boat is a tow boat, not a race boat, I'm just curious if this is to be expected. It's the 310 carb motor and standard 3 blade prop.

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GreenMan

Dad, might be a good idea to use the GPS in your phone or a TomTom or something to validate the speedometer. The speed might be reading too high and requires adjusting with the dash switch.

Remember if you're using the boat in a current such as a river you will have to allow for the current speed...

Does it have Prefect Pass that you can check the speedo against...?

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Daddyodaman

Cool, I hadn't thought of that, thanks. I do use it on a river, so I might need to factor that in. I'll have to verify the speedo calibration with gps next time I'm out. I Don't have PP or any kind of cruise. It happily gets up to speed but then seems to top out below 4300rpm, which according to everything I've read about modern boats isn't as high as most. I'm assuming either the prop or the hull is limiting the engine revving higher?

Edited by Daddyodaman

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GreenMan

I expect you would normally expect higher max RPMs. I'd say it's very unlikely that you would have a true water speed of 49 MPH at only 4200 RPM. Do you know what pitch/dia prop is fitted and if it's in good nick?

When on the river, use your GPS while the boat is drifting with the current to establish the speed of the current. Then add the current speed to your indicated GPS speed when going upstream or subtract when going down stream. The GPS will show 'ground speed' but won't account for actual water speed of the boat. If the current is 3 MPH your GPS will read 3 MPH slower than the speed of the water under your boat when going upstream and vice versa. You should have a 'SPEEDO' switch on the dash that you can jog to adjust the speedo gauge while under way.

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Sailvi767

It's unlikely the boat is doing 49 with that engine. A more likely top speed would be in the 43 to 44 mph range. The boat should be propped to turn 4600 to 4800 rpm flat out drag limited. If you will be mostly boarding or surfing I would prop it down to hit 4800 at 40 mph. You need to check both the speedo and tach. Either or both could be inaccurate. Many direct drives used a 13x13 prop back then. RPM normally was almost the same as speed up to about 36 mph.

G

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Daddyodaman

Thanks guys, I'll try that in a couple of weeks when I get back up there. The boat is at a shop getting some gelcoat redone on a bow rub at the moment, but it'll be good to go Friday week. How do I adjust the speedo? I'm assuming the speedo button just clicks up and down but is there an ideal speed to travel at to adjust it?

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REW

Thanks guys, I'll try that in a couple of weeks when I get back up there. The boat is at a shop getting some gelcoat redone on a bow rub at the moment, but it'll be good to go Friday week. How do I adjust the speedo? I'm assuming the speedo button just clicks up and down but is there an ideal speed to travel at to adjust it?

I set mine at 10, 20 30 and flat out against the GPS. Each adjustment got a little smaller and now it is pretty close, well it was until I hit the speedo switch while trying to cycle through the display.

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GreenMan

I set mine at 10, 20 30 and flat out against the GPS. Each adjustment got a little smaller and now it is pretty close, well it was until I hit the speedo switch while trying to cycle through the display.

Don't ya hate it when you do that...?!!!

... and then try to remember how many times you just toggled it so you can revert?

Edited by GreenMan

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67King

Could be the tach?

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TallRedRider

What does your ear tell you the RPM's are? A boat sounds different at 5K vs. 4K RPM for sure.

On my old Tige of that same vintage, there was a setting on the back of the RPM meter that adjusted the RPM for 6 cylinder vs. 8 cyclinder. I am sure someone can chime in how the meter counts RPM. But my boat dealer noted I was running low on the RPM guage. He flipped the switch and the RPM's were right where they were supposed to be.

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Daddyodaman

Thanks guys, yeah I'll have to verify the speed with gps and also have a closer look at the tach. It sounds like it's revving a fair bit, but maybe both the gauges are out? The main part is that it's still completely fine to drive and ski behind, but it would help if I knew what ACTUAL speed I was doing when towing a boarder/skier

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