87 zilla.  Hpr bored and modified 87 TM38 carb.  In Michigan.  Worked great this past summer or I didn't notice the problem.  After a long hard run or some aggressive riding, when stopping and letting the engine idle, it takes about 5 seconds to get to the proper idle speed.  It does not rev super high, just seems to come back to idle real slow.  It had a 25 pilot in it for the summer and when it got cold it idled really high. So I turned the air screw in to 3/4 turn out and that helped for the all the time high idle (assuming that was from the winter temp change).  SO I put a 30 pilot in it now and have the air screw at 1 1/2 turns out and it is still hanging for about 5 seconds after a long hard run. I can't even get to the air screw on the carb without loosening the carb and turning it due to the elka rear shock on it.   Is this normal or do I have a problem?  Quad starts ok and idles for as long as you let it.  Maybe I am being too picky.  It is very consistent and does it every time. 

maybe fatten up the main it sounds lean. when the motor just kinda hangs many times it want more fuel. you may want to raise the needle also.

87 zilla.  Hpr bored and modified 87 TM38 carb.  In Michigan.  Worked great this past summer or I didn't notice the problem.  After a long hard run or some aggressive riding, when stopping and letting the engine idle, it takes about 5 seconds to get to the proper idle speed.  It does not rev super high, just seems to come back to idle real slow.  It had a 25 pilot in it for the summer and when it got cold it idled really high. So I turned the air screw in to 3/4 turn out and that helped for the all the time high idle (assuming that was from the winter temp change).  SO I put a 30 pilot in it now and have the air screw at 1 1/2 turns out and it is still hanging for about 5 seconds after a long hard run. I can't even get to the air screw on the carb without loosening the carb and turning it due to the elka rear shock on it.   Is this normal or do I have a problem?  Quad starts ok and idles for as long as you let it.  Maybe I am being too picky.  It is very consistent and does it every time. 

 

A momentary high idle and or the engine is taking too much time to return to a slow idle is one of those undesirable characteristics of a two stroke, especially large displacement single cylinder engines.  A two stroke engine that is doing the ring-da-ding-dings before returning to an idle is usually lean while it is doing it's ring-da-ding-dings.  The phenomena that causes this undesirable problem is complex but I will try to explain it.  I hope this does not get too long and boring.

During periods of high RPM wide-open throttle, all of the surfaces inside of the crankcase tend to "dry out".   These surfaces inside the crankcase are comprised of the ports in the cylinder, crankshaft, connecting rod, all of the bearings, reeds and surfaces surrounding the crankshaft.  All of these surfaces are wetted by a very thin layer of fuel and oil.  The degree of wetness depends upon the velocity and time the air and fuel mixture has been flowing over these surfaces. 

The thickness of this layer of fuel and oil on the crankcase surfaces are thickest at an idle and thinnest at high RPM wide-open throttle.   As an engine is decelerating and the throttle is closed, the velocity of the air fuel/oil mixture flowing over the crankcase surfaces, decrease in proportion to the RPM.  The layer of fuel and oil becomes thicker as the RPM decreases.  It takes anywhere from 25 to a few hundred engine revolutions for the layer of wetness to reach its saturation/stabilization point.  The saturation/stabilization point occurs when the layer of wetness will not accept any more fuel or it cannot get rid of any more fuel on the crankcase surfaces. The saturation/stabilization levels change and depend upon the RPM and throttle position.  The RPM and throttle position controls the velocity of the mixture moving over the surfaces inside the crankcase.   When saturation/stabilization of these surfaces occurs, all of the fuel the carburetor is putting into the air stream makes it to the combustion chamber.

A two stroke engine can have three different air fuel ratios that end up in the combustion chamber at a given throttle position and RPM without changing anything in the carburetor

When any two-stroke engine begins to accelerate after the throttle is opened, the thickness of the wetted surfaces inside of the crankcase starts to diminish.  As the thickness of the wetted crankcase surfaces diminish, fuel is being lost from the wetted crankcase surfaces and is added to the mixture flowing through the crankcase.  A rich condition exists during this period until stabilization occurs.  Anyone that has ridden a two stroke has experienced this period of stabilization.  The engine will often misfire after opening the throttle and the engine RPMs are increasing.  As the engine gains RPM and after a few seconds the engine begins to run without any misfires occurring.   While the engine is reaching stabilization, it is often referred to as “the engine is cleaning out”. 

Before the wetted surfaces in the crankcase stabilize and the engine is accelerating, the mixture reaching the combustion chamber will be richer than what the carburetor is metering into the air entering the engine.  This is air/fuel ratio # 1.  After the crankcase wetted surfaces stabilize, the air/fuel ratio that is currently being metered by the carburetor makes it's way to the combustion chamber.  This is air/fuel ratio # 2. 

When an engine is decelerating and the throttle is closed or almost closed, fuel is being lost from the mixture traveling through the crankcase and is added to the wetted crankcase surfaces.   Crankcase mixture continues to loose fuel to these wetted surfaces until they have stabilized causing a leaner condition than what the carburetor is currently metering. This is air fuel ratio #3.   

Once we understand the above phenomena, we can see why an engine may run different from one minute to the next without making any jetting changes to the carburetor.  A carburetor meters fuel according to the air velocity passing over the fuel circuit’s discharge hole.  A carburetor cannot determine whether the engine has been running at a steady RPM and throttle position for one second or 10 seconds or is accelerating after idling for 1 second or 10 minutes.  We do not have a way to make the carburetor compensate for how the crankcase surfaces can alter the mixture as it travels from the carburetor to the combustion chamber. 

As we increase the power of a two-stroke engine, the precision of the mixture delivered to the combustion chamber becomes more critical.  A carburetor meters an air/fuel ratio controlled by the jets and needle at different throttle positions.  The crankcase surfaces are always adding or taking some fuel from the crankcase mixture as it flows from the carb to the combustion chamber.  The safest most common tuning technique for high performance two strokes is to tune them for the worst-case situation, which is to them a little rich. 

We all have expectations of having an engine that runs like a top fuel dragster one minute but can idle like a sewing machine for hours.  Four-strokes can do a much better job of running clean and crisp at any RPM and throttle position.  This is due to the fact they have a very efficient scavenging cycle and have much less wetted intake port surface. 

Two-strokes have very inefficient scavenging cycles as compared to four-strokes but have a power stroke every engine revolution.  These two characteristics make two-stroke engines prime candidates for high performance application when competing with four strokes of equal displacement.  There is always some sacrifices we have to make when tuning carburetors on high performance two-strokes.  When we get the carburetors tuned to provide good throttle response and a mixture that is not too lean at high RPM and partial throttle, the engines usually do not always return to the same idle RPM and or not want to idle more that 10 seconds or so before loading up and or dying.  If we tune the carburetor on a high performance two-stroke so that it will idle for 10 minutes without loading up the crankcase, it will usually be too lean during high RPM closed throttle deceleration.  If we size the needle jet so that it pre-wets the crankcase surfaces as the engine returns to an idle RPM it will be too rich at around 1/8 to 1/4 throttle when riding at a RPM that is "off the pipe"

Nathan:

As you know, cold weather requires richer jetting. Initially, I would increase your needle jet diameter 2 to 4 steps.  If you have a R-2 389 needle jet, I would richen it to an R-4 or R-6 and a 30 to 35 pilot jet for the cold low humidity during winter. You may need to lean the pilot as the needle jet is made richer as well as lower the throttle stop/idle speed to get an acceptable idle.  Remember more air will flow under the same slide stop setting with cold dense air.   

The Mikuni and Keihin tuning manuals show charts on what circuits are responsible at different throttle position and steady state running.  They usually show the needle jet having an effect from about 1/8 to 1/4 throttle position.  Manufactures charts do not show the needle jet flowing any fuel when the throttle is closed and the RPM is high.  The needle jet does flow fuel at closed throttle when the RPM is sufficiently high as well as the two transition holes that are feed by the pilot jet.  At high RPM there is sufficient air flow over the needle jet to cause fuel to flow when the throttle is closed.  As the RPM continues to decline the fuel flow from the needle jet ceases and the engine is getting all of it's fuel from the two pilot jet fed transition holes that are located on both sides of cair flow controlling edge of the slide.  As the RPM continues to further decline to an idle RPM, the transition hole between the slide and the needle jet ceases to flow fuel and the transition hole on the high vacuum side of the slide is supplying the fuel to the engine at a very slow idle.

As the throttle is slowly opened, the transition hole between the needle jet and slide starts flowing fuel.  As the throttle is opened further the needle jet starts flowing fuel controlled by the area between the needle jet and the needle.  Opening the throttle further, the taper of the needle is controlling the fuel flow between the needle and needle jet.  As the throttle opening is increased further the area between the needle jet and needle taper becomes larger than the flow area of the main jet.  At this point the main jet is controlling the majority of the fuel flow to the engine.  The pilot jet continues to flow fuel at wide open throttle but the amount is small as compared to the amount of fuel flowing through the main jet

I think your engine needs a little more help from the needle jet to help start wetting the crankcase surfaces as the RPM is returning to an idle RPM after a hard WTO hard run.  The wetting process needs to occur before the engine gets close to its idle RPM.  The crankcase surfaces during your wide-open throttle hard runs; stabilize at a "dryer" level than what they will be after 5 to 10 seconds of idling.  Your pilot jet may be set to give the engine the proper mixture to idle but some fuel is being robbed from the mixture traveling through the crankcase on its way to the combustion chamber.  The fuel that is being robbed from the mixture traveling through the crankcase is being used to return the wetted crankcase surfaces to idle saturation levels.  The engine is often lean while the wetted surfaces are being replenished to an idle saturation level.  It appears it is taking about 5 seconds for your crankcase surfaces to stabilize at idle saturation level.   If the pilot jet size is overly rich it will usually help the condition you are experiencing but may cause the engine to load up and want to die after a short period of idling.

Four-strokes also have the same problem I have been describing in this article but is hardly noticeable because the surface area of the intake port between the carburetor and intake valve is over 10 times less that the crankcase surfaces in a two-stroke.  The amount of fuel wetted surface area in any engine determines how long it takes the wetted surface to stabilize and allow the carburetor to have complete control over the mixture that is burned in the combustion chamber.  Two-strokes also have more sharp turns that the mixture has to negotiate and hiding places for fuel to accumulate than what a four-stroke intake port has. 

Thank you Jerry for the very detailed explanation.  And Q2W, I assumed that the slide stop screw was the idle adjustment screw like it is on my keihin carbs.  I am not used to dealing with  mikuni carbs. All my other 2 stroke quads have keihin carbs on them.  So the needle jet is a new one for me.  I checked it and it has a R-4 in it.  Next time I get into the local dealer I will pick up a R-6 and try it out.   Also it sounds like this is normal to a certain extent.  So I am ok with it.  Wont be able to mess with it too much this week anymore.  So when letting off the throttle at higher engine RPM's like decelerating in 5th gear, is it easier on the engine or more desirable for the motor to not fire when coasting?  Right now it is just like hitting the kill switch until you pull in the clutch.  I think before I could hear the engine still firing in the pipe.   I just want what will make the motor live as long as possible.  Thanks.. AND after tinkering around with this mikuni, I think I might try one on one of my hondas.  Looks like keihin carb jetting for the zilla is pretty similar to what I have in my hondas.........

Thank you Jerry for the very detailed explanation.  And Q2W, I assumed that the slide stop screw was the idle adjustment screw like it is on my keihin carbs.  I am not used to dealing with  mikuni carbs. All my other 2 stroke quads have keihin carbs on them.  So the needle jet is a new one for me.  I checked it and it has a R-4 in it.  Next time I get into the local dealer I will pick up a R-6 and try it out.   Also it sounds like this is normal to a certain extent.  So I am ok with it.  Wont be able to mess with it too much this week anymore.  So when letting off the throttle at higher engine RPM's like decelerating in 5th gear, is it easier on the engine or more desirable for the motor to not fire when coasting?  Right now it is just like hitting the kill switch until you pull in the clutch.  I think before I could hear the engine still firing in the pipe.   I just want what will make the motor live as long as possible.  Thanks.. AND after tinkering around with this mikuni, I think I might try one on one of my hondas.  Looks like keihin carb jetting for the zilla is pretty similar to what I have in my hondas.........

Mikuni makes R-6, R-8 for a lot of different carburetors.  Make sure that you get a R-6 of the same series as your R-4.  I think that your R-4 is a 389 series.

If you cannot feel the engine firing when you let off, that is usually a good sign that it is rich when the throttle is closed.

Keihin PE, PJ, and PWK carbs do not have a replaceable needle jet.  All needle jets ware out making the carburetor get richer in the zero to about 1/2 throttle position.  When the needle jets ware on a Keihin carb you throw it away or find another needle that has a larger diameter before the taper to compensate for the ware.

Mikuni uses anodized aluminum needles and brass/bronze needle jet.  When parts have to rub each other, the least amount of ware occurs when the two parts are made of different metals.  Honda has Keihin make  anodized aluminum needles for all of their OEM carburetors I have seen.   All of the after market Keihin carburetors use a  brass/bronze needle  and a brass/bronze needle jet.  I think using similar metals for these two parts is very poor engineering because the Keihin needles and needle jet ware quickly as compared to anodized aluminum needles.  The decision for the choice of metal for Keihin needles may have been made by the marketing department and not the engineering department.