330 carb tuning

[Powerstroken]

I know there are a lot of threads on jetting and I have been looking at a lot of them the past few days trying to get my motor jetted correctly. I'll start out with what I am using, I started with a 55 slow, 185 main and ddj needle on the middle clip. I started the motor took a few kicks, now the 0-1/4 throttle was running pretty good but once above that it was sputtering awful. So I dropped to 180 main still the same thing. So I dropped the needle a clip made it a little better but still not good. Dropped the needle again so now I'm at the first clip on the needle from the top and it's running a little better up top but still not right. I am out of needle now but I feel if I drop into the 170's that would be too small of a main correct? I am at sea level and it's in the mid 80's. Should I get a leaner needle and bring the main size back up or would it be safe to drop into the 170's on my leanest setting on the needle? Also I have been trying to get the air mixture screw set right, I read that you need to bring the idle up then adjust but then when you bring the idle down won't it affect the mixture yet again? Sorry for the noob questions but never got into tuning carbs this much

[rk88r]

A ddj in the first groove is equivalent to a dej in the second groove.

[Rupp250]

Plug chop will help you on the main. My 363 has a 162 main. 40.5 Kehein. Sea level as well.

[Powerstroken]

I am just scared to death of not getting enough gas or oil in that motor

[Powerstroken]

Ok messed with it this afternoon and got the main down to a 175. It is to the point where it's not breaking up on the role during any more but seems like it still might be rich. I have the idle screw all the way in and air screw out 1.75-2 turns. It idles pretty good and not much hesitation when stabbing the throttle from an idle. The only thing is that it's still taking maybe 5-6 kicks to start even when hot. Not sure if it is just from such high compression or if I can still adjust something? What should I look at to help starting it

[rk88r]

I would order a leaner needle just to have more range of adjustment. Give it ride in the morning when it's cooler out just to get a feel for it. One of R's I run a dej in the second groove in the summer, the middle groove in the spring and fall, and the fourth groove in the winter.

[Jerry Hall]

Ok messed with it this afternoon and got the main down to a 175. It is to the point where it's not breaking up on the role during any more but seems like it still might be rich. I have the idle screw all the way in and air screw out 1.75-2 turns. It idles pretty good and not much hesitation when stabbing the throttle from an idle. The only thing is that it's still taking maybe 5-6 kicks to start even when hot. Not sure if it is just from such high compression or if I can still adjust something? What should I look at to help starting it

With the idle screw all the way in, the slide is probably open 1/8" or more.  The air velocity over the pilot jet transition holes is too low to draw adequate fuel from the pilot jet, especially when the air screw is at two turns and the slide has 1/8" or more of opening.  Increasing the number of turns on the air screw decreases the suction on the pilot jet, resulting in less fuel being delivered to the pilot jet transition discharge holes.  

The choke is actually a separate enrichening circuit (a little carburetor inside the existing carburetor) where air must take another path and over a discharge hole that is connected to a very rich jet.  If the slide is open 1/8" or more the choke will not work very well because the air flowing through the carburetor takes the path of least resistance.   Adequate air flow will not go through the choke circuit to pick up the proper amount of fuel when the engine is cold and the throttle is open 1/8" or more.

Your pilot jet is too rich and or the fuel flow area between the needle and needle jet at close throttle is too large requiring the idle stop screw to be all the way in to make it idle once you get it started.  

I would recommend that you set your side to be open about 1/16" open and reduce the pilot jet size a couple of sizes, set the air screw at 1 1/2 turns and see if idles at a reasonable RPM.  You may need to optimize the airscrew and idle stop screw to get the best idle.  

The needles last letter in the 3 number sequence on the needle, designates the diameter of the needle from closed to about 1/2 throttle opening.  On most new Keihin PWK, PWM or PJ carbs we use on the 250s or 250s with big bore cylinders, the diameter designator is usually an "H" xxH or "J"  xxJ.  As the carburetor needle jet wares with mileage, this straight or constant diameter portion of the needle must be increased to keep the mixture (fuel flow area between the needle and needle jet) optimized.  If you end up needing to using a needle diameter designators that is a K, L, M, or N that is usually an indication the carb has appreciable ware in the needle jet.  I have bore gages that can measure the needle jet holes and they usually ware into an oval shape with the largest diameter  parallel to the air flow through the carb.  

The main jet size depends primarily on the type of air filter and the back pressure in the exhaust system. The back pressure is controlled by the stinger and muffler inside diameter, number of degrees of bend in the stinger and the inside diameter, the number of degrees of bend of a turn down if it exists.

[aberegg05]

With the idle screw all the way in, the slide is probably open 1/8" or more.  The air velocity over the pilot jet transition holes is too low to draw adequate fuel from the pilot jet, especially when the air screw is at two turns and the slide has 1/8" or more of opening.  Increasing the number of turns on the air screw decreases the suction on the pilot jet, resulting in less fuel being delivered to the pilot jet transition discharge holes.  

The choke is actually a separate enrichening circuit (a little carburetor inside the existing carburetor) where air must take another path and over a discharge hole that is connected to a very rich jet.  If the slide is open 1/8" or more the choke will not work very well because the air flowing through the carburetor takes the path of least resistance.   Adequate air flow will not go through the choke circuit to pick up the proper amount of fuel when the engine is cold and the throttle is open 1/8" or more.

Your pilot jet is too rich and or the fuel flow area between the needle and needle jet at close throttle is too large requiring the idle stop screw to be all the way in to make it idle once you get it started.  

I would recommend that you set your side to be open about 1/16" open and reduce the pilot jet size a couple of sizes, set the air screw at 1 1/2 turns and see if idles at a reasonable RPM.  You may need to optimize the airscrew and idle stop screw to get the best idle.  

The needles last letter in the 3 number sequence on the needle, designates the diameter of the needle from closed to about 1/2 throttle opening.  On most new Keihin PWK, PWM or PJ carbs we use on the 250s or 250s with big bore cylinders, the diameter designator is usually an "H" xxH or "J"  xxJ.  As the carburetor needle jet wares with mileage, this straight or constant diameter portion of the needle must be increased to keep the mixture (fuel flow area between the needle and needle jet) optimized.  If you end up needing to using a needle diameter designators that is a K, L, M, or N that is usually an indication the carb has appreciable ware in the needle jet.  I have bore gages that can measure the needle jet holes and they usually ware into an oval shape with the largest diameter  parallel to the air flow through the carb.  

The main jet size depends primarily on the type of air filter and the back pressure in the exhaust system. The back pressure is controlled by the stinger and muffler inside diameter, number of degrees of bend in the stinger and the inside diameter, the number of degrees of bend of a turn down if it exists.

So would a motor with a larger I.D. stinger need a larger main jet than a standard size stinger?

[Jerry Hall]

So would a motor with a larger I.D. stinger need a larger main jet than a standard size stinger?

I need more information to answer that question.  I will give some examples and trends in jetting with changes in pipe back pressure.

Engine package #1:  
We have a highly developed engine package where the ports, pipe, head, etc are all optimized to work with each other.  This engine package produces good power over a wide RPM range.  The back pressure in the pipe for this engine package is optimized so that reducing the inside diameter another 0.5mm will cause the engine to experience detonation.  Lets say that this engine needs a 180 main jet with a 26.0 mm id stinger

Engine package #2 as compared to engine package #1
We take engine package #1 and increase the ID of the stinger and silencer to 27.0mm. The 180 main jet will be too lean and it may take a 190 main jet to optimize the wide open throttle mixture and power.  The  power may come on 100 to 300 RPM lower, peak HP may go down and over rev may suffer as much as  300 to 500 RPM after the main jet is optimized.

Engine package #3 as compared to engine package #1
We take engine package #1 and reduce the ID of the stinger and silencer to 25.5 mm.  The  power may come on 50 to 100 RPM later,  peak HP is about the same and over rev may increase as much as 50 to 100 RPM.  The 180 main jet produces the best power but 5 to 10 seconds of full throttle will drive the engine into runaway detonation.  Installing a 182 main jet makes the detonation go away, the front side (power before the power peak) of the power curve looks good like the power curve from engine package #1 but be loose 300 to 500 RPM on the over rev.

Engine package #4 as compared to engine package #1
We take engine package #1 and reduce the ID of the stinger and silencer to 25.0mm.  The  power may come on 100 to 300 RPM later,  peak HP is down, over rev  is down as much as 100 to 400 RPM with a rich misfire at extreme high RPM.  We have to reduce the main jet to a 170 to get rid of the rich misfire at the extreme high RPM range but now the engine is experiencing detonation at wide open throttle, engine coolant temperatures have risen substantially, piston crown temperatures can rise to the point of caving in the top of the piston or burning a hole in the middle of the piston in as little as 10 to 15 seconds of full throttle.


In summary:
The restriction or back pressure on a pipe has to be optimized for the amount of power and RPM an engine produces.  Go to large on the stinger for you engine package and it will need larger jets and peak power and over rev will be reduced.  Go slightly too small on the stinger and the power does not usually change much but the engine is very sensitive to jetting and is always trying to hurt a piston.  Go extremely small on the stinger or instal an ill-designed spark arrestor or have a spark arrestor that is partially plugged and you will have to reduce the jet size to keep it from having a rich misfire at the high RPMs.  Once the main jet size is reduced to make it not misfire at high RPM the engine seems lazy and is trying to hurt pistons.

[Powerstroken]

Thank you for the information

[Jerry Hall]

So would a motor with a larger I.D. stinger need a larger main jet than a standard size stinger?

The definition of a standard size stinger depends what size engine/ HP the engine is and the total length of the stinger and silencer.

Most 250 single cylinders up to around 55 hp need something around 26 mm ID stingers.  Big bore singles up to around 70 hp need stingers with an ID around 29 mm.   Big bore singles up to around 100 HP need stingers with and ID of around 32mm.


If you have a 29mm Id stinger on a 40 HP single you will  need a larger main jet than if you had a 26mm stinger on the same pipe.  Even with an optimized larger main jet needed to produce max power with the larger stinger,  the engine will not make as much power and there will not be enough over-rev resulting in a narrow power band.

An engine package needs the correct restriction (length and or ID stinger) for the RPM and power the engine produces.  You cannot compensate with jetting for the wrong restriction for a given engine package!!!!!!!!!