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2005 250f low compression

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I've just picked up a 2005 KX250F for a price I couldn't walk away from.  Luckily, I know the owner and the back story to the bike.  The bike currently has low compression and wont start, it'll kick over just fine but no fire.

Previous owner was on the dunes in 3rd or 4th gear, kinda lugging the bike then it blew the decompression plug out of the head.  He got it back home and put a new plug in, and no fire ever since, as well as low compression.

I've been doing some internets research and i'm figuring a leak down test will yield some answers.  I brought the bike home today and attempted to screw in the hose for the leak down tester but it wouldn't thread.  In fact, it got a little cross threaded and was a struggle to get it back out.  Spark plug still threads in just fine, so no permanent damage.

Anybody know if there's a special adapter for the cylinder on the KX250F to screw in a leak down tester hose?

Aside from that, I took the valve cover off, check the auto-decompression lever and it moves freely. Checked the timing and it looks to be perfect. Checked spark on the plug and that's fine.  Plug was wet so I know it's getting fuel.

Tomorrow i'll check the valve clearances to make sure they're in spec.

I know I still need to do a leak down test, just gotta figure out the proper adapter to fit the plug threads.

If that's ok, then where do I look to find the problem?

 

Thanks for your help....

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I've also been seeing stuff about cam gear spinning on the camshaft.  Are there marks on the cam and gear so I can check for alignment?

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The marks on the sprocket are at 90° and the lobe pokes up at 45°.

A leak down isnt testing compression.

Heres a cheap way, pour a tbsp of oil into the sparkplug hole. Put plug back in and kick. Did compression go up? Then its rings and or cylinder.

Do your own leak down. Buy a brand new head gasket. Take your head off. Pour gasoline into manifolds. Intake first or exhaust first, one at a time. Is it leaking past the valves?

But always look at what you changed.

How are you compared to the manual?

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Got everything figured out today.  Main culprit for it not running is a broke piston, it broke nearly in half. Got the leak down issue sorted out, the plug hole had a some corrosion and rust in it.  A little WD40 and patience and I finally got it to thread in and seal so I could do a leak down.  Which, with the Harbor Freight set I bought was reading 65%, which I assume is 35% leak or loss? Either way it's getting a top end rebuild as well as the head taken down and valves and seats checked and cleaned.

 

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Now that the motor is torn down, i'm cleaning up everything and will be cleaning and checking valves and seats. Ordered up a wiseco top end rebuild kit, which claims 13.5:1 compression.  I know this is a bit of a bump from OE, so now i'm curious as to what octane fuel I should be running?  I've done a little searching online for some sort of calculator regarding octane and compression ratios, but, that doesn't seem to exist.

 

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That whole octane thing...

Ive taken hi comp bikes into Mexico where they dont even have 87!

Ive used 91 and Sea Foam for every bike Ive ever owned. I have NEVER had a motor under my balls ping.

But if getting a higher octane isnt inconvenient for you then get it. I stay 91 cause its everywhere. 

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thanks for the info Notorious.  Looks like my compression will only be bumped a little bit.  I have steady access to 91 and I think 93 locally.  If i really wanna get crazy I can also get 110, which seems a bit overkill and will probably take away performance.

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Ive ran a few motors in Tijuana and topped off with their piss gas no problems. Not saying it doesn't benefit from a higher octane. Just that if I can get away with their 85 octane then 91 will be fine. 

Ive got two exact same bikes. Ones stock rebuild. Ones bored up and a hi comp. They both use 91

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Anybody have input into air filters?  This whole washable/reusable foam filter concept is foreign to me.  I've always used K&N's (on my old quads).  Is one filter better than another?

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I've used the No Toil in the past, already have the cleaner stuff for them.  I've heard good things about the Twin Air also.  I live in the pacific NW so my riding conditions can be anything from sand to dirt to fine powdery silt.  I'd like to stick with a twin filter design.  Does the Uni have a twin design?

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Now I have a question about the exhaust, specifically the spark arrestor.  I'm not sure if I have a full exhaust or just a slip on, but my bike has a Leo Vince x3 exhaust on it.  The spark arrestor was more torn than intact, so I removed it.  Apparently these replacement arrestors are hard to come by and are quite expensive.  Anybody have insight as to where I can buy a sheet or flat piece of stainless mesh to make a new one?  Once I get some material, it should be pretty straight forward.

 

Thanks

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An industrial metal supply. Theres one even named that, IMS. But I dont know where you live. So google your area and find out where plumbers buy pipe and welders buy metal. Millions of ideas will come to you the second you step in there if you've never been to one

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Finally got all the parts I need to get this bike back together.  Had time today to actually make it look like a bike again now it won't start.  Here's what i've done and double/triple checked.

Just finished a top end rebuild on my 05 kx250f.  New piston, honed cylinder, base gasket and head gasket.  I completely disassembled the head, cleaned all the valves, lapped the seats, cleaned up the intake and exhaust ports and put it all back together and checked valve clearances, all looks good.  While it was apart I pulled all the jets out of the carb, cleaned them all up, cleaned out the carb and put it all back together, replaced the OE fuel screw with an adjustable one and set it at 2 turns out.

  I was meticulous with proper torque specs and order of bolting stuff down.  After a frustrating hour of messing with the timing chain, I was finally able to get it back around the crank gear, then worked on the cams and proper timing.  Installed the tensioner, double checked the timing marks again, everything looks good.  Put it all back together, fuel in the tank, fresh oil and coolant, new spark plug (E3 spark plug) turned the fuel on, choked it, gave the throttle a few turns to get the squirter going and kicked and kicked and kicked, absolutely nothing.  

Pulled the plug out to check for spark, it appears to have plenty of spark to me.  The carb was initially dribbling fuel out of the over flow, but it stopped after I replaced the tank for the second time.  Pulled the valve cover off and quadruple checked the cam timing and crank timing, everything looks to be where it should.  I also put a rod into the cylinder to rest on top of the piston to double verify the crank is at TDC.

What am I missing here?  I know it's got air and spark, I don't see any reason it wouldn't be getting fuel?  Since you can't check compression on these bikes, what's a good gauge for compression?

 

Put it back together again, and same story.  

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Before installing the piston, did you put a glaze of oil on the cylinder? If not, take it back apart and do so, checking for scratches in the cylinder since it would've been dry. If you did, then hold your finger (probably your thumb to seal it) over the spark plug hole, and have someone kick the bike over, or push down the kick start with your free hand. One kick of the kickstart might not be enough, try three pushes but one might do it considering it's a fresh top end. If it blows your finger off, and is just short of hurting, you have enough compression and something else is going on. If you can hold your finger over the hole without air escaping, compression is a problem. Check for correct torque on all head bolts. Also, check for air escaping around the spark plug considering you had to mess with that a little. If you think compression is getting past the rings, take the fill plug for the oil out, set it on top, and kick over the bike. If the plug falls because it was blown off, you may have a cracked ring from installation.

I'm only 15, but have done 6 complete rebuilds.... just listing some of the things I have had happen to me that may have occurred in your situation. Just my 2 cents...

Good luck! Reply with your findings!

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Thanks for input Brock.  I did oil the cylinder before installation, so i'm not concerned about that.  I'll try your compression check idea and see what happens. I'm going to go back and check the valve clearances again with a new set of feeler gauges. Also, going to check the cam gears haven't spun on the camshaft.

 

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