Six Stroke
05-02-2011, 06:40 PM
Over the past six years I've done some swapping and tweaking to my YT125, because I apparently can't leave well enough alone. They were all fairly simple and cheap mods, as most involve using other stock Yamaha parts. At this point, I think, I should share my findings in case anyone else desires to follow suit.
Exhaust: The first thing I changed (after a new Wiseco top-end) was the stock, non-expansion pipe. The factory YT175 pipe bolts on with only a minor mod required to the rear mount, as the 175 mount is further back. I got around this by welding a piece of 1/8" strap steel straight forward off the pipe tab, then drilling a hole through the other end so it would bolt up to the stock 125 mount. Note: in order to mount up the pipe, you'll need the 175 exhaust flange that attaches to the cylinder.
Carb/Intake: Next, I switched to the YT175 carb/intake/reed block. The carb is considerably bigger, and the intake flange is a more modern, sculpted design. To use the carb, you'll also need the lower throttle cable off the 175, as it's a bit longer than the stock 125 cable.
Reeds: Boyesens (ordered a set for the DT125, as Dennis Kirk doesn't list Moto reeds, and the DT reeds are the same) were the last mod that I did until recently, and it ran great for several years with the 175 pipe/carb/Boyesen configuration.
Beyond: Just this last weekend I swapped on an air-cooled YZ125 cylinder (a great eBay steal: only $11 for a useable jug, piston, and head!) and threw in a set of Boyesens I had laying around for the larger YZ reed cage. The YZ jug has considerably larger ports, while the two engines have the same bore, stroke, base gasket, and wrist pin. A pretty easy swap, and all the old 175 parts went back on. The initial results were...inspiring. Low and mid range is suffering a bit, mostly from poor baseline jetting combined with the larger ports. Top end is a whole different story - at the RPM I normally would have shifted at before, the thing now comes alive. Like, insanely alive.
Carb#2: I have an '82 YZ125 carb on the way, which will be the carb I do all the serious jetting changes to. I'll update when I have some solid results.
Exhaust: The first thing I changed (after a new Wiseco top-end) was the stock, non-expansion pipe. The factory YT175 pipe bolts on with only a minor mod required to the rear mount, as the 175 mount is further back. I got around this by welding a piece of 1/8" strap steel straight forward off the pipe tab, then drilling a hole through the other end so it would bolt up to the stock 125 mount. Note: in order to mount up the pipe, you'll need the 175 exhaust flange that attaches to the cylinder.
Carb/Intake: Next, I switched to the YT175 carb/intake/reed block. The carb is considerably bigger, and the intake flange is a more modern, sculpted design. To use the carb, you'll also need the lower throttle cable off the 175, as it's a bit longer than the stock 125 cable.
Reeds: Boyesens (ordered a set for the DT125, as Dennis Kirk doesn't list Moto reeds, and the DT reeds are the same) were the last mod that I did until recently, and it ran great for several years with the 175 pipe/carb/Boyesen configuration.
Beyond: Just this last weekend I swapped on an air-cooled YZ125 cylinder (a great eBay steal: only $11 for a useable jug, piston, and head!) and threw in a set of Boyesens I had laying around for the larger YZ reed cage. The YZ jug has considerably larger ports, while the two engines have the same bore, stroke, base gasket, and wrist pin. A pretty easy swap, and all the old 175 parts went back on. The initial results were...inspiring. Low and mid range is suffering a bit, mostly from poor baseline jetting combined with the larger ports. Top end is a whole different story - at the RPM I normally would have shifted at before, the thing now comes alive. Like, insanely alive.
Carb#2: I have an '82 YZ125 carb on the way, which will be the carb I do all the serious jetting changes to. I'll update when I have some solid results.