DT 125 4BL Jetting Confusion
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Hello Guys,
Currently trying to tune my carb on my 4BL
It seems like it’s the ?small? Version because of the 22.5 pilot jet
However, if I try the 210 or even the 220 jet it runs lean and bogs down
A 250 was a bit too rich and I wouldn’t rev out right
Setup is stock new cylinder and piston, stock exhaust with Athena silencer and the snorkel removed
De-restrictedI still don’t quite understand how some carbs came stock with a 240 main and some with a 210
And don’t quite know where to fish for the right main jet now -
@dominik22 I'd start by refitting the airbox snorkel and the stock silencer if you have one. My French '93 4BL ran very well running the jet sizes you mentioned (i.e 240 main) with either a full stock exhaust system or a Tecno expansion/DEP silencer combo but it didn't like the stock expansion/aftermarket silencer combination.
Sometimes the tailpipe can affect performance as the stock expansion chamber and silencer are designed to work together; if you change one it can alter back pressure and mess with the pressure waves going backwards and forwards. These evacuate exhaust gases from the cylinder, assist in drawing fresh mixture up through the transfer ports and act to keep the fresh charge from escaping straight out the exhaust port. I never thought the tailpipe alone would affect this but once I tried a brand new DEP silencer on my otherwise stock French '93 4BL with 3NC CDI; it immediately refused to rev beyond 9k. I rode it around for about an hour to eliminate wind direction etc., came home and refitted the stock silencer and straight away it revved to 10k+ again. This is only personal experience, I don't have access to a dyno or anything but it made a big impression on me. A Tecno expansion chamber would probably work well with that Athena silencer.
Tecno expansion is identical to the Athena apart from the clearcoat and under half the price. Only downside is the clearcoat melts around the header pipe and is difficult to sand off making your bike look like a Crème Brûlée. If I bought another one now I'd paintstripper it and paint it heatproof black before fitting it. Very good quality pipe for the money, gives a more usable mid-range hit than the DEP which delivers peak HP revs about 600rpm later.
