quick jetting question

kdxracer577 · Dec 15, 2002

i have a 2002 honda cr 250 and a 2000 kdx 200 that i run at 32:1. i plan on tryin a 40:1 in both and was wondering if they will run richer or leaner. thanks

CJ Rider · Dec 16, 2002

I also changed from 32:1 to 40:1 recently. The only thing I've noticed is that I get a little better performance and a little less spooge coming out of my pipe, so it's now running a little leaner. However, I haven't had to change jetting at all from the look of my sparkplug. So to answer your question... leaner, but not by much.

darringer · Dec 16, 2002

Since the carb flows a certain amount of atomized fuel through the jets, they will run RICHER, because you have more gas and less oil flowing through the carb at any given throttle position.

luvtolean · Dec 16, 2002

Originally posted by darringer
Since the carb flows a certain amount of atomized fuel through the jets, they will run RICHER, because you have more gas and less oil flowing through the carb at any given throttle position.

This is correct, because there is less oil in the fuel, it's viscosity drops; it's thinner. Thus, more can flow through the jets making the bike run richer.

Maybe you noticed less spooge because it was colder, or you were riding at lower elevations??

ryanfoley · Dec 17, 2002

i have always thought that running richer only depended on the amount of oil not gas.

Roland C · Dec 19, 2002

The following is from Canadian Dave's JustKDX site (http://justkdx.dirtrider.net):

"Before we get into the different parts of the carburetor and how they effect gasoline delivery I want to stop for a second and define the terms RICHER and LEANER. I know these terms can cause some trouble for those who are new to the sport or new to carburetor tuning and they are often used incorrectly. The terms RICHER and LEANER refer to the amount of GASOLINE being delivered to the engine and not the amount of oil. If you’ve done a plug reading at wide open throttle and the plug indicates you are running rich ( dark brown to black ) this is an indication that too much gas is being delivered to the engine and not too much oil. I know there are people that will say "You’re running too rich, try to change your premix ration from 42 parts gas: 1 part oil ( 42:1 ) to 50 parts gas : 1 part oil, that should lean things out a little ". This is in fact increasing the amount of gasoline ( 8 more parts of gas for each part of oil ) and causing the engine to run RICHER rather than leaner. If you remember richer and leaner are referring to the amount of gasoline being delivered this will all make much more sense."

Hope that helps!

jboomer · Dec 19, 2002

I know I've read that probably 50 times, but what I don't understand is: because there is less oil in the fuel then MORE of it will pass into the engine at a given instant...therefore, although less oil is in each part of gas there is so much more fuel entering the engine that it actually allows MORE oil into the engine at that instant (along with the increased fuel)? This has to be true because gas alone won't turn a plug black and goopy or cause your engine to spooge. If the 50:1 is richer than 25:1, then say 100:1 is even richer than that? It can only hold true up to a certain point because the jet will only allow so much fuel to pass through it in an instant. So, although the bike would technically be running richer, you would be getting less lubrication to your rings, etc. So are you still rich or are you lean?

That's why none of this makes sense. I've tried to just accept it as truth, but it has literally kept me up at nights thinking about it. Maybe someone with a bigger brain can shed some light on this. I only ask because I've never seen anyone challenge this theory.

CJ Rider · Dec 19, 2002

Interesting. I'm with ya'll on the "richer" being a gas thing and not an oil thing. I went with the 40:1 to alleviate an exhaust spooge issue, not to address a rich/lean issue, so I was just guessing. However, now I'm wondering why it worked? No change in temp nor altitude and I didn't have to re-jet. What happend?

darringer · Dec 19, 2002

It worked because there was less oil overall to burn in the mixture. Therefore, you are going to get less spooge. It is only possible to pull a certain amount of premix through a jet at any set rpm, so when you lessen the amount of oil by raising (numerically) the ratio (32: 1 to 40:1), more gas has to be drawn through the jet to compensate for the reduced amout of oil. So, you get less spooge with a richer condition.

jboomer · Dec 19, 2002

So what most people (including myself) consider a rich condition (minus the plug chop -- strictly speaking from a spooge point of view) is not really a rich condition but rather too much oil in your premix? So, with that in mind, if my jetting is pretty much spot on, I could run a richer FUEL mixture to cure the spooge issue? It's starting to make more sense now. I've always assumed that smoking and spooge were rich issues and there wasn't a difference between a jetting issue and a fuel mixture issue. Thanks!

bermcon · Jan 16, 2003

I think people are talking about two different things. there is pre mix ratios which is gas to fuel ratio and there is the fuel to air ratio determined by carb jetting if the ceramic is to dark you need to leen out your main jet

Papakeith · Jan 16, 2003

I think you are right. There are two conversations going on about two different topics trying to make sense out of each other.

