Yamaha Tri Moto YT125 - 1981

This article will start with some carburetor setup information. At least for me, data seemed hard to locate compared to say Honda ATC models.

Basic settings - I found a "Condensed Service Manual" that had basic tuning data for both the YT125 and YT175. The online Service Manual I found used YT125 in the title but was actually a YT175 manual. The Owners Manual I found did not seem to have detail tuning data in it.

HOWEVER...my YT125 is fitted with a replacement Mikuni carburetor, with the only markings on left side of throttle valve body of "Mikuni" with small "2Nz" stamped near air bleed screw on flat spot, and large "MIC" on right side of throttle valve body. Dimensions are 24mm bore, with 35mm spigot and 43mm air inlet outside diameters. It appears to be a Mikuni VM24, but with idle air screw in same position as found on the YT125 VM20 and YT175 VM24. Determining Float Height was a challenge, as the Condensed Service Manual shows 21mm for the YT125 (VM20) and 25mm for the YT175 (VM24). But...current specs for a VM24-512 is 21mm float height, while a VM26-606 is 22-24mm float height. Mine was 23mm and I left it there after trying both 21mm and 25mm.

Initial Air Screw setting - 1 1/2 turns out. I found this setting worked best for my YT.

Main Jet : 110 stock YT VM20, while my VM24 is fitted with a 115 and ran great at full throttle.

Jet Needle: 3rd groove stock, while mine ran best with the clip in the 4th groove down from top (slightly richer than stock).

Choke Plunger: Mine was broken off and I had to replace it. I found a Mikuni MK-412 plunger kit which included all parts at the carb except for the choke cable itself. But....the plunger itself was longer than stock and the cable takeup brass threaded fitting was not as long as the original curved piece. It turns out the MK-412 kit is made primarily for VM30 and larger carbs so the plunger VM15SC4/85 that comes in the kit is "long" while there is supposed to be a "shorter" Mikuni plunger part number VM15SC4/86 that is available from NicheCycleSupply.com. The VM15SC4/86 turned out to be identical in dimension to the plunger that came in the MK-412 kit I purