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330 carb tuning
Quote from: Powerstroken;55511
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.

June 26, 2015, 09:27:55 PM
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Re: UPP Intake Boot Ripped F-Red:

Using an OEM type carb clamp will make life easier on any intake boot.  The clamp that you are using above, makes life a little more difficult on the hose because that type of radiator clamp tends to "bunch up the rubber" on one side of the hose.

June 17, 2019, 03:57:28 PM
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Re: UPP Intake Boot Ripped
This is for you Myles. The pic is a little misleading. But there is a significant difference.





There is a huge amount of development that goes into the OEM rubber type connector between the reed and carburetor.  There are 3 things that I feel are very important about this engine component that is viewed by most as just something that connects the carburetor to the engine.  The air flow characteristics, the length and the proper amount of heat isolation/vibration damping for the float bowl carburetor.

I have seen enough burned up pistons to fill a few 55 gallon barrels because of improper vibration damping when using old hardened oem manifolds,  billet manifolds with the short 2-bolt rubber flanges and reed assemblies that incorporate the carb to reed adaptor as a one-piece casting with a short hose and 2 hose clamps.

These one-piece reed assemblies have excellent air flow characteristics for engines below 55 to 60 hp that use carburetors smaller than about 37 to 39mm.  It has been my experience that the special short molded hose that connects the one-piece reed casting to the carburetor, does not offer enough vibration damping on most engines especially the ATC/TRX Hondas that have the big bore kits.  The rubber durometer appears to be too hard and the 5mm or so of rubber between the one-piece casting and carburetor is too short for the necessary damping to prevent fuel "frothing" in the float bowl.

I have observed the fuel "frothing" problem many times when doing steady state dyno testing or performing long run time acceleration dyno test.  If the fuel frothing problem occurs, it occurs at one and sometimes two points on the usable RPM scale while conducting a test.  The three vehicle applications where I see the most problems are on recreational vehicles, flat track and land speed racers.  Drag racers, Stadium racers, and motocross racers seldom experience a burned piston because these vehicles cross through the problematic RPM range in a second or less. 

Flat track racers may dwell for a few seconds at one particular RPM in the corners. Recreational vehicles often cruse down a road or trail at a constant RPM. Land speed racers accelerate slowly due the the high speed gearing.  Land speed racers accelerate through the RPM range where fuel "frothing" may occur, slow enough that the carburetor will go lean long enough to burn a piston.  Flat trackers and recreational riders carburetors can also go lean when running at the RPM where fuel "frothing" occurs.

Fuel "frothing" is obvious when dyno testing.  When the engine reaches a certain RPM where the carburetor vibrates at a frequency that causes "frothing", the main jet sucks a foam of air and fuel,(very lean)  the floats drop to the bottom of the bowl causing more fuel to enter the float bowl and is instantly turned to foam and fuel gushes out of the vent lines while the exhaust gas temperature goes much higher.

The first time I observed this phenomena it did not make sense.  How can the exhaust temperature suddenly go much higher while the floats were stuck open.  I though the fuel level had gone extremely high due to the fuel gushing out of the vent lines.  I removed the carb, cleaned the fuel flow/float valve passage way, and checked the float level.  I conducted another dyno run and had the same problem.  I repeated the dyno test again but this time I touched the carburetor with one finger when fuel started gushing out of the vent lines.  Fuel immediately stopped gushing out of the vent lines every time my finger touched the carburetor.  I could not feel a change in the intensity of the carburetor vibrations from any of the other RPMs where fuel frothing did not occur.

I have also seen the fuel frothing problem occur when the carburetor is allowed to touch the engine cases or frame.  I have also seen the fuel frothing problem occur when the carburetor is not properly supported on the air filter side of the carburetor.  I have also seen the fuel frothing problems occur with some of the aftermarket rigid intake tubes and air boxes. 

Every engine and chassi many have a slightly different resonance frequency.  Some guys may not ever experience the problems described above while others are having problems burning pistons and have not found out why.  Focusing a camera on the vent lines while riding may help eliminate one more possibility

June 18, 2019, 06:39:40 PM
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Re: ESR exhaust flange O-ring size I gave up on experimenting/searching for the magical sealing compound for sealing pipes many years ago.  I do not know of any non metal compound other than asbestos that will take the average temperature of the exhaust flange and pipe flange for very long.  If there is something out there.............it probably is not affordable. 

Most new owners of the ATC250r and TRX250R never experienced how well OEM pipes sealed because they immediately removed the OEM pipes and installed an aftermarket pipe.   The majority of the aftermarket  pipe flanges were made of seamed tubing that was rarely round, the tubing was usually a sloppy fit on the exhaust flange and the welded seam of the tubing prevented any hope of any type of a controlled leak.

I think the Honda engineers had the solution on their original OEM exhaust flanges and pipe flanges.  These flanges had a metal piston type ring with the asbestos type packing on the end of the exhaust flange.  If the aftermarket pipe builders would copy the pipe flange that Honda had, use the long lasting OEM steel exhaust flange with the metal sealing ring and use the packing on the end of the flange it would eliminate the majority of the mess at the exhaust flange.


July 16, 2019, 01:50:13 PM
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Re: Update from Power Apt on cast smart carbs                                           Survey


I would also like an update on those that bought smart carbs back in 2015 or earlier. 

