2-Stroke basics

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2-Stroke basics

Post by canyncarvr »

How about this?

What are the chances that a 2-stroke can breathe so well as to somewhat negate a properly operating scavenging effect?

Consider the intake stream as a measurable column length of 'fuel'. If the bike takes in six inches of air and the scavenging effect puts back only five inches of that column...you have 1" of raw 'fuel' sitting in the pipe.

Sounds like a mess waiting to happen, don't it? Kind'a like my bike! :cry:

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Post by fuzzy »

Could be....never really thought about that much...
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Post by Green Hornet »

Look up how a Super Charger works, since the POWERBAND HIT on a 2 stroke is similar to the HP INCREASE of a SUPER CHARGER. I'm curious about that question tooooo.
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Post by canyncarvr »

A supercharger is pushing against a (usually) closed exhaust valve.

The question is in regard to continued severe drips I have out of my SA and pipe/SA junction....and KIPS actuator..and everywhere else possible. It's not a too rich situation. In fact, the richer I jet (to the point of a good bit of loss of power) the less it drips. Although not an indication of correct jetting in the current situation, the jetting I'm running happens to be the same I had before the cylinder was ported, and it ran clean as a pin. Powder dry.

I've been lean enough to lean-roll stick this cylinder..more lean is not the answer.

I repeat that because if you saw my bike, you wouldn't think I had a clue what a 'jet' is. Maybe something flying in the air???

My bike has a problem due to either porting or plating. Not sure which. It's not jetting. Still, it's quite curious. The ram effect part just occurred to me as a possibility.

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Post by fuzzy »

The fact that the scavenging seems to get better the richer it gets is very odd....
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Post by m0rie »

If you think its breathing too well why don't you restrict it a bit and see if it improves? A couple of washers with different sized holes at the pipe to cylinder junction would let you experiment with restricting the flow different amounts.

*edit* Rereading your first post I wonder if you super soaked your filter with oil (vs. lightly oiling) if that would negate some of that breathing ability and get things back to normalish?
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Post by dave04kdx »

Is it possible that something changed in the KIPS system when the cylinder was repaired? Height of exhaust ports??

Maybe the KIPS actuator is causing the valves to be open or closed too long?

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Post by canyncarvr »

That's a good thought. I'll try soaking a filter real good..see what heppens. Some plastic wrap covering the airbox holes is another possibility.

I've repacked the SA 4-5 times in the past few months, btw.

I don't know that scavenging changes with it being richer..but it drips less.

Recall the thread with the pic of the fender and the spark plug? Fender dripping black, plug stark white.

re: KIPS change.

The detonation I got with the cylinder change blew out a subport drum..that was a 'change'. But it's changed back, now. Ha! It idled MUCH better with the subport drum missing, btw.

The exhuast port was not part of the porting job. It wasn't touched.

I have another cylinder. I could know right quick that a non-ported cylinder would get me back to 'normal'. Well, I still question all the holes in the plating. I'll send my ported cylinder to Langcourt to see if they can fix it before I give up on it.

I may well be missing something...but I don't see it. ..uh..guess that would be the missing it part, 'eh?

I've been up and down all over the place in every circuit in the carb. I've changed fuel and mix ratios. I guess it's time to wonder about it all less..and get it done. Put on my OEM cylinder and send the ported cylinder to Langcourt.

I'm a procrastinator! :oops:

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Post by Rhodester »

I've quit worrying about my spooge. After a good hard ride the trail of spooge runs clear back up the outside of the silencer (FMF Q) to the front screw (upstream-wise). I don't consider this too bad, but I can't seem to be able to have good power everywhere and no spooge at the same time. I'll take the good power....
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Post by Indawoods »

CC... ditch that ported cyl on eBay! :grin:
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Post by Mark W »

I think it is that rare air that you have in that mythical State of Jeffersonia that is your problem. If you had your bike here in Minnesota with that good old "Land of 10,000 Lakes" air going into the cylinder, you could run whatever jetting you like and not get any mess.

