Im far from any kind of expert, but my bikes seems to run well enough that I was asked to put my thoughts into print.. .so here they are....
These thought's appeared in various threads.... here is a synopsis...... Most of it applies to stock motors, running 39mm PWK carb.
Most CR500's I have seen appear to share similar conditions...
1. They wont idle
2. There too rich from off idle to 1/2 throttle
3. Their too lean at 3/4 throttle
4. Their too rich WFO
What to do.... What to do......
The pilot... This controls the fuel ( in concert with the air screw) to set the idle mixture. In math 8+2. 7+3. 6+4, 5+5 all equals 10 right? Well, the pilot and the air screw work the same way to achieve the goal. Most PWK's like a high 30 to low 40 pilot, PJ's seem to prefer a high 40 to mid 50 pilot. Depending on the temp, altitude, humidity, etc..etc. you fine tune the mixture with the air screw to get the mixture right for the current conditions where your riding. When you have the correct pilot, this screw becomes pretty sensitive. 1/4 of a turn should do allot. If it doesn’t, or if you are more than 2 turn out, you probably have the wrong pilot. Also if the engine idles about the same with the choke on or off, you probably have the wrong pilot.
ZipTy makes this killer " Air Screw" that makes adjusting it a breeze when out on the trial.
http://www.ziptyracing.com/products/pro ... d=MTA5OQ==
When everything is as it should be, it should start easy (cold) with the choke on, then as its warming up it should start running crappy until you shut the choke off. Then it should run smoothly, and predictably ( No stalling as an example) for the rest of the day. If it starts easy (cold) with the choke off, your way rich. It wont surge during deceleration or run "choppy" at part throttle settings when its set right. It should run fairly "four stroke" like . On my bike with a PWK I run a #40. In the high desert my A/S is about 1.5 turns out to lean things, down at sea level is about .25 turns out to richen it. Start wrenching and congratulations once you reach this point. NOTE: Changing the pilot and airscrew setting will have hardly any effect on other throttle settings, don't goof up your pilot settings chasing other issues!
The needle, your best friend.....
First off, understand the needle does not control the fuel flow at most throttle settings... Its the difference in area between the needle diameter and the fixed hole machined into the carb body that it slides up and down in that controls the fuel flow. I mention this as people assume that what works for one bike will work for another. WRONG!..... If the carb you have has a larger hole than your buddies ( due to age, wear, or different tooling used during its manufacture) your bike will be forever "richer" if you run the same needle he does. Understand this important fact? Mukini's have a screw in "needle jet" and are superior to Keihins in this regard.
I assume you do, so moving on heres the rest of the story. As I stated, most CR500's seem rich down low. This causes lazy-ness, plugged pipes ( I ve seen them weigh it at 25 lbs...) the dreaded "spooge", and poor mileage. The cure? Increase needle diameter. Stock Honda needles ( R1368) are 2.68 mm thick. Something in the low 70's ( 2.72 as an example) really cleans up the mess.
The other parameter of the needle we are interested in is the taper, or how quickly it gets pointy for those board members in Virginia. Under a good load, on a hot day, allot of bikes ping at around 3/4 throttle. Increasing the taper from 1.3 degrees to 1.4 ( or even 1.5 for you duners) richens up the mixture to about 3/4 throttle and helps reduce detonation problems.
So, what’s the "Bob's Magic Needle" number? As you can see, there isn’t one. Its depends heavily on your bike. I good place to start is a 1472 ( DGN) , and start in the 3rd clip from the top.
Main jet. Once you crank that throttle OPEN and get the dang needle out of the way, the main jet comes into play, AND NOT BEFORE! Folks often keep playing with the main jet trying to fix anything from hard starting to fouling plugs. These same folks probably never pin it either and are accomplishing nothing. Main jet sizes from the mid 160's to mid 170's are the norm. If you find you are going beyond these numbers in either direction,something is probably wrong elsewhere.
Other points to ponder....
1. Use a fuel filter.
2. Use quality 2 stroke oil at 32 / 44:1 ( as you lean out the bottom end, you are also reducing the amount of oil entering the engine.)
3. ONLY use REAL Keihin parts. They cost more because their worth more. As an example, the main jets are drilled, then flowed, then stamped with the proper number. Aftermarket jets are simply drilled, then stamped, and are very inconsistent.
4. DONT GET FANATICIAL REGARDING JETTING! I have seen guys waste a whole day when they could be riding playing with their carb. Guess what? These carbs are low tech, marginal mixers at best and its rare you ride in identical conditions day to day. YOU WILL NEVER HAVE YOUR CARB SET PERFECT. Live with that statement, and have fun instead of frustration.
If any of this is confusing, use this form as intended and ask questions!
My Thoughts on Jetting
My Thoughts on Jetting
Last edited by AlisoBob on January 7th, 2008, 9:14 pm, edited 1 time in total.
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Ported&Polished
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My pwk at around 6000 feet,temps in the mid 80's, ported jug, new rad valve.Ok, so today I spent the whole day trying to get my bike to idol. I had a 42 slow jet, a 160 main, and tried alot of fooling with the air screw and idl adjuster. The idol adjuster had zero affect on the idol, or rpms at all. I finnally used the throttle cable adjuster to make it idol. As soon as I was able to make it stay running without my hand on the throttle, I turned the air adjuster in and out, 1 turn, 2 turns three turns, allllll the way out, nothing, not one change in the way the bike ran or any increase or decrease in the idol. So my buddy said try a different slow jet. I only had a 60, and it did nothing for me. He said try a 40 or a 38, which I don't have, but will get. I put in fresh c12 mixed with caster at 32 to 1. My main jet is ok, because the bike works great while riding, just want to make it idol properly without adjusting the throttle cable to force it.
Don't Clyde it, ride it!
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Ported&Polished
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The plastic one seems to raise the slide slightly higher than the metal one.AlisoBob wrote:The word is idle....
The difference between a 38, 40, and 42 pilot is squat....
Do you have the metal, or the plastic, idle speed adjusting screw?
To idle on a 500cc motor, the slide screw on the PWK needs to be just about all the way in.
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dannygraves
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Redbull500
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2strokeforever
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