2005 2006 Yamaha 4 strokes 250 motors .i replaced gas filter ,heard vacuum leak

The engine in a 2004 JD Buck should be a Rotax 650, which is a 2 cylinder 4 stroke water cooled engine generating 62 hp. With that said, the most likely cause of oil in the air filter and leakage from the top of the motor is excess pressure in the crankcase. A cracked block is a possibility, but the most likely culprit is worn piston rings. A compression test should help narrow it down, but in either case you are looking at a complete tear-down of the engine.
https://www.rotax.com/en/products/rotax-powertrains/details/rotax-650.html

timing may have jumped and bent the intake valve causing it to blow back into the carb on the compression stroke.

You will need to replace the head gasket. Tightening the head will not work once the oil has found a path to leak out.

lots of reasons check spark again and fuel to what filter carb? engine problems basically are gas and spark so tells me its gas pull plug squirt some gas 2 stroke make shure its oil/gas 4 stroke gas only in cylinder use a eyedropper or a clear gas line of fuel to put fuel in cylinder put plug back in fire up if starts problem in carb if no start may have jumped timing check air filter may even have a mouse nest in it most people don't drain gas out of carbs for winter drop carb clean bowl with pure white vinegar clean all jets pilot / main reasemble

I apologize - my answer to the previous question didn't really clarify your actual question - Your VIN can be found along the frame rail - left hand side - about a foot or so ahead of your left foot guard -



If it's still there - your model number is here:

If you can read your VIN - and here's a tip to know you're getting the whole number and it's not rusted away a digit; your second digit should be a (Y) - if you write down your VIN, second digit is Y so you know the placement is right - the 10th digit is the year. And here are the breakdowns for that 10th Digit:

(For all Yamahas - not just the Timberwolf):
88 - J, 89 - K, 90 - L, 91 - M, 92 - N, 93 - P, 94 - R, 95 - S, 96 - T, 97 - V, 98 - W, 99 - X, 2000 - Y, 01 - 1, 02 - 2, 03 - 3

From 1992 - 1998 the Yamaha 250 4-stroke, shaft drive, trail bike was the Timberwolf.

From 1999 - 2004 the Yamaha 250 4-stroke, shaft drive, trail bike was the Bear Tracker.

*****SAME ENGINE, SAME TRANSMISSION, SAME REAR END, TIRE SIZE, PULL START - THINGS THEY DEFINITELY CHANGED IN 1999 WHEN THEY STARTED CALLING THEM THE BEAR TRACKER: - PLASTICS BECAME A LITTLE LONGER I BELIEVE; AND THE SEAT WITHOUT A DOUBT BECAME LONGER. THE CARB WENT TO A CONSTANT VACUUM - INSTEAD OF THE SLIDE CARB WHERE THE THROTTLE CABLE WENT THROUGH THE CAP LIKE THE TIMBERWOLF. HARD TO REMEMBER - BUT I THINK THE TIMING CHAIN TENSIONER BECAME UNADJUSTABLE - AND NOT MUCH ELSE CHANGED. IN 99 THEY EVEN KEPT THE BRAKING SYSTEM THE SAME. SOMETIME AROUND 2000, 01 OR SO THE BRAKE SYSTEM CHANGED FOR THE FRONT WHEN YAMAHA WENT TO DISC BRAKES. HOPE THIS HELPS. ANYTHING ELSE YOU NEED TO KNOW ABOUT YOUR TIMBERWOLF JUST LET ME KNOW - I'VE OWNED, REBUILT, BOUGHT, SOLD, TRADED A TON - I'VE CURRENTLY GOT A 99 BEAR TRACKER THAT IS THE MOST DEPENDABLE 250 4 STROKE I'VE EVER OWNED AND A 1996 YAMAHA WOLVERINE 350 4X4 - I'VE REBUILT THEM FROM THE BOTTOM TO THE TOP. THOSE BIKES ARE GOLDEN IF YOU TAKE CARE OF THEM AND DO A LITTLE ROUTINE MAINTENANCE. ALL THE BEST - JUSTIN MCCARTNEY

THE 99 I MENTIONED THAT I STILL HAVE:


A 98 I BOUGHT WITH THE ENGINE IN A MILK CRATE THAT I BOUGHT, REBUILT & SOLD:


A 2004 I BOUGHT, REBUILT AND SOLD:

All the best - let me know if you're ever in a jam with yours; not too much I haven't seen, fixed or had to deal with.

Check air filter and clean if needed or replace, change out plug, how old is gas? gas filter replaced? is it the 2 stroke or 4

A cylinder efficiency test would be the quick answer. Set the engine to top dead centre on the compression stroke if it is a 4-stroke or any top dead centre if it is two stroke. Lock the crankshaft or engage high gear and apply the brakes to prevent the engine rotating during the test (locking the crank is better). Jam the throttle open and remove the air filter.
Remove the spark plug and fit a commercially available adapter or a modified spark plug which allows compressed air to be fed into the cylinder without leaking.

Once the cylinder is fed with compressed air, leaks through the induction indicates a faulty inlet valve or seat, air escaping through the exhaust indicates similar problems with the exhaust valve or seat and more than the usual dribble of air escaping through the piston ring gaps indicates problems with the piston, rings or cylinder bore.

This type of test is quick and reliable and far superior to wet-and-dry compression testing but the diagnosis is slightly different for a 2-stroke as there are no valves but the process is quite intuitive and a valuable aid to fault finding but stick to low pressure on these engines..

can you tell me what year and make of atv sounds like you might have a timing problem were you do have spark but at the wrong time need more information to know for sure and also is it 2 stroke or 4 stroke

Replace idle jet with a new one OEM is $5.00. If won't idle still the compression is so low that it won't stay running Or possibly the main seals are leaking (a 2 stroke has top end & bottom end compression.)

.003 " on intake .005" on exhaust at TDC on compression stroke (cold motor)