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Air leak kx250f

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ok so im having similar issues as MOTODAD with his 07 kx250f..i bought a non working 250f that had a blown gasket and leaked coolant into the cases....checked the crank bearings and seal, water pump seal, replaced gaskets. Crank has no up and down play within spec for side to side play..crank spins freely in the bearings. Removed the valves and changed the oil seals and also removed the carbon build up on the seats. valves are in spec and lined up correctly..wire harness shows good flow and resistance. I have also cleaned the carb, the jets, the float bowl and valve...did a liquid test and only one exhaust valve leaks a few drops an hr..have not performed a leak down test.but will construct a DIY leak down tester.....Bike has spark, has fuel, and good amount of compression...we will see after the leak down..but I can not get the thing to fire...not even a back fire..A leak down test will help me see the overall seal of the engine..

Im leaning towards a compression leak..i ruled out the gaskets cause as i kick the starter over no air seems to come out of the rad. cap..However upon opening up some passage ways to try to hear for air leaks..i noticed air coming out of the ignition cover through the timing plugs..now IDK if it's model specific, but according to the service manual there is no oil seal next to the crank bearing on the flywheel side..I have tried searching for air leaks on the flywheel side and all i get is bad crank seal and or bearing...

I took of the cover of my 2010 250f and also get some air passing though the timing plugs..but that bike runs like a champ...

Can anyone please tell me if this is normal for a kx250f..? maybe if someone with a strong running bike can just take their timing plugs out and kick over the bike to see if air comes out..im trying to rule out possible air leaks..Im pretty set on getting a new valve train and sending the head to get the 3 angle cut and install the valves myself..but if it does seem to be a crank bearing..

Edited by Racer24

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Lets  do a bit of basic detective work.

1. when bike does not fire up after lots of cranking , how is the appearance of the plug ? Is it wet dry or ??

2. have you put your hand over the carb manifold with carb removed and tried to crank it ? It should have a good bit of suction on the intake stroke and not much or any on the compression stroke.

3. Im pretty sure there is a oil seal on both sides of the crank. so maybe the seal is leaking a bit. This is crankcase pressure though and the bike should still start. borrow a adapter hose and pressurize the cylinder head. Beat to remove the cams or lock out the compression release and test at tdc. ( you can stop the engine from turning over with a 17mm socket on a breaker bar to the flywheel nut ) pressure to 30-45 psi and listen for leaks at the exhaust exit and also the carb intake. Also at the oil filler plug.


I am not a big fan of the fluid leak test w/o having a bit of pressure behind it. I usually flip the head over and fill it as best possible compression area ) with wd 40 and pressurize the intake and then the exhaust ports with a rag and blow gun. you don't want to do too much pressure as the valves may unseat due to the low valve spring pressure but with a bit , say 10-20 psi they should hold and not leak.

If the spark plug is dry, then the engine is not making enough suction to pull the fuel charge in.

if the plug is wet then you may have a timing issue or over fueling carb.

hope this helps.


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1. Plug is wet not too bad but definitely not dry.

2.yes I have there is a decent amount of suction

3.in the serrvce manual it shows two bearings but only one seal in thr primRy gear side..

Also I noticed some air coming through the oil filler capp..couod be rings..but I checked and they are all gapped correctly..

Will be doin thr leak down test tuesday when I get the parts..I also tried bump starting it still no luck

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Wet plug means you have enough "pump" to suck in fuel.

it also means :

1. over fueling at start. Could be 1. high float level or stuck float valve 2. incorrect jetting 3. fuel screw set too high


Also: 1. Ignition, not strong enough to ignite the fuel charge 2. ignition timing way off. 3. sparkplug fouled


do this before you disassemble or test.

make sure your fuel switch is off

crank bike w/o choke but wide open throttle

crank for 4-8 times and see if you get any response or fire.


just to test if carb is over fueling.

remove the plug and check for condition.

let me know


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