If my .02 is worth anything, this is how I would proceed.
The oil fuel ratio should be decided upon 32:1 or whatever you want to run. This is then set stop worrying about it.
Now We only have to deal with the air/fuel ratio. Richer would be too much fuel. Leaner would be too little fuel for a given amount of air.
Jet accordingly.

You can tweak your jetting by adjusting your gas/oil ratio. But, I think the preferred method would be to set gas/oil and jet for air/fuel.
BTW, kdxracer577 I believe that by going from 32/1 to 40/1 you are in fact Richening your air/fuel mix.

Someone else might want to verify this for me, but I was under the impression that jets didn't meter by viscosity, but rather by volume. Is this correct?

MrLuckey · Jan 16, 2003

Originally posted by jboomer
therefore, although less oil is in each part of gas there is so much more fuel entering the engine that it actually allows MORE oil into the engine at that instant (along with the increased fuel)?

jboomer - I could be wrong but I think that is where you are getting confused. Changing from 32:1 to 40:1 is NOT going to significantly increase the amount of premix flowing through the carb. I believe the previous comment about the viscosity changing and "allowing a significant change in fuel flow" is a little off base.

I am fairly sure that you would still have 'pretty much the same amount' of pre-mix flowing at the same throttle position.

jboomer · Jan 16, 2003

So, same amount of premix=less oil and more gas right? More prone to plug fouling (like dumping a bucket of gas on a match...it'll probably extinguish the fire instead of exploding) but less oil to cause smoke and carbonization? It would be rich with fuel, but possibly lean of oil (if really high ratio 50:1 or 100:1)? See, I am EXTREMELY confused!

MrLuckey · Jan 16, 2003

In my mind your ref to dumping a bucket of gas on a match is probably a fairly good analogy. On the subject of spooge, keep in mind that if you are too rich, you are probably spewing unburnt fuel AND oil out the exhaust , so even though you have less oil in the premix, its not all combusting properly and thereby causing a spooge problem.

Hopefully Rich will stop by and either confirm or deny.

jboomer · Jan 16, 2003

That makes sense "unburn fuel AND oil" coming out of the exhaust. I'm just trying to figure out how to control the spooge. If jetted properly, then you shouldn't have spooge (jetted properly, running the ideal premix ratio, and the correct plug heat range). But, I see time and time again someone claiming their bike is jetted properly, yet they still get spooge and others saying this is normal. If you are jetted correctly, but still getting spooge, then couldn't your gas to oil ration be wrong? If you're running 32:1 and bike is jetted correctly, but still spooging, couldn't you run 40:1, jet for this ratio and possibly eliminate spooge? What if you are running 32:1, bike's jetted right, and you're getting spooge; could a hotter plug eliminate the spooge? Could spooge be a product not of being too rich, but having too much oil in your ratio?

I'm focusing on spooge, but it all ties in together.

Rich Rohrich · Jan 16, 2003

Originally posted by TTRGuy
On the subject of spooge, keep in mind that if you are too rich, you are probably spewing unburnt fuel AND oil out the exhaust , so even though you have less oil in the premix, its not all combusting properly and thereby causing a spooge problem.

Exactly. Spooge is just an indication of incomplete combustion for whatever reason. While the oil ratio can certainly influence this it is RARELY the root cause. Jetting (and with it engine load and heat), fuel properties, like the heavy low volatility components in pump gas that won't vaporize, and oil properties (not quantity) are the most common culprits in spooge. If part of the mixture (oil & fuel) that enters the chamber can't be burned it will end up in the exhaust, simple as that.

As for premix, maybe this wil help some. A jet is just a fixed size orifice that flows a fixed VOLUME of fluid (lets call it 10cc) . If you had a 1:1 ratio 5cc of fuel and 5cc of oil would pass through that jet under a standard pressure. If the ratio were changed to 2:1 then 6.6666 cc of fuel and 3.333 of oil would pass, but it would still be 10cc total. This of course ignores the influence of viscosity, but when we are talking about the difference between 32:1 and 40:1 viscosity has such a minor influence that it's easier to ignore it for the sake of clarity.

