First 18' race for me this year cut short. Engine

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3/18/2018 6:20 AM

Try to make this short

Bought a 16 350sxf a couple months ago not sure on how many hours but I'd guess 60ish. Stripped it to the frame regreased everything changed all the fluids and bearings. I've put 10 hours on it since with no issues.

First race last night. Had a hard time starting the bike in the morning before we left, never had any issue before. Got to the track unloaded and the battery died on first attempt. Got a push start, fired right up and ran fine. Let the bike run on the stand for a few minutes while I finished getting gear on to hopefully charge up the battery. Got on went to the line where I continued let it idle until they let us go (maybe another 5 mins). Right before taking off I noticed a few Puffs of white smoke float up from the radiator over the gas tank. Thought maybe from it sitting running a little overflow Steam is all and took off. On the last lap of practice started hearing a load chattering, rattle or ticking coming from the motor the higher the RPM the louder faster the chatter seems to be.

Turned it off and on a few times, no change (battery charged now). After sitting for an hour and cooling down I started it before loading it up and no noticable nosies coming from the bike at all. Took everything in me not to just race it but I didn't.

Any opinions or advice world be greatly appreciated. Grew up on 2 strokes and am decent with a wrench. Just not very knowledgeable on thumpers. Where do I start?


3/18/2018 6:29 AM

I'm a two stroke guy too, and I am not familiar with the layout of the plumbing on new 4 strokes, but the puffs of white smoke from the gas tank area, was it smoke, or steam from coolant. Did you smell antifreeze, or did it smell like oil smoke. Only reason I ask is, if you smelled antifreeze, it might be possible you have a leaking head gasket, and combustion pressure is building up in the coolant system/temp build up, and it is venting out of the relief hose. Otherwise, is there a crank case ventilation hose around that area. I'm no expert, but just throwing something out there to think about. Also, if you have a leaking head gasket, you could be getting coolant into the combustion chamber, and with that in the chamber, you could have less air space causing increased compression preventing the starter from turning over the motor. I'm sure some others will chime in with info.


3/18/2018 6:35 AM

You let it idle a super long time so I’m betting it overheated. Might have then blown the head gasket which let water in the motor.


3/18/2018 6:36 AM

Mine makes more noise than I think it should but it keeps on running fine. All I can do is ride it until it breaks or overhaul without actually knowing if anything is wrong.


3/18/2018 7:02 AM

I assume it has a oil filter.

Remove it and cut open carefully with razor. Unfold.

If something is failing the metal will show up there. It is a way to start.

I cut open my filters ever time to see how things are living. The debris will be consistent when there is more time to go looking.

You can also run a magnet thru the debris to see if it is steel or aluminum.


3/18/2018 7:22 AM

Sounds like valve clatter from overheating. Never let a non fan assisted engine run more than long enough to warm up.


3/18/2018 7:30 AM

Years ago, I had a 4-stroke moto bike. I used the most popular motorcycle engine oils in it. I noticed when the oil was hot that the engine was noticably louder. Not sure if this helps at all.


3/18/2018 7:32 AM

That is a very long time to let a 4 stroke sit at idle. Owners manual even (at least mine does) warns against it.


3/18/2018 7:37 AM

MudPup545 wrote:

That is a very long time to let a 4 stroke sit at idle. Owners manual even (at least mine does) warns against it.

This. Have you ever seen your head pipe glowing red idling in your garage! Charge your battery by riding. These boys get hot!


3/18/2018 7:42 AM

Cooked head gasket followed by further damage...rebuild time before you end up with lots of little pieces. Drain the fluids and look for water in the oil etc


3/18/2018 7:45 AM

Let any modern 4 stroke (1998-now) idle for 30 seconds to1 minute and its over heating.


3/18/2018 11:29 AM

Thanks for the replies.

Yeah, I know this was self inflicted for sure and I know better than to let it sit at idle that long. Was running a little late, trying to charge the battery and get dressed before hitting the track.

Going to be a costly mistake I think but live and learn I guess.

Really just trying to figure out where to start looking for issues. Probably due for a top-end anyway so I guess I'll start there.