Maybe you need one of those "swirlers" in between the carb and cylinder. How about swapping out the air stryker and putting on a different carb? Doesn't the air stryker behave slightly differently than the OEM carb. More breather hoses I know but since I have never seen nor hooked one up, can't speak too intelligently about them.

I'm with Inda, me thinks the real problem is in that ported head.

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Post by IdahoCharley »

CC wrote "Consider the intake stream as a measurable column length of 'fuel'. If the bike takes in six inches of air and the scavenging effect puts back only five inches of that column...you have 1" of raw 'fuel' sitting in the pipe. "

I assume you realize that is exactly what is suppost to happen - correct? :neutral:

FISHEAD wrote this up back in Dec of 03. I think it was a year or so after he went to KTMs but long timers should recognize his name since he was very instrumental in helping CC, JD, and RB Designs come up with the current jetting used in the RB'd carb modification. Procedure has been written up quite a few times and seems to have come from the people associated with Cart/Kart racing.

Throttle position jetting utilizing wet oil line dimension

“Give a man a fish and he eats for a day. Teach him to fish and he’s likely to get into the fishing business and feed hundreds.” It’s apparent that there is a need for some basic info to get started jetting bikes based on the number of comments that start with “How do I?”

This is a technique for optimizing jetting at various throttle positions utilizing the wet oil line boundary measurement in the exhaust port of 2 stroke engines. Since fuels, oils and riding techniques and conditions vary widely throughout the world a method of optimizing jetting for specific conditions would be useful. Plug reading has benefit primarily at wide open throttle positions and readings can be skewed by oil choice, fuel additives and plug heat range. In addition a reliable way to evaluate jetting at part throttle positions is helpful. Ideally data acquisition equipment would be best but it may beyond the resources of the enthusiast. It also may be too fragile to be used in an off road environment. Additionally the lag time between rider noticing changes and reacting may be too slow to prevent serious engine damage. The ride and feel method is widely used and those who are adept at that type of jetting may find this technique will allow a more accurate assessment of the jetting package. With a minimum amount of fuss it is possible to pull the pipe after riding a particular section to evaluate your jetting under those specific conditions.

The wet oil line is the boundary between the hot dry exhaust gas and the fresh charge in the exhaust port. Since two stroke engines utilize exhaust port supercharging with expansion chambers the fresh charge is drawn through the cylinder into the pipe and stuffed back into the exhaust port by the return pressure wave. The speed of the pressure wave is affected by the temperature of the exhaust gas. When the mixture is too lean the hot gas enters the combustion chamber and huge pressure rises or detonation occurs. If the mixture is too rich the return pulse is weak and lost power results. Fuel and oil choice as well as timing affect speed of the pressure rise during combustion so those variables should not be changed until an adequate baseline is established.

The JDJetting .com guide is an extremely valuable tool to aid in making adjustments to the fuel curve. James provided invaluable insight into the nuances of altering the fuel curve. HGNR.com enthusiastically provided the basic information and is a great resource for those wishing to delve further into the subject. Oil choice is a key component to the success of this procedure so you don’t want to overlook the value of clean burning oil.

Getting started

Fresh fuel, oil and silencer packing are a must. Enough room to accelerate up to 3rd or 4th gear at ¼ throttle increments and hold till speed stabilizes is necessary as well. The engine should be brought up to temperature and several hard acceleration runs made to duplicate racing conditions.