In short. As you run LESS OIL in your premix the same jets will pass MORE FUEL all else being equal. Call it rich call it lean call it sally for all I care. I'll leave that idiotic semantic arguement to someone who gives a damn. I'd rather TUNE :confused:

jboomer · Jan 16, 2003

You say "with engine load AND heat." So, if you're woods riding (low engine loads) and the bike is jetted properly, you can correct a spooge problem (gas and oil) by raising the heat within the cylinder? With a hotter plug right? So, since I'm running a BR8ES, I can go to a BR7ES and eliminate at least some of the spooge? This wouldn't affect my jetting would it?

jboomer · Jan 16, 2003

RR, I'd rather get it right so I CAN RIDE!

Rich Rohrich · Jan 16, 2003

Originally posted by jboomer
You say "with engine load AND heat." So, if you're woods riding (low engine loads) and the bike is jetted properly, you can correct a spooge problem (gas and oil) by raising the heat within the cylinder? With a hotter plug right?

NOPE, but that's the common misconception.
Going to a hotter plug will raise the operating temperature of the plug insulator not the operating temperature of the engine. So the nose of the plug will stay cleaner and help prevent fouling, but it won't change the overall combustion characteristics other than to lower the number of cyclic misfires that come with a deposit laden plug insulator.

Light loads with low heat is a tough situation in a two-stroke. The scavenging is so erratic that you aren't likely to ever get it 100% spooge free. Using an oil designed for these lowered temp situations, jetting really sharp, and using fuel that will vaporize COMPLETELY at these lower temps (pump fuel WILL NOT BTW) will go a long way towards keeping spooge to a minimum.

jmics19067 · Jan 16, 2003

Rich,

While you are in this thread could you explain what happens to the oil in the premix. My understanding is that when the premix fuel air mixture enters the crankcase the oil drops out of the mixture to lubricate the crank and piston. Then mostly straight gas/air towards the combustion chamber produce power. With everything else being equal a premix of 20 to 1 would have more oil to lube the bottom end and less gas to burn compared to 40 to 1 which would have less oil to lube and more gas to burn. This is how I explained the leaner premix ratio eguals richer fuel air ratio to myself. Would this <in all simplification> be a decent way to start understanding what is going on inside my engine?

Also what happens to the oil? I believe the oil haphazardly gets thrown up into the combustion area with no real purpose except to hopefully burn clean enough not to carbon or gum up the top of the piston or powervalve. After the fact of lubrication is this one of the major purposes of bike specific two stroke oil compared to what they sell at the Lawn & Garden supply store.

MrLuckey · Jan 16, 2003

Some good questions there Jmics. I would have thought that the crank would get its lubrication from the bottom end though. Would it not have to get down past the rings to lube the crank?

jboomer · Jan 16, 2003

Bottom end meaning the fluid your clutch is swimming in? NO. It would hold more than a quart of fluid if your bottom end used the same oil. Good question! How is the bottom end lubricated? Excess oil sliding down the cylinder walls into the lower end? Sheesh....I can't study for all the questions being asked and no answers!

Faded · Jan 16, 2003

Originally posted by TTRGuy
I would have thought that the crank would get its lubrication from the bottom end though. Would it not have to get down past the rings to lube the crank?

If it's getting past the rings then you have a problem ;)

If I'm not mistaken...As the piston rises it draws in an intake charge. As the piston moves down in its stroke the piston skirt seals off the intake port. The trapped intake charged is forced down into the cases by the piston. This is where the lubrication part is taking place. At the same time the piston is moving down it is exposing the transfer ports (on the top side of the piston). The intake charge is pushed from the cases (bottom end) through the transfers where it ends up on the top of the piston. After BDC and the piston starts traveling upward it seals off the transfers and starts compressing the intake charge to be fired. In a 2 stroke the top of the piston never travels down far enough to openly expose the intake port.

The bottom end of the motor (crank, crank bearings, rod bearings, piston pin bearings) is lubed by the oil in your premix. Your clutches are in the same case, but sealed off and have their own dedicated oil cavity.

jmics19067 · Jan 16, 2003

Originally posted by Faded

In a 2 stroke the top of the piston never travels down far enough to openly expose the intake port

Although every bike "I" have seen this would be true but the Detroit Diesel 2 stroke does.

That would be a completely different animal though since it is supercharged and has camshaft operated exhaust valves. Not too go off ona different tangent but I thought you would like to know

quick jetting question

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