3/18/2018 11:56 AM

Daddyfatsac wrote:

Thanks for the replies.

Yeah, I know this was self inflicted for sure and I know better than to let it sit at idle that long. ...more

Before teardown check valve clearance ,oil for coolant,coolant for oil and the filter for metal like I posted. Just so you know where you stand.

Also,the crankcase vent tube may be where the smoke came from. It vents into the airbox on some bikes and some folks pull it out of there and let it vent to atmosphere to avoid puking oil into the airbox.


3/18/2018 1:08 PM

That bike, if being raced is due for a top end anyhow. Especially since you’re not the original owner. But yeah, you may have overheated a bit. I’ve overheated mine before and it resulted in white smoke puffs. Ended up needing new valve guides and top end. Had a shop do the valve stuff for me and replaced the piston myself. It’s not too bad of a job. Make sure to check clearances too once together. Mine got SUPER hot though... had a small tip over and ripped the coolant hose off the radiator and didn’t realize it til a few minutes later in the Moto.


3/18/2018 1:40 PM

Sounds expensive to me.


3/18/2018 1:47 PM

KX450f’s don’t do that kind of stuff, so I have no idea.


3/18/2018 2:19 PM

Daddyfatsac wrote:

Try to make this short

Bought a 16 350sxf a couple months ago not sure on how many hours but I'd guess 60ish. Stripped it to ...more

Sounds like normal day with KTM , lucky that the frame stayed intact!


Born in Prague,living in LA,so if you don't like my spelling,blow me!

21 CRF450R
16 Yamaha YXZ 1000R
17 Yamaha MT-10 FZ-10 165 Hp RWP
06 CRF450X street legal supermoto
96 CR250R smoker
2012 CR500AF
2003 KLX110 mod

3/18/2018 2:20 PM

Pretty sure the cam chain tensioner is hydraulic on that bike. It may have failed or failed to have sufficient oil pressure once the oil thinned out, or the cam chain guide may have broken. I'd check both before doing a complete teardown on the top end.


You kids get off my lawn!

2013 KTM450 Factory Edition
Projects......lots of projects!

3/26/2018 3:33 PM


Drained the coolant and oil with no mixture between the two.

Oil looked good with very little shavings, it was pretty much black though (rotella T6 probably 3hrs). The oil screen did have a little more shavings on the top side than I've seen in the previous two oil changes. Nothing in or on the oil filter. I don't think the amount shavings was really abnormal.

Coolant was clear (well green and transparent) and free of any debris.

Checked the valves.

Intake - factory spec .10 - .15mm

shifter side .12 somewhat snug
Brake side .11 somewhat snug

Exhaust- factory spec .13 - .18mm

Shifter side .13 tight maybe even should say very tight
Brake side .14 some drag but. 15 wouldn't go

I think I measured everything correctly. Measured between the cam lobe and black arm.

Should I go further into this thing or fill fluids and test it out?


3/26/2018 5:20 PM

I see dudes all the time, starting their 4strokes up in the parking lot and letting them run for several minutes while putting their gear on. I just shake my head and laugh. It's not a damn corolla, these things overheat quick when not moving.


3/26/2018 8:22 PM

The hard starting could have been valves slightly out of spec or maybe needs a new inline fuel filter.

If you have a shop that can do a leak down test, that will tell you how your compression is looking.


3/26/2018 8:39 PM

kkawboy14 wrote:

KX450f’s don’t do that kind of stuff, so I have no idea.


-'09 KX500AF

- '19 KX450F

3/26/2018 9:19 PM

You need to have a leak down test performed. If the motor passes the leakdown test, then you'll be ok. If it doesn't pass, then you could have one of many issues, if not several different issues. Hopefully it didn't get too hot. When the head is removed, it needs to be check for warpage.

Compression tests will only tell you the compression in the cylinder. A leak down test will tell you how the engine is sealing (valves, rings, head gasket.)
Also a coolant pressure test could be performed to check and make sure the cooling system is 100% sealed. Just because you didn't see any coolant mixed, doesn't mean that it didn't get burned in the combustion process.