Dep exhaust jetting
Wondering if anyone can help I’ve got a 2001 Yamaha Dt 125 r working power valve model with a full dep exhaust runs and rides fairly nice bit boggy when warming up but other than that it rides okay although when changing my spark plug recently it was black but no excessive build up, the air box is standard and has the snorkel I’m assuming it’s running a bit rich but I’ve heard that the standard 240 main jet is correct for a full dep system on a Dt so I don’t know if it’s my fuel/air screw that needs adjusting which is currently at 1 and a half turns out.
any advice would be appreciated
@jacobx5sv With out doing a proper plug chop, new plug, riding at the throttle position you want to test and then cutting the threads off, to the measure the smoke ring. The problem with just taking a used spark plug out, and checking the colour, is at best just giving you a 'average' reading.
You could be very rich on the pilot say, but lean on the main/power jet. The plug could look dark from riding at the lower throttle openings (pilot/needle) for longer and not show the leaner condition, for the occasional sprint to full throttle.
If your bike is running ok and has been jetted that way for a while. I probably wouldn't be too concerned. It is much safer to run slightly richer.
Calum last edited by
@jacobx5sv standard jet is 210, 240 on earlier models.
With those mods, 210 main is about right. But really jetting to be done to suit. Anything, even just a change of piston quality, can have an effect on jetting.
If you aren't sure, get it on a rolling road with an Air Fuel Sensor connected to dial it in. Otherwise its just guess work.
Dry & black, or wet & black?...
The 2003, according to Haynes, has a 240 main jet, a .25 idle/pilot jet, a 5J25 needle, a Q2 needle jet, a 2.5mm cutaway on the slide, and 1 & 1/4 turns out on the airscrew...
That’s assuming the carb is original
PS. The needle clip should be in the fourth groove down from the top...
The airscrew only affects the mixture from idle to about 1/8 throttle...
I personally don’t believe that an aftermarket exhaust needs upjetting the carb. A carb is made (& jetted) to deliver fuel at the ideal mixture (it’s about 14% petrol and 86% air) once it’s right, it’s right!...
I think it’s an old wives tale, myself...
An airfilter on the other hand can. I’d try washing the filter with brake cleaner and then oil it with clean engine oil. Then wring it out gently...
I’d also dismantle the carb and check all the sizes/designations match your year. If they don’t, return it to standard...
I’m pretty sure everything’s on the stock set up on the carb although the pilot screw is at 1 and 1/2 turns out I’m planning on doing a top end rebuild and maybe getting the cylinder wall honed if it needs it but I’m gonna keep the standard 56mm bore also putting some boysen reeds in it to make it a bit more responsive
my mates got a 1988 model with a hand welded pipe from Portugal and with reeds and no snorkel and I think he went Down to 230 main jet and standard on everything else so I’ll keep mine on the 240 and see what’s it’s like after I’ve done it he said the reeds make it a bit richer so may have to go down a few