Were you able to tune them to your satisfaction?

Were you able to tune them to run more reliable, better fuel mileage, fewer seized  or holed pistons than a Keihin or Mikuni carb?

Are you still running them?

If you had it to do over, would you buy another Smart Carb? ...............and why.


July 17, 2019, 08:57:45 PM
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Re: Pingel petcock I instruct all of my customers to ALWAYS run their petcocks in the reserve position.  Petcocks obtain their fuel from the lowest level in most fuel tanks when positioned in the reserve position.  Petcocks obtain their fuel from a stand pipe that may be 2 or more inches from the bottom of the fuel tank when placed in the on position.  High performance two strokes that are highly tuned, can burn a piston in less than a couple of seconds or so when the air fuel mixture becomes too lean. 

I have seen a large number of pistons torched on engines that had the petcock positioned in the on position.  The fuel level in the float bowl does not stay consistent when the fuel tank level becomes low enough to not keep the stand pipe on the petcock completely submerged 100% of the time. 

As the fuel level in the fuel tank becomes lower and lower and the fuel is sloshing around, the stand pipe is not providing fuel to the float valve 100% of the time.  The inconsistent fuel supply to the float valve causes the fuel level in the float bowl to stay lower for longer periods of time until the lean condition causes a piston failure. 

A two stroke will need a fuel flow rate through the carburetor of at least 6 gallons per hour when developing around 70 hp.  You will never use 6 gallons per hour if your engine develops 70 hp but the fuel supply past the float valve must be at least 6 gallons per hour any time the engine is developing 70 hp to keep it from going lean for the period of time it is at that power level. 

A flow test with a full tank will provides a lot more fuel pressure and flow, so always perform this flow test when the fuel level in the tank is 1 to 2 inches from the bottom of the tank.

August 05, 2019, 11:56:29 AM
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Re: New LED HPI ignition If the kick starter is trying to blow off your foot when kick starting, something is drastically wrong.  When this happens it is an indication that the ignition timing in the starting RPM range and or the moment of inertia of the crankshaft assembly is too small. 

I have not used or tried to tune an engine with a HPI ignition.  If it has a very small rotor, that could be part or all of the problem.  If it has a rotor that has the same moment of inertia or higher than the OEM flywheel, the timing is at least 5 degrees too advanced in the starting RPM range.

If the HPI ignition is a programmable ignition I would retard the timing 5 deg in the 200 to 800 RPM range, leave the rest of the curve alone and see what happens.  If the HPI has a universal timing curve similar to the basic very small rotor ignitions and other non programmable ignitions boxes, and the timing is adjusted by rotating the stator plate, rotor or reluctor plate, you may have a real dilemma.  The choice is retarding the whole ignition curve so that the timing is correct at starting RPMs or correct at one RPM at the RPM of your choice. 

My experience has not been good with ignitions timing curves that were not engineered for a particular engine package. 

September 18, 2019, 09:37:59 PM
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Re: Not your typical oil question: Castor oils I started writing a response to the post by wilkin250r  but it was getting too long and had not even started to answer the questions.  I will try to edit my response and post it in a few days.  This is not a question that can be answered by a simple yes or no answer.
December 03, 2019, 11:52:56 PM
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Re: piston size Since your piston is for a 68 mm bore and assuming your piston to cylinder wall clearance was set correctly when it was last bored, using your current piston diameter measurements your current piston to cylinder wall clearance is 68.00- 67.85 = .15 mm  or 0.0059".  A new 68mm piston should have a diameter of 67.92 to 67.95mm 

If you can still see the cross-hatch I would not hone or de glaze the cylinder.  It will take about 15 minutes longer for the rings to seat when you do not hone the cylinder.  The cylinder bore will last longer, the piston and rings will last longer and the piston to cylinder wall clearance will remain tighter when you do not hone the cylinder when replacing rings and or piston on a two-stroke.

A ball hone, flapper hone, 3 stone-spring de glazer will make the bore look great but it will only make the cylinder taper and or out of round worse.  If you are bent on honing the cylinder take it to a shop that has a Sunnen hone that uses the long mandrels.  A shop that has invested in a Sunnen hone will also have the measuring equipment to properly measure the bore and piston to cylinder wall clearance.  The piston to cylinder clearance of a Wiseco should be .0030" for a new bore and .0040" would be ok with a new piston on an  old bore that has not been honed.  The piston to cylinder wall clearance on a cast piston should be .0026 on a new bore and .0035 would be ok with a new piston on an old bore that has not been honed.


February 06, 2020, 12:05:34 PM
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Re: Silencer turn downs made easy If anyone is needing production quantity tube bending, I have 3 mandrel tube benders and a lot of dies for a wide assortment of tubing diameters, wall thicknesses, and center line bend radi.  Two of the semi automatic machines will make bends on tubing up to 2.00" OD and 15' long. anywhere between zero and 180 deg. and up to 10 bends with zero 359 deg clocking between bends.  Minimum spacing between bends depends upon the length of the clamping die.  Clamping dies are usually in the 3" to 6" in range The CNC tubing bender will do anything the semi-automatic machines will do, but a lot faster and up to 3.0" OD tubing.
February 11, 2020, 07:45:12 PM
2