4 stroke smoking...any ideas why?


44 replies to this topic
  • mxjosh

Posted August 10, 2008 - 02:08 PM

#1

Just put a new piston in my KX450 and checked the valves. Went ok for my 1st time. But I'm getting lots of smoke out of the exhaust. White smoke. Went to the track today to see if it would burn off but its still smoking just as bad when I 1st started it. Like I said this is my 1st time working on a 4stroke. I don't know why its doing that and where I should start looking. Bike idles perfect and runs great too. No misses or hiccups.

I tried the KX450 forum but that one seems to be pretty dead and I haven't received any responses in 3 days. Please help.

  • JS264

Posted August 10, 2008 - 02:12 PM

#2

Does the smoke have a sweet, make you want to gag smell or a burning oil smell?

  • mxjosh

Posted August 10, 2008 - 02:13 PM

#3

Well I don't feel like gagging so it must be a faint oil smell.

  • rmxtgh

Posted August 10, 2008 - 02:21 PM

#4

sell it and go yellow like your boy reed.....!

that'll solve the problem......:)

  • WALKINGWOUNDED

Posted August 10, 2008 - 02:25 PM

#5

Did you change the piston because there was a problem, or was it just for maintenance? Also how did you stagger you oil ring scrappers? When you installed the cylinder over the piston, did it go easily, or did you have to force it? Also make sure it's not overfilled with oil.

  • mxjosh

Posted August 10, 2008 - 02:25 PM

#6

Funny guy! Don't really have anything Reed has...just a fan not a fanatic.

  • Sean_Lane513

Posted August 10, 2008 - 02:26 PM

#7

That's productive advice. Sounds like oil is getting by your rings.

  • mxjosh

Posted August 10, 2008 - 02:27 PM

#8

Did you change the piston because there was a problem, or was it just for maintenance? Also how did you stagger you oil ring scrappers? When you installed the cylinder over the piston, did it go easily, or did you have to force it? Also make sure it's not overfilled with oil.


Stagger oil ring scrappers? Sorry not familiar with that terminology. No it did not go easily. It was a pain to get in.

Drained oil and refilled it according to what it said on the bike to make sure it wasn't the oil amount.

  • WALKINGWOUNDED

Posted August 10, 2008 - 02:28 PM

#9

Also, did you happen to install the new rings in the cylinder and look at the gaps?

  • mxjosh

Posted August 10, 2008 - 02:34 PM

#10

No I did not. Bike has 50 hours total and cylinder looked almost new. Didn't really have a measuring device at the time of install. I do now though.

So it looks like my rings are the problem...not sealing?

  • WALKINGWOUNDED

Posted August 10, 2008 - 02:34 PM

#11

Well, I don't know for sure what's going on, but it sounds like an oil control problem, so you may have to pull the cylinder and look at the rings. Other than it being a pain to get the cylinder in place, if you had to force the cylinder you may have damaged a ring(s). Make sure you breather / pcv hose is not pinched shut, other than that you may be looking at pulling it apart.

  • mxjosh

Posted August 10, 2008 - 02:36 PM

#12

alrighty, well it looks like I'm going to get quite familiar with the bike then, haha.

Thanks guys. I'm sure I'll have more questions once I get into the motor.

  • WALKINGWOUNDED

Posted August 10, 2008 - 02:39 PM

#13

I personally have not had a bike that smoked until the rings seated. If the gap was correct and the rings were "snug" in the piston grooves, it did not smoke from the beginning.

  • mxjosh

Posted August 10, 2008 - 02:42 PM

#14

yeah its definitely not right. I noticed it right away. So did everybody at the track, lol.

  • Honda_250f_rider

Posted August 10, 2008 - 03:53 PM

#15

If it is smoking blue, it is because you didn't hone the cylinder and didn't assemble the motor dry. So you have bad ring seal at the moment. You need to hone your cylinder and put new rings on the piston and assemble it dry. Here is the procedure: ---- If it is white smoke that would be coolant, did you replace your head and base gaskets?

A 250f motor with bad ring seal can loose up to 1.5 hp... For a 450f you will need a 4.25" ball hone tool.

Hone the cylinder (ball hone or brush hone) in soap/water. Clean out with break clean and paper towels until there is no black on the paper towels. I have a 3.5” ball hone in 180 grit.

Wash the piston/rings in soap/water, spray off with break clean and let air dry.

Install the rings on the piston. Use proper ring staggering - follow the manuals staggering method - Be really careful not to get any oil residue on the piston, rings, or cylinder. Keep your hands wahsed

Install one clip on the piston. - the right side one.

Install the piston on the rod, use a light coat of oil on the wrist pin only.

Install the other clip, be sure not to get any oil residue on piston/cylinder.

Install the piston in the cylinder. Compress the rings with your hands and very carefully install it. Be sure that the oil ring doesn't bend over.

With the motor together, and no crankcase oil rotate it 50 times. Then fill with oil and check all fluid levels

Do not start the bike until you intend to ride it. Start it up, let it warm up good, go ride it hard with engine breaking, this will ensure the ring seal.

