what causes 2stroke to smoke too much?

i would rebuild the carb and lean it out i would also run redline at 40:1 on 91 or 93 octane.

Does it smoke when you run it hard or does it smoke when you're putting around? 2-strokes do get kind of smokey if you putt them around a lot. Then, they really smoke when you go full throttle after putting around a lot. Between 32:1 and 40:1, you're not going to see much of a difference in the amount of smoke you make. Even at 20:1, the bike should make reasonable amounts of smoke. The type of oil does. Amsoil Interceptor is, by far, the least smokey oil I have run, followed up by Redline and Bel Ray HC-1 in 3rd.

It's hard for me to tell you what is wrong with your bike without taking a look at it and watching you ride. How much smoke is excessive? Are you noticing any loss of coolant or transmission oil? How do your carb settings compare to stock? What kind of numbers are you getting from a compression test? How does your plug read?

I have a friend with an 88 KX 250. That bike smoked so bad it would make you cry if you stood behind it while it was running. It smoked because the slide in the carb was worn out and letting air leak by. We compensated by installing a larger (richer) pilot jet and raising the throttle needle (richer) one position from stock. We had to do this or the engine rev out of control and not return to idle. The bike ran great afterwards, it just smoked a lot at idle.

If it smokes when you start it, but clears up as you start riding, that's fine. All 2-smokes do that.

A KDX is going to be between 120 and 170 psi. MX bikes are 150-210 psi.

podfish said:

it's an old (87) KDX200; it has as much power as I'd expect it to have, and seems to run great. But it puts out a lot of smoke. I run 32:1 as per the manual. I have a compression tester but I haven't gotten a consistent answer as to how to tell good vs bad compression on a 2stroke - and even if it was bad, I don't see how that would make it smoke. I hesitate to do a full top-end when it's running fine, but on the other hand it seems the smoke's a sign something's not right. Even though I don't see any need to rebuild it just to squeeze out every last bit of power, I don't want to neglect it to the point where it'll kill it prematurely. So how do I judge when it really needs rebuilding? and where should I start looking for the cause of the excess smoke?

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You could have bad seals.
Your Carb could be jetted too rich.

If you have good compression, it's likely a jetting issue.

Jetting and Fuel to Oil Ratio each determine whether or not a bike is running rich or lean.

The terms Rich and Lean can be confusing when dealing with Fuel to Oil ratios and when talking about Carburetor Jetting in the same sentence.

If your bike smokes too much, a lot of people will recommend leaning out your fuel to oil mixture. This does not necessarily fix anything and, in fact, can cause major problems with your piston seizing or breaking into little pieces that like to scatter themselves to all corners of your engine, thereby killing other parts of your engine in the process..

Think of Rich and Lean as being applicable to two completely separate, and different aspects of tuning, Mixture and Jetting, and should be thought of in the following manner:

Mixture – Rich and Lean Mixture applies to how much oil you mix with your gasoline. 32:1 is a richer mixture than 40:1.

If your Jet size is 150 (example size only) and you run 40:1 gas, more gas will pass through the small hole in your jet than if you had mixed your gas at 32:1. This is because you have more gas in the premix when it’s mixed with less oil at a leaner (40:1) ratio than when it's mixed at a richer (32:1) ratio.

At 32:1 ratio, there is more oil in your pre-mix so it needs a little bit of a bigger hole in the jet to pass through. Increasing the amount of oil in your gas, WITHOUT rejetting to the proper sized jet reduces the amount of gas that makes it through the hole in the jet to your cylinder. As a result, you actually run leaner and the engine gets less lubrication.

When you get too much gasoline into your cylinder, this can foul plugs quickly and, if the pre-mix ratio is too lean, it can cause detonation or seize your piston because there is not enough oil making it to your cylinder to lubricate it properly. You may smoke a little and still seize because you just didn’t provide enough lubrication.

Jetting – Rich and Lean Jetting applies to how the carburetor handles the altitude you ride at, weather conditions like temperature and humidity, as well as the jet sizes, needle shape/size, needle clip position, and carburetor slide shape inside your carburetor.

If you properly jet your carb for your location and weather, and pre-mix ratio of 32:1, you will produce little if any smoke, and your engine will make maximum reliable horsepower because your engine is receiving plenty of lubrication.

There’s a difference between Jetting and Mixture when talking about if a bike is rich or lean. Always remember that.

The key is to pick a pre-mix ratio (32:1 is best) and stick to it. Properly tune your carb jetting for your pre-mix ratio, your altitude and your weather conditions. If you do that, you’ll ensure long engine life and maximum power.

I have an 07 ktm 200 xc-w and it smoked like crazy when I first got it. I took it back, had it re-jetted, changed the ratio from 32:1 to 50:1 (ok by owner's manual) and it's been fine ever since (400+ miles). I use Maxima 927 synthtic smokeless oil. It only smokes when it's first started.

mandark1967 said:

Mixture – Rich and Lean Mixture applies to how much oil you mix with your gasoline. 32:1 is a richer mixture than 40:1.

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Last time I checked, more gas yields a richer mixture, so 32:1 would actually be a leaner mixture than 40:1... right?

Yes, but we like to mix our definitions around some.

Mandark1967,

You have raised a few good points, however it's just too confusing to propose that the term 'rich' can mean a relatively high concentration of either oil or gas. IMO, 'rich' refers to fuel/air and not gas/oil.

For me, 'rich' refers to fuel in the fuel charge delivered by the intake and (32:1) means 32 parts of fuel to one part of oil in the fuel tank. Otherwise, you will get endless corrections of the terms.

(edited for clarity)

In conclusion, it's normal for your 2-stroke to smoke a lot until you get on it and clear it out. It shouldn't take more than one trip through the gears to clear it out. Afterwards, there should be minimal smoke but you may still have some depending on conditions. If your single tracking around a lot, expect some of the smoke to come back. It shouldn't be billowing, but there will probably be a puff every time you get on the throttle while single tracking.

you gott a go woth il specs on mixing not manufactourers

You should follow the manufacturer's because the bike is jetted to run with that spec. Everybody wonder's why their 125's are jetted way rich from the factory. It's because they're jetted to run at 28:1 but everybody runs them at 40:1.

ktm300mxczz said:

you gott a go woth il specs on mixing not manufactourers

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You should follow the manufacturer's because the bike is jetted to run with that spec. Everybody wonder's why their 125's are jetted way rich from the factory. It's because they're jetted to run at 28:1 but everybody runs them at 40:1.

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I smell a debate :yeehaw:

Well, no debate. Just throwing some stuff out there. Go ahead and run it at 40:1 if you want. Honda says to run my CR250 at 20:1, no way I'm going to do that. That would be way to expensive. Run whatever mixture you want, but that's not why you're smoking too much.

76GMC1500 said:

Honda says to run my CR250 at 20:1, no way I'm going to do that. That would be way to expensive.

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Just out of curiosity, I did Exactly that after my last rebuild. Used 20:1 927 oil. Engine never ran crisper. Did smoke a bit more than normal. BUT, OMG, when I pulled the cylinder off, I have never seen a more perfect piston. It didn't have one bit of carbon, machine lines all still visible on the sides. It looked like it had been polished and oiled. Had about 10 hours of hard trial riding on it when I pulled the cylinder.

Well, so much for that experiment. It did get a bit expensive and hard to keep that much oil onhand. So I'm back to 32:1 and carbon-coated piston.

The only draw backs were a little more smoke -- and not really that much -- a little more spooge and cost/hassle of using so much oil. The benefits seemed to be a strong running and well oiled engine.

That elsinore is probably jetted to run at 20:1. Lean it out a tad and you'll probably have the same clean running at 32:1. Good quality oils also have lots of detergents and the more oil you run, the more cleaner you run through the engine. At 32:1, I get a bit of carbon on the crown, but none on the combustion chamber, in the ring grooves, or on the exhaust valves.

I've leaned this bably out as much as I dare. I can tell I'm running out of gas when I start to lose power, then engine starts to reeeeeeeeeevvvvv.
I believe those studies that show the more oil you mix, the more HP you can produce.

But it was quite expensive and a bit of a hassle. And I like rebuilding my engine occassionally.

Try running Amsoil instead. I started running Amsoil and mix it 50:1. My RMX 250 runs a lot better and it seems to have cleaned out my exhaust valve and it hardly smokes at all now once the engine warms up. I was using Moterx with a 32:1 but Amsoil works better. IMO

A lot of smoke coming from an old 2 stroke is usually a sign of a bad crankshaft seal. It will smoke more than usual because it's sucking tranny oil thru the old seal on the clutch side.

one other thing you need to check is your silencer. if it has really old packing in it, it has become soaked with oil. when the bike gets warm some of that oil is from the packing itself. ive seen cases where it actually caught fire after really leaning on a motor for an extended period of time.

if it is a seal going bad, you will notice a harder crustier deposit on your sparkplug. over time your oil level will drop. if the seal is really bad, the plug will foul.

havent you heard 2 strokes called "smokers" :laugh: but i have a question for yall. i have a 2003 kx125 and everyone is saying on here run like 32:1 or even 20:1. i run 50:1 on mine because thats what it said on the ams oil bottle. is that bad?

i couldnt really tell you if it runs any beter or worse because i just got it this year and dont have that much experience with dirtbikes yet.

There are many 2T oils on the market today that can allow the rider to use a higher concentration of fuel and still see adequate protection.
Synthetic esters and advanced additive packages have increased the integrity of 2T oils dramatically in the past 10 years or so. And when using one of these better made oils, you can run higher ratios of fuel/oil, such as 50-60:1
There are even products that could allow you to run a 2T at a mixture of 200:1 and still protect every bit as good as the conventional products do at 20:1...although none of these products are marketed for use as a 2T premix.

However, it is a fact that different motors will require a different rate of protection.
An AF500 will be fine using a quality oil at a 50:1 ratio, and with all it's abundant power, it is probably overkill to use more oil than 40-50:1
The smaller bore bikes require a bit more protection, due to their creating higher revolutions, and the style in which they must be ridden. 32:1 is probably the rule of thumb limit for 125's...although like I stated, there are products available that will protect at 40-50:1 even with the high revving engines.

Testing concludes that more power is produced with a higher concentration of oil in the mix. This may in part be due to better sealing of the rings/cylinder walls.
I use ~20-26:1 in all 2T engines.

No matter what, you should always pick the ratio suited for the bike and style of riding, and only then start to dial in the carburetor to find the optimum mixed fuel/air ratio.

On the issue of bikes being jetted for certain ratios at the factory...
The settings on factory bikes are determined by an average, as there is no way to tell just what conditions the consumer has. Is he at sea level, or 10,000 feet? Is the buyer riding in the desert or in the jungle?
See, the mfg has no way of knowing and providing a catch-all setting. So an average is computed, and that is what we have.
It is not really that they jet the bikes for a particular ratio. That simply doesn't make sense, if you understand the dynamics involved.

Thank you gentleman... I learned a lot reading this thread. (tips his hat in your direction(s))