Moto Mods

[Six Stroke]

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.

[taz]

what year was the yz jug from

[trizilla]

what year was the yz jug from

curious to know this also...

keep it coming six stroke.....

[Six Stroke]

what year was the yz jug from

About 1977. All '76-'79 YZ jugs should fit, though the later the better.

I have a theory about the rest of the engine as well, but have yet to find one for testing. The DT engine (as well as earlier YZ and IT engines) have the same mounting pattern, thus will bolt into the moto frame. I'm pretty positive that the recoil and other necessary parts from the moto will work on the DT/IT/YZ engine, so you'd have a manual clutch moto. That's my next major step, either when I find one or my current YT bottom end self-destructs from the abuse.

[Vealmonkey]

I can tell you this, any bike engine, even though it will bolt in to the yt, won't work as the shaft that the sprocket sits on is too narrow and won't line up with the rear sprocket.

[trizilla]

I can tell you this, any bike engine, even though it will bolt in to the yt, won't work as the shaft that the sprocket sits on is too narrow and won't line up with the rear sprocket.

you must have ran into this with the dt motor in the pro tech tri moto?

[Six Stroke]

I can tell you this, any bike engine, even though it will bolt in to the yt, won't work as the shaft that the sprocket sits on is too narrow and won't line up with the rear sprocket.

Hurm...but, one could (possibly) swap the YT countershaft into the DT cases to get the spacing right, provided that it'll swap over. That unfortunately rules out the IT/YZ engine though. Dang, I wanted that six-speed.

[RubberSalt]

I was talkin to an older guy up here at Mid America Power sports as i droped my ported 175 cylinder off to get bored. He says they used to ride a heavily modified 175 back in the day. Differnt front forks, rear suspension, DT cylinder, pipe to match, bigger carburator, manual clutch....6 SPEED TRANSMISSION.

He couldn't remember where all the parts got robbed from

[Six Stroke]

A six speed would be real nice. The pre-'80 IT175 cylinder would be a marked improvement over stock, as should an IT125 jug.

On another note, does anyone know where I could get a non-stock size rear sprocket? I'd really like to go up a couple teeth.

Update on the YZ Moto: I made a makeshift reed spacer out of an old reed block (from a later reed valve 433 sled engine - turns out the reed blocks are the same) by cutting off the cage and enlarging the hole in the plate to clear my cage. It's not a very big spacer, but it seemed to smooth out the power transition from mid to top end a little. It still hits hard, though, as the trike came out from under me on a strong 1-2 shift. Kenda Scorpions can grip flesh pretty good!

[old-yellow]

thanks six stroke for you sharing your findings

[RubberSalt]

Thats good stuff.