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What causes a bike to smoke on Startup? (09 KX250F)

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Hey guys

Just looking for some opinions on why a bike with maybe 20 - 30 hours on the motor would smoke on statup. Quite heavily actually, but it seems to go away when warmed up. Apparently the bike has been doing it sinse new.

The owner has been taking very good care of it ito clean airfilters etc.

Only done 2 oil changes so far though.

I cannot comment on the colour of the smoke, so please any opinions are welcome and whether you think it is serious or not.

Thanks ! :busted:

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Sounds like oil is getting into the top of the barrel some how? Maybe when it sits there the oil builds up but when your riding the oil doesn't drain down fast enough to blow out smoke when its burnt.

Would it be possible oil could get in via the valves?

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When an old engine smokes on startup it's often a symptom of worn valve guides - oil trickles down 'em into the ports when the engine's not running, then gets burned when you fire it up. When this is the case they usually also smoke when you crack open the throttle after the engine's been on the over-run.

But as you've done so few hours (and it's done it since new) I'd say it's unlikely. More likely that it's just blowing a little 'til the oil level in the crankcases stabilises (it's a semi dry sump motor I believe). As off-road motors don't take a lot of oil if you overfill them just a little they can smoke too.

Is the smoke blue or black? If it's black it could just be the engine's running very rich while the choke's on.

If the bike doesn't use any oil then it might be perfectly normal.

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I know the bike doesnt have many hours on it but has the piston been replaced recently? When I threw a new piston in my quad it smoked for a minute or two after startup. Once the break in period was over it quit doing it though.

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Smoking on startup that goes away once he engine heats up is a sign the barrel needs attention and the piston needs new rings. It stops smoking because the rings 'grow' due to heat expansion.

If it were valve seals, it would still smoke after decel as the vacuum would suck oil past worn seals.

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Could be a rich condition when the choke is on. my 06 kx does it with the choke on but once its off its fine

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its a 2 stroke, that's why they're called smokers, unless it's really excessive and doesnt stop when the bikes warmed up it should be normal. if youre really worried about it take it to a kawi dealer i know the one i use wouldnt have a problem if i dropped by and asked him something like that nor would he charge me

Hey guys

Just looking for some opinions on why a bike with maybe 20 - 30 hours on the motor would smoke on statup. Quite heavily actually, but it seems to go away when warmed up. Apparently the bike has been doing it sinse new.

The owner has been taking very good care of it ito clean airfilters etc.

Only done 2 oil changes so far though.

I cannot comment on the colour of the smoke, so please any opinions are welcome and whether you think it is serious or not.

Thanks ! :busted:

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its a 2 stroke, that's why they're called smokers, unless it's really excessive and doesnt stop when the bikes warmed up it should be normal. if youre really worried about it take it to a kawi dealer i know the one i use wouldnt have a problem if i dropped by and asked him something like that nor would he charge me

He is talking about a kx250F :busted:

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Smoking on startup that goes away once he engine heats up is a sign the barrel needs attention and the piston needs new rings. It stops smoking because the rings 'grow' due to heat expansion.

If it were valve seals, it would still smoke after decel as the vacuum would suck oil past worn seals.

Not necessarily. From what I've seen, if the bike's been sitting for a few days, and you have leaky valve guides, enough oil will leak onto the top of the piston, then on start up, poof, all that oil burns up right away in a puff of blue smoke. Generally, a guide leak isn't fast enough to notice burning any oil after initial startup. If your guides are warn enough to cause smoking on decel as well, you've got more problems than smoke on start up (will run like crap!). I would say smoking on decel would be more likely from warn rings.

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do an oil change to many hours for just 2 changes. use belray 20-50 and see if it gets better.

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valve problem. Guides are worn or remotly the crankcase vent hose or the vent screen behind flywheel is plugged. Usually this will manifest itself in the oil sight glass launching off side of motor

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Thanks for the responses guys :busted:

This is a bike I'm looking to buy to replace my KX250 (2 Stroke - I think I need to spell that out for some here :banana: )

I just don't want to buy an almost new bike (completely stock) and then have troubles with it. My DRZ400 does not smoke at all, ever.

So a rich condition is nothing to worry about for me, but when is comes to opening up the top end on a 20 hour bike, that's a bit much.

The problem is how do I know....? :thumbsup:

I was expecting mixed opinions, but am still unsure how to proceed.

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Well I have a nice 09 KX250F I'd sell for 3999.00 just let me know

If you pay shipping, deal !!!

:busted:

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Thanks for the responses guys :busted:

This is a bike I'm looking to buy to replace my KX250 (2 Stroke - I think I need to spell that out for some here :banana: )

I just don't want to buy an almost new bike (completely stock) and then have troubles with it. My DRZ400 does not smoke at all, ever.

So a rich condition is nothing to worry about for me, but when is comes to opening up the top end on a 20 hour bike, that's a bit much.

The problem is how do I know....? :thumbsup:

I was expecting mixed opinions, but am still unsure how to proceed.

You'd need your head examined if you're seriously considering buying a used 250F that saw 2 oil changes in 20-30 hours of run time.

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depends on the color..most of the time its because of oil getting into the combustion chamber because of worn rings or leaking valve oil seals/guide

White smoke:

Piston oil ring worn

Valve oil seal worn

Valve guide worn

Engine oil level too high

Black smoke:

Air cleaner element clogged

Main jet too large

Fuel level in carburetor float bowl too high

Brown smoke:

Main jet too small

Fuel level in carburetor float bowl too low

Air cleaner duct loose

Air cleaner poorly sealed or missing

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You'd need your head examined if you're seriously considering buying a used 250F that saw 2 oil changes in 20-30 hours of run time.

Thanks, I have to mention that the bike was not raced and used in more of an enduro environment. It also has a flywheel weight on it.

Yes, I know 2 oilchanges is stretching it a bit.

Think it is a tragedy?

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depends on the color..most of the time its because of oil getting into the combustion chamber because of worn rings or leaking valve oil seals/guide

White smoke:

Piston oil ring worn

Valve oil seal worn

Valve guide worn

Engine oil level too high

Black smoke:

Air cleaner element clogged

Main jet too large

Fuel level in carburetor float bowl too high

Brown smoke:

Main jet too small

Fuel level in carburetor float bowl too low

Air cleaner duct loose

Air cleaner poorly sealed or missing

Thanks Racer, like I said, I unfortunately cannot comment on the colour of the smoke. The bike is not anywhere near me.

On the items that you marked as "worn", do you think it is reasonable to assume so on a low hour motor?

I know anything is possible but maybe it is unlikely to be the case...

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Thanks, I have to mention that the bike was not raced and used in more of an enduro environment. It also has a flywheel weight on it.

Yes, I know 2 oilchanges is stretching it a bit.

Think it is a tragedy?

I don't care if it's been used in a parade every other sunday the oil should be out of these bikes before the 5 hour mark, we change ours every 3 hours. Personally you are now talking about a bike with questionable maintenance and an unknown smoking issue, I'd pass.

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