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Hey guys



What jetting is recommended when running the exhaust without the spark arrestor insert? I plan on cutting off the wire and perforated tube insert from the endcap, then enlarging the hole in the encap and tig welding my own custom tube into it.





Thanks



John
 

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Hey guys



What jetting is recommended when running the exhaust without the spark arrestor insert? I plan on cutting off the wire and perforated tube insert from the endcap, then enlarging the hole in the encap and tig welding my own custom tube into it.





Thanks



John
 

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I sweated some copper plumbing fittings, added a s/s mesh screen, and didn't need to re-jet the carb. More air flows out and the noise level is a little louder.
 

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Whatever jetting yields a 14.7:1 air:fuel ratio by mass. When you change engine operating parameters, either mechanical or environmental changes, it is wise to pay attention to the butt dyno and make adjustments as necessary. Adjustments may not be necessary.
 

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Whatever jetting yields a 14.7:1 air:fuel ratio by mass. When you change engine operating parameters, either mechanical or environmental changes, it is wise to pay attention to the butt dyno and make adjustments as necessary. Adjustments may not be necessary.




Querty I added an 225 piston to my bike some time ago and the jetting remained the same. Is this normal? I thought I would have to mess with the mix all over again. I am adding a YMF 225 High comp piston, do you think the jetting will need some work?



Ronnydog
 

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Querty I added an 225 piston to my bike some time ago and the jetting remained the same. Is this normal? I thought I would have to mess with the mix all over again. I am adding a YMF 225 High comp piston, do you think the jetting will need some work?



Ronnydog


You'll find out when you ride it. The notion that predicting jetting changes before doing mods without previous tests is not science. Generally, a moderate increase in displacement has little to do with changes in air and fuel flow through the intake, head, and exhaust. Raising compression may require a higher octane fuel, which may affect jetting because hotter and larger volume exhaust gasses will interact differently with the exhaust tuning, which can easily change the actions of gas movement during valve overlap.



Changes to exhaust usually have more affect on jetting than changes in compression, IF the limiting factor on air flow through the engine is exhaust tuning. This is not the case with TWs, the limiting factor in power output seems to be cam timing. That's why exhaust mods generally make little or no difference in power output, but even a mild performance cam makes a significant output increase. I'd expect a cam change would be more likely to require a jetting change (richer off idle) than most anything else on TWs.
 

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You'll find out when you ride it. The notion that predicting jetting changes before doing mods without previous tests is not science. Generally, a moderate increase in displacement has little to do with changes in air and fuel flow through the intake, head, and exhaust. Raising compression may require a higher octane fuel, which may affect jetting because hotter and larger volume exhaust gasses will interact differently with the exhaust tuning, which can easily change the actions of gas movement during valve overlap.



Changes to exhaust usually have more affect on jetting than changes in compression, IF the limiting factor on air flow through the engine is exhaust tuning. This is not the case with TWs, the limiting factor in power output seems to be cam timing. That's why exhaust mods generally make little or no difference in power output, but even a mild performance cam makes a significant output increase. I'd expect a cam change would be more likely to require a jetting change (richer off idle) than most anything else on TWs.




Qwerty, God gave you a brain a special brain!!!!! I often need to read what you wrote more than once to fully get what you are saying! Thanks. Ur the bomb!



Now I will add a cam to the list, Ronnydog
 

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Ronnydog, on the subject of jetting, as Qwerty says it is almost impossible to predict before you start to ride the new engine.

Here is a good example. I have converted two 125 engines, one to 200 high comp. and one to 225 standard comp.

Now in the 125 carb MV28mm TK same as 200, the 125 engine has a 134 main jet and a 34 pilot, this is actually too rich for a 225 engine and in the 200 high comp it's very lean.

It just seems wrong that a little 125 requires bigger jets than the 225, I think it must be to do with the swept volume that the shorter intake stroke sucks in less fuel.

But I'm only guessing.

Keihin 393 series mains fit the TK carb and are easier to find but the numbers don't cross reference so if you go this route you will need to find a base setting number first.
 

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Ronnydog, on the subject of jetting, as Qwerty says it is almost impossible to predict before you start to ride the new engine.

Here is a good example. I have converted two 125 engines, one to 200 high comp. and one to 225 standard comp.

Now in the 125 carb MV28mm TK same as 200, the 125 engine has a 134 main jet and a 34 pilot, this is actually too rich for a 225 engine and in the 200 high comp it's very lean.

It just seems wrong that a little 125 requires bigger jets than the 225, I think it must be to do with the swept volume that the shorter intake stroke sucks in less fuel.

But I'm only guessing.

Keihin 393 series mains fit the TK carb and are easier to find but the numbers don't cross reference so if you go this route you will need to find a base setting number first.


You are right, the 125 engine doesn't draw as much vacuum, and vacuum is what lifts fuel from the bowl. An engine rebuild with a radical cam won't hardly idle because of the lack of vacuum, no matter what jets are installed. On the other hand, a more efficient, higher output engine will also need bigger jets because more air is flowing through the engine at high rpm and WFO, because of the low vacuum conditions.



EDIT: When I get around to rebuilding Tdub, first order of business will be mildly porting the head, port matching, and a bigger carb, along with more cam and stiffer springs. I think with the stock cam and head, once jetting is right, other changes will move the powerband up and down the rpm range, but not really change the amount of thrust generated. I think cam and head work are the first effective steps to more power, with a bigger carb next in line, followed by freer flowing intake and exhaust. Only then will a bore and/or stroke be able to make significant power gains. XR100.com used to build TTR230 engines bored to 250cc to 28hp, and the first step in the process was headwork and cam. The potential is there, but I'd worry about durability and the narrowing effect on the powerband, especially with the TW trans ratios.
 
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