Step 1) Quarter throttle

With the engine hot at normal tempeture accelerate up to 3rd or 4 gear and hold the throttle at the ¼ throttle position till rpms stabilize for several seconds. Pull in the clutch as you hit the kill switch and hold the throttle at ¼ till the engine dies. After stopping pull the pipe and look at the port and head pipe. You should see a dry area in the head pipe and a wet area closer to the piston. The safe dimension is 2-1/2 to 3 inches from the piston skirt. If it is closer than 2-1/2 inches you will want to richen the jetting, more than 3 inches you may want to lean it slightly. Pilot jet, needle straight section diameter and clip position all have an effect in this area. If you are using multi taper needles you may want to test the next increment before making a change. With single taper needles clip position has the greatest effect so you may want make that adjustment first. With ¼ throttle sorted out which may take several runs move to step 2

Step 2) Half throttle

Get the engine hot again and repeat the test in the same gear as step1 holding throttle at ½ open. Check the wet oil line dimension and make adjustments to the clip position as necessary to get the wet oil line to 2-1/2 to 3 inches. If your ideal clip position is different from the ¼ throttle you will have to make adjustments to straight section diameter and pilot to get ¼ throttle in line. If you are using multi taper needles it may be necessary to change pilot and or throttle slide cutaway to adjust ¼ throttle readings.


Step 3) ¾ Throttle

Repeat you basic procedures but this time using ¾ throttle. Needle taper is the main variable but main jet will have an effect. You may change taper or get the main optimized and return to this section


Step 4) Full throttle

This test is about the main jet. With the main jet right you will be surprised at the overev hiding in your engine. You will want this test to closely resemble your riding conditions as much as possible. If you are riding in the woods you are likely to be on the main for a few seconds at a time or longer if you are climbing a hill. Desert riders may be on the main for much longer periods so your test should be as close to real riding conditions as possible. Sand riders will use larger mains still.


Danger signs

Knocking, pinging, rattling, overheating are signs that you are lean, too lean, and damage is occurring rapidly and in a big way. Signs of scuffing on the exhaust side of the piston or a bright stripe from the crown of the piston to below the ring lands or a horizontal smile on the skirt are signs that you are lean somewhere. You may find this occurring as you make changes to your fuel curve going thru the steps so you will have to back up and correct your jetting before you go further. The attached image shows a bright stripe resulting from a lean condition at 1/4 throttle.
http://ktmtalk.com/addons/photogallery/ ... 030141.JPG

Sound advice and other considerations

Like all empirical methods this techniques has limitations. In conjunction with good notes and observations it will make a higher level of performance possible. Ignore temperature, barometric pressure, altitude or make fuel, oil and timing changes and your results may be expensive failure. It is prescriptive in that it measures what is happening and tells how to improve your state of tune. Proceed cautiously and you will reap the reward of better performance. The increments used are arbitrary so you may use any increment that makes sense to you and can give repeatable readings. Once you are correctly jetted you may find yourself riding a gear higher and getting better fuel milage as the result of improved efficiency.

James Dean of JDJetting.com Stuart and Andy of HGNR.com and Rod Faulker of Dumonde Tech racing oils have provided an invaluable amount of encouragement and help. Their help has made this bit of information possible and we all benefit.

Good Jetting!

Fishhead

This post has been edited by Fishhead on May 1 2004, 02:14 AM

CC- Of course this is referencing as to a jetting aid and the 2 1/2 - 3 inches is the length of fuel column being pumped into and out of the exhaust pipe. But with proper jetting and doing a coventional plug chop with closing the throttle you should still have a "wet" fuel mixture area in the first 1 or so of the exhaust pipe. This is what prevents the smile on the surface of the piston facing the exhaust port (indication of being lean).
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Last edited by IdahoCharley on 11:45 am May 10 2006, edited 2 times in total.
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Post by Colorado Mike »

The more I learn, the more I know I don't know... :blink:
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Post by canyncarvr »

I've read the 'wet line' method, wasn't thinking of it when I started this thread, hence I used misleading figures.

Yes. It is supposed to be that way (wet). All of the above write-up has to do with jetting and how to use the wet-line observation method to check it (jetting).

My question was (and still is) to what extent the breathability (sic) of the 2-stroke engine has an effect on that wet line. I understand the wet line to be symptomatic of jetting, correct or not, but that does not preclude the possibility of other factors also having an impact.