  • mxjosh

Posted August 10, 2008 - 04:08 PM

#16

Whats the reason for installing dry? Isn't metal on metal bad?

  • Honda_250f_rider

Posted August 10, 2008 - 04:24 PM

#17

doing it dry (along with honing the cylinder) ensures proper ring seal which is hard to get on these race motors.

Rotating it 50 times w/out crankcase oil helps initially seat the rings. As soon as you start the bike the oil mister hits the piston and lubes it. And do not start the bike until you intend to ride it - let it warm up good and go ride it hard with engine breaking.

Doing the dry method does not hurt anything. You have everything to gain.

  • NomadicDR650

Posted August 10, 2008 - 04:29 PM

#18

Just put a new piston in my KX450 and checked the valves. Went ok for my 1st time. But I'm getting lots of smoke out of the exhaust. White smoke. Went to the track today to see if it would burn off but its still smoking just as bad when I 1st started it. Like I said this is my 1st time working on a 4stroke. I don't know why its doing that and where I should start looking. Bike idles perfect and runs great too. No misses or hiccups.

I tried the KX450 forum but that one seems to be pretty dead and I haven't received any responses in 3 days. Please help.


Piston/Rings replacement is kind of an exact science unless you're working on an old push lawnmower. First off, the cylinder bore. If you replaced the rings and piston but didn't hone the bore then you probably are getting oil moving by the rings into the combustion chamber. In a new engine or one properly rebuilt, the cylinder bore will be honed with a swirl pattern. This does a couple of things. First, in the rebuild scenario, it breaks down the glazing which normally occurs during wear over time. Kind of like the same thing when you do a brake job. Putting new pads on glazed rotors won't help much without breaking down that surface and allowing the two to mate properly. In the cylinder bore, the honing essentially prepares the bore for proper break-in of the rings. No cylindrical space is perfectly round, nor are any piston/rings combo. New rings are designed to break in and accomodate the differences in the bore since it ain't perfect. New engines have the same preparation on the bore for that purpose. Which is why it is ill advised to use synthetic oil during break-in. You actually want the rings to wear and "seat." Once this occurs, then compression will be nominal and oil burning shouldn't be an issue.

Secondly, if you replaced the rings, did you specify any oversize? I would wager not since you indicate your lack of measuring tools and thus I doubt you mic'd the bore diameter. OE rings usually come a bit fat for break-in but if you have a worn bore, you may need oversize after honing. If the cylinder had a lip at the top edge perhaps about 1/4" down into the bore of any significance, that would indicate a worn bore which probably needed to be turned and honed; and thus oversize rings would be in order.

I would wager based upon your post that if you didn't hone the bore, the rings are not seating properly and the probably won't. You can keep running it and see if it gets better, but I would doubt it. I would perform a compression check on the bike and then a leak down test would be in order. The latter requires special tools and compressed air. That'd tell you definitively whether your rings aren't seating. Here is a page which explains a typical leak down procedure: http://www.aa1car.co...ry/leakdown.htm

Another possibility is valve stem seal leaking, which is most prevalent at idle when the vacuum is highest. The bike sitting and smoking at idle but smoking less at higher throttle is a sign of this, as is smoking upon start-up after sitting awhile which tends to clear with running the engine. But if you're getting smoke under load and virtually all throttle positions, considering the work performed I'd wager the rings simply aren't seated. Hopefully they will. Hope this helps.

  • toyota_mdt_tech

Posted August 10, 2008 - 04:31 PM

#19

White smoke means coolant. Blue smoke means oil, black smoke means fuel. Coolant will smell sweet, oil will smell like burnt matches, over fueling will smell like spray paint.

Problem started after top end, we can narrow it down to probably coolant or oil past rings.

Did you install all the rings properly? Usually 4 strokes will have 3 rings, top will be compression and should be chrome, second can also be chrome, but could also be cast iron. Its called a scraper. Works like a squeege. 3rd ring is the oil control ring. It basically lubes the cylinder. Was the oil ring a 3 piece or single part? Ring end gaps need to be anywhere except 6 and 12 oclock (cylinder wears oval, so this is usually the widest part) and not at 3 or 9 oclock. More like 8/4 and 10/2. IF the oil control ring was 3 peices, expander gap goes over the wrist pin, each ring on the oil control will but on just each side of the wrist pin

  • KJ790

Posted August 10, 2008 - 04:32 PM

#20

Assembling it wet won't hurt anything normally, but you do run a greater risk of having a poor ring seal. A dry assembled engine will tend to perform better (not really noticeable) and will often last longer. Your problem sounds like either a bad head gasket (if the smoke is white and smells sweet) or a ring issue (if the smoke is more blueish and smells like oil. Whenever you rebuild and engine always hone the cylinder, no matter how good it looks. Also make sure to stagger all of the ring end-gaps around the perimeter of the piston, say the top ring end-gap at 12-o-clock, the next ring down end-gap at 4-o-clock, and the oil ring end-gap at 8-o-clock.





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