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 Post subject: Beginners jetting
PostPosted: Mon May 29, 2017 2:13 am 
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Joined: Tue Jan 17, 2012 3:19 am
Posts: 7698
Location: Hope, B.C Canada
I don't know if anyone even wants to read this but it is a procedure I use when I jet a new carb that has never been on the machine. I got this off the internet and it is a bastarsized version of the research I have done on it. It works real well for me.

Beginning jetting:
1) You will need one of those small induction type tach's (pic below). The reason is because it is impossible to hear a few hundred rpm change in a two stroke when you set the air screw. Setting the correct air jet and screw seems critical to an oddy. If it's wrong then the machine is impossible to drive.
2) Get on the carb manufacture web site and find the stock jets for your carb. The ones they put in it when theu sell it to you. You will find that when this is all over you will be within about two jet sizes of what they recommend and have installed when they sell it to you.

The air screw: The airscrew is a fine-tuning adjustment designed to allow the carburetor to be slightly adjusted for variances in atmospheric conditions. The air screw works with the pilot jet, mainly affecting the engines initial starting, idling and initial power delivery. Proper adjustment of the airscrew can offer direct feed back on pilot jet sizing. When adjusting the air screw, tightening the screw is richer and loosening it is leaner.
If you get no RPM fluctuation when adjusting the air screw there is a good chance that there is something clogging the pilot system or the wrong size pilot jet is installed. In most cases, the pilot jet should be the same as original stock. Clean the system thoroughly with carb cleaner and blow out with compressed air.
A lean setting will cause your Engine to surge at very low RPM’s, bog or cut-out when the throttle is opened quickly and have trouble idling down.
A rich pilot setting will result in hard starting, plug fouling at low RPM’s, "sputtering" as the throttle is cracked opened.
Set your air screw to 1 1/2 turns out and start the machine. You want your machine to idle as slow as possible without stalling. With an eye on the tach turn the screw out 1/4 turn at a time until the rpm don't increase anymore. You also have to give it a bit of time for the rpm to stabilize each time you have turned the screw. You want to be at 1 1/2 turns out on the air screw when this is all done. If you are less or more than that then change the jet.
Make a jet size up or down adjustment based on what you have just read above.

The needle: The needle clip position affects performance in the 1/4 to 3/4 throttle position. Raising the clip will lean out the mixture. Lowering the clip will richen the mixture.
When the clip setting is lean the bike will be very zingy sounding . Lean in the midrange will rob power and cause the machine to run hot and seize easily or even hole a piston.
Set it to the factory middle clip position and leave it there. Most riders couldn't tell the difference if it was slightly rich or lean anyways so don't gamble by screwing around. The factory setting is goddamn close.

The main Jet: Main jet size affects 3/4 to full throttle performance. Plug chops can help determine correct jetting. Put the factory recommended main jet in it. You will be close.
When the main jet is lean the Engine will experience detonation or "pinging". Exhaust note will be of a higher, tinier type note. Engine will over heat easy and can be down on horsepower. A moderately lean main jet can cause Engine seizures. A severely lean main jet can cause the Engine to burn a piston (hole in top). Shitty fuel octane or poor fuel supply (amount) will also hole a piston faster than you can say FK.
When the main jet is rich the Engine will be a bit flat or lazy at ¾ to full throttle, giving off a flat, dead sounding exhaust note. When the main jet is severely rich the Engine will "sputter" in the high RPM’s and have a lot of trouble making power up top.
The safest way to get the main jet setting as near correct as possible is to richen the main jet setting up until the Engine begins to lose power and sputters at WOT (Wide Open Throttle). Then, reduce jet size until sputtering disappears and WOT (Wide Open Throttle) is clean and crisp.
A plug chop can be done to fine tune main jet size.

Note: On the same bike, a bigger carb will need bigger main jets because it will "see" less vacuum. Vacuum signal and air flow are two completely separate things and any change of carb size on the same bike or Engine size with the same carb can not be directly compared.
I'll give an extreme example. The '82 Huskys all had the same 38mm carb from the 125 to the 430. The 430 has 440 main stock and the 125 had a 470, and the 470 in the 125 is SUPER lean, it needs a 530. Why does the 125 need 100 sizes bigger than a 430? Its not because the 125 needs more fuel or flows more air, its because the vacuum signal the 125 Engine sends to the carb is so weak it needs a much larger main jet to draw enough fuel. I have not run into this issue yet but I thought I would post it because it is a reasonable theory. The factory recommended jets were very very close for my oddy's. Since my brother and I go from just above sea level (about 200') to over 5000' in a single ride, I put in the altitude corrected main jet and air screw setting. That is usually two main jet sizes smaller and one turn more (out) on the air screw. I do this because the engines are dogs at the high altitude if you don't. The risk is if you were on the lean side to begin with and then go two jet sizes down on the main, then you could seize a piston. Make this adjustment with caution and based on where you normally ride. When in doubt leave the factory recommended main jet in it and do plug chops to verify. But do open your air screw that extra turn.

Last couple of comments here: We now run the Mikuni TM38 flat slide carb. I have to recommend this carb to all oddy owners. It's good. MAKE FKN SURE YOU KNOW WHICH TM38 YOU GOT. There are 3 different TM38's and they have different size jets. In fact there is a 100 point difference between two of them. You will know which one you got by looking at the stamp on the carb. It will say TM38-***. Those numbers at the end tell you which one you got so when you are looking at the Mikuni site make sure you are looking at the right carb.
Last but not least run good octane fuel. I honestly believe the oddy needs higher octane fuel than what the factory recommended. Honda had a lot of recalls on this machine and I think they could have avoided most of their problems if they would have just told owners to run race gas. They could have made a marketing gimmick out of it by saying these are "RACING MACHINES" and need "RACING" gas.
Your fuel supply is also critical. Your fuel pump MUST put out about 4.5oz in 10sec.


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