Plug reading does tell you a lot about jetting, timing, etc of an engine. BUT...it's all out the window if you're looking at an incorrect heat range plug.

Likewise (possibly) a modified cylinder complete with a pitted plate job will result in a too-deep wet line...even if the air/fuel ratio is perfect.

While I can't say for sure to what extent the ported cylinder is superior to an unported cylinder (it was not installed independent of other changes), I doubt I want to summarily off the thing.

All of it has answers...I'm merely pondering some possibilities. And, as said, I could get those answers if I spent a lot more 'quality time' in the garage.

Yeah...well....... :roll:

Thanks for the info, IC. Good stuff!!

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Post by fuzzy »

Well, there is one thing.....One ported cyl can/will be better than another ported cyl depending on application. RC's CR250 cyl installed on your woods-friendly CR250 would surely result in an absolutely worthless configuration for woods riding regardless of what's in the carb...
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Post by IdahoCharley »

If I were you CC I would also be pondering the spooge problem.

I don't have an answer and while I did not like the looks of your plating it seems difficult for my little brain to comprehend how the cylinder's poor plating could contribute to your spooge woes. I thought when you first found the sub-port drum - you found your problem - but this was not to be the case. Maybe (tongue in cheek) you should remove both sub-port drums since it ran better with one out of the way; it would likely "sing" with both removed. :grin:

I just don't see how you can stuff the engine with so much fuel that your running the engine lean and the pipe is spooging from the excess fuel. Not saying it is not happening - just I can not comphrend how it is happening.

My only other thought was your crankcase seal on the main gear side would be leaking contributing transmission oil to the fuel mixture and spooging your pipe but then you would be seeing a drop in the transmission oil and of course when you kissed the exhaust pipe goodnight - it would/should smell different. :lol:
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Post by canyncarvr »

>|<>QBB<
IdahoCharley wrote:If I were you CC I would also be pondering the spooge problem.

I don't have an answer and while I did not like the looks of your plating it seems difficult for my little brain to comprehend how the cylinder's poor plating could contribute to your spooge woes.
USChrome referred to the holes as good places for oil to 'live'. Maybe it lives there until it gets blown out the pipe!

Obviously I have no clue myself. Anyone looking at the situation would certainly have reason to doubt the 'facts' of the whole mess (pun intended) as I've presented them.

IC wrote: I just don't see how you can stuff the engine with so much fuel that your running the engine lean and the pipe is spooging from the excess fuel. Not saying it is not happening - just I can not comphrend how it is happening.
Me neither.
IC wrote: ...it would/should smell different. :lol:
Oh, it smells plenty! With the toluene it's downright nasty.

OBviously the reason I'll be bringing up the rear in a couple of weeks down south! It's only my concern for other's well being and happy riding.

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Post by Matt-itude »

how about a different pipe in regards to the theory of returning exhaust pulse speed. The only other thing I come up with is a rich condition in the jetting somewhere that allows a build up in the pipe that is blown out (not burned) when you run it hard, yet doesnt show with a plug check. If this spot were in a throttle position that didnt get checked by plug chop it wouldnt show on the wet oil line using the same throttle positions. just my .02 good luck.
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Post by canyncarvr »

I have a -35 FmF. Have thought about putting it on just to see what happens. That involves another round of jetting to suit the pipe...and frankly I'm getting weary of that.

The bike runs so well, I've bracketed the jetting I have to satisfy myself that what I ended up with works well, it seems to me the jetting is correct. It's not a 'too rich' situation (imo), but too much of a correct mixture in the pipe, not getting burned.

That's what brought the question to mind in this thread.

I've read about the wet-line method of jetting...can't say as it's clear to me how the pressure waves of the pipe reflect that. Seems the pipe volume and cone shapes/sizes (and heat) would determine the wet-line. Well, yes I can say..that it's not clear to me at all! :wink:

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