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need help diagnosing motor problem

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i picked up a 2008 brute force today (750 vtwin) that was not running. the guy said he had a "mechanic" look at it and he said that it was the crank bearings. well its not locked up. he said that it gradually lost power and he just parked it. well when i got home i noticed the front plug boot is off. im not sure if the "mechanic" did that or if that is the problem.... so i guess my question is would one of the boots being off cause a gradual power loss? if not what else could it be? my initial thought was piston/cylinder but it wasnt smoking.... i am getting a battery tomorow so i'll see if it cranks with the plug boot on....

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Do a compression test first and then look at the oil. Make sure its not milky, glittery, or burt.

After that run it and see if its carb related. Maybe the main jet is clogged, maybe the floats are bad,maybe there's junk in a circuit... etc.. does the problem change with throttle position. If that's not it and it doesn't seem ignition related, personally I would pull the heads and have a look see. Might as well check bearings too while its apart incase he ran it after sucking mud. That's personally my guess for a 4x4 that has lost power. They don't run em long or hard and the motors are very conservative. Not gonna run across many with a worn out motor that arent ancient or... sucked mud.

Take that with grain of salt, I am no mechanic :smirk: and headgaskets are expensive. I just like to skip straight to the heart of the matter and also see what shape the insides of a used machine are in.

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Thanks! It is fuel injected and the is a little milky. What does that mean?

Milky oil means there is water in it. There are a few causes for that. Could be something simple like it sucked some water in the vent tube when running through deep water, could be a water pump seal going bad, or something major like a blown head gasket or even a cracked cylinder. Best bet is to warm it up and change the oil. Warm it up and change the oil again and see what it looks like. Also check the coolant level. Another trick is to remove he rad cap and look for any bubbling in the coolant, that indicates a blown head gasket as well. With a blown head gasket, on the intake stroke you'll suck some coolant into the cylinder and on the compression and combustion stroke, you'll push "air" into the coolant system.

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The milky oil could be from condensation from not being run long enuf. If its crank bearings it will rattle, knock etc when running and is expensive to fix because you have to buy a crank and cases as they dont sell the bearing. Its fairly common on the vtwin Kawasaki whether in a quad or Teryx. Lost power could be a fuel pump problem.

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You have to put a pressure guage in the line and run it. If i remember right it should have 42lbs but dont quote me on that. The other way is to find another known good one and swap and see if the problem swaps also.

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check for spark. check for compression. if you have those two things, take a propane weedburner torch and place it into the intake/airfilter (unlit of course) the propane is pre-atomized and it will light off. if it does, you now know you have fuel issues to work out. if not, then you have timing issues and possibly fuel issues too. pretty easy to get to the bottom of it.

to rule out crank issue, bring it to tdc and place an indicator on top of the piston, roll it over and back on the top of the stroke, there should be no noticable stop in the sweep of the indicator.

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