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Old 23-03-2004, 01:19 PM   #1
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xr2 WOT

whats the max rpm for a merc xr2?? my mate has one with a 21" cleaver prop and revs to 6400 but keeps blowin it up!

ta gt
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Old 23-03-2004, 01:30 PM   #2
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depending on the age 6500 is fine
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Old 23-03-2004, 01:33 PM   #3
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someone told him 7200, sounds a bit much though. engine is in top notch condition, hes spent thousands on it
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Old 23-03-2004, 01:42 PM   #4
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you can go over 6500 but you run the risk of blowing it up or needing a rebuild once a year
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Old 23-03-2004, 04:35 PM   #5
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The XR2 will run at 7500rpm no problem.
This is the same engine as the S2000 engine that is used on a light weight cat.
The limiting factor however is the weight of the boat that it is pushing.
The motor blowing up is probably nothing to do with the rpm that your pulling.
It is more likely to be one or more of the following:
Low water pressure.
Low Fuel pressure.
Blocked fuel pickup in tank.
Carb float levels incorrect.
Carbs jetted incorrectly.
Compresion to high for fuel being used.

I had a XR2 that kept blowing during the running in period.
After the third rebuild I found that the fuel pressure was to low under load.
If you suspect that it is fuel pressure, turn up your regulator as much as you can without the carbs flooding.
This can be checked by removing the air box and ensuring that no fuel is running out of the carbs.
I set mine at 5.5psi with no problems.

Jet sizes:
Standard for XR2 0.86"
Running in Jetts 0.94"
If your motor has been tunned I would up the Jett size to about 0.90
If you what ultra reliabilty then use 0.94 however this will loose you some performance.

Note ! check the height of the engine which reference to the bottom of the boat. Make sure it's not to low.

Also worth checking the impellor.

Hope this helps.
These engines are very good when the are set-up properly.

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Old 23-03-2004, 04:54 PM   #6
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When you say your mate you mean you really, dont you!!!

What ratio of fuel to oil is your mate :wink: running?
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Old 23-03-2004, 08:41 PM   #7
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What Cyco said - got a workshop full of the bloody things - shouldn't "blow up" at those revs - if it's not for racing put 94's in it - can't hurt it at all - Probably shouldn't ask but who has he spent the thousands with and how come this person has kept on rebuilding it without sorting the problems?

I think I would find someone else to fix my engine and my problems although there are so many reasons for these to break (see above!) it might take a while!

Out of curiosity what exactly is blowing up? is he seizing pistons? top or bottom and which side - you can have an idea what the cause is from that ...........

If want to pm me I can recomend someone to sort but he's down in deepest darkest Cornwall and he's always busy!
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Old 23-03-2004, 11:44 PM   #8
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no it really is my mate. i promise.

its on a phantom lifted up very high with surface piercing prop. Hes killed two gearboxes and melted several pistons, just had a new power head efi stylee fited as last one died. ill try and find out some more detail, all I know is rough figures from a friday night down the pub!!! urgh! He was told its over proped on the 21" but i had my doubts,(seems i may be wrong) it does fly!
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Old 24-03-2004, 10:33 AM   #9
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Can you write in english so we can all understand.

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Old 24-03-2004, 11:01 AM   #10
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WTF

IMHO it's a death warrant if you run a v. high transom height without.

Explain in English Not Korean!!!!
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Old 24-03-2004, 05:57 PM   #11
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Sounds like a cooling problem. If the prop is surface piercing and the gearbox keeps blowing up chances are he's running it so high a large percentage of the gearcase is out of the water and so isnt being cooled sufficiently. The water pickups are probably so high the motor is sucking a bit of air up along with the water and so the power head isnt getting the cooling required, hence the melted pistons.

You most likely need to lower the motor on the transom.

Better still, get someone who knows how to set up race boats to do it!!!
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Old 24-03-2004, 06:05 PM   #12
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Theres alot of what if's in that statement.
He has'nt stated what height the motor is running at Just High.
He also has not stated which type of gear case he is using.

Cyco
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Old 25-03-2004, 12:17 AM   #13
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So 'what if' there is!!!

I was just thinking (I know I shouldnt, but I cant help it) if its well high as he bounces over the waves the water intakes maybe in and out of the water alot, even with a bullet box.

Hey, what if its a short shaft motor and he hasnt cut the transom down
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Old 26-03-2004, 03:53 PM   #14
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all try and find some more details and get back to you guys, save on the claws comin out

thanks
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