Posts
878
Joined
9/29/2009
Location
CA
Edited Date/Time
1/27/2012 3:19pm
I have a 2008 KX250F.
So I was trail riding on a beautiful day and came across innocent looking wet dirt... So I decided to make it through but there was water underneath the top soil and the rear end quickly sank. The rear wheel was pretty much completely in mud but everything else was out as the subframe was on a dryer part... So I called up friends and we dug it out... Kickstarted it and ran great all the way back home - I atually stopped at a friend's house and restarted the bike on my way home. Then I washed the bike.
I got geared up for a ride on the following day; started kicking.. got it fired up - then it made a weird sound on a revolution, then was fine for 5 more seconds, then it made that weird sound again and stopped... Wouldn't start - making backfires...
So I checked the carb and timing - everything looked fine. I thought the vavle clearance was too tight but the team manager at Motosport Kawi (got the bike from them) told me that they were fine at 0.005. So I had the camshafts taken off- put them back on but then after making the engine turn a few times 2 of the valves were a bit too tight. I was with my father and we decided to try and start it anyways... but then as I was kicking there was oil leaking from the hose that connects behind the clutch. It still backfired...
The spark plug works fine- did a compression test and I only had 40 psi.
So we took the cylinder head off..and the cylinder. Piston and rings are top notch as well as cylinder. Problem is there's a shitload of carbon deposit on top of the piston and where the valve heads are. The inside of the pipe is all black, too. So could a piece of carbon deposit be the problem? I put the head sideways and filled it with gas. There was the equivalent of 1 drop/5 minutes going through both exhaust valves so I take it that's OK. The deposit is quite thick and really compacted on some stuff so it couldn't have been 1 screw up deal..probably just me running the fuel screw a bit too open all summer long...? But why won't it start?
Let me know if you have any ideas...
So I was trail riding on a beautiful day and came across innocent looking wet dirt... So I decided to make it through but there was water underneath the top soil and the rear end quickly sank. The rear wheel was pretty much completely in mud but everything else was out as the subframe was on a dryer part... So I called up friends and we dug it out... Kickstarted it and ran great all the way back home - I atually stopped at a friend's house and restarted the bike on my way home. Then I washed the bike.
I got geared up for a ride on the following day; started kicking.. got it fired up - then it made a weird sound on a revolution, then was fine for 5 more seconds, then it made that weird sound again and stopped... Wouldn't start - making backfires...
So I checked the carb and timing - everything looked fine. I thought the vavle clearance was too tight but the team manager at Motosport Kawi (got the bike from them) told me that they were fine at 0.005. So I had the camshafts taken off- put them back on but then after making the engine turn a few times 2 of the valves were a bit too tight. I was with my father and we decided to try and start it anyways... but then as I was kicking there was oil leaking from the hose that connects behind the clutch. It still backfired...
The spark plug works fine- did a compression test and I only had 40 psi.
So we took the cylinder head off..and the cylinder. Piston and rings are top notch as well as cylinder. Problem is there's a shitload of carbon deposit on top of the piston and where the valve heads are. The inside of the pipe is all black, too. So could a piece of carbon deposit be the problem? I put the head sideways and filled it with gas. There was the equivalent of 1 drop/5 minutes going through both exhaust valves so I take it that's OK. The deposit is quite thick and really compacted on some stuff so it couldn't have been 1 screw up deal..probably just me running the fuel screw a bit too open all summer long...? But why won't it start?
Let me know if you have any ideas...
But anyway double check your valve clearances.While you have the head off pour some wd40 or something else down the intake and exhaust port and check if the valves are sealing.After 30 seconds or so if you see ANY leaking past the valve then its time to replace them and recut the seats.
I have $5 saying with no compression all of a sudden and it ran that its the valve clearance.
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About 40 hours on the bike since I got it from the Motosport race team.
If you've got the cylinder off, which you must to be able to comment on ring condition, you need to replace all the top end gaskets anyway. At the very least, put in a new set of rings. They never seat back like they were originally.
Adjust the valves. Did the team manager at Motosport check them this time while you are in the bike or did he say they were fine when he sold it to you?
The oil pumping from the bottom end sound like either the engine has too much oil in it or you've got a blown head gasket, which will correct itself with new gaskets on assembly.
I'm waiting on some parts to come in before I put it back togather and I'll be busy this nect week so I dunno if I'll ahve time but I'll keep you guys posted... just so you know who 'won' haha. Thanks for the help.
Cleaned up piston used air compressor to clear out carburettor also, nothing found.This all happened at around 47 hrs.
Went riding and sure enough 10 min. into moto started coughing backfiring again.
Pulled down a 2nd time...checked shims all OK... ,putting plug cap back noticed frayed orange wire leading to cap..This was the problem. Electrical Repaired with tiny heat shrink sleeve ........ran perfect for about another 10hrs and same problem occured only different section of the wire.....
This time bought new sub wiring lead ...........120hrs later still running sweet
I'll check the wires to make sure there are no apparent problems with them and go for a ride in a bit...
Thanks for the help everyone... I'm happy the bike is running again but I'm a bit disappointed not to know what was wrong...
As for valves leaking a drop or so....they should leak nothing in 5 minutes, not even a weep in one hour....this must be a 100% seal at all times or drastic performance/driveability issues will result, no leakage at valve seats is the acceptable limit on any 4 stroke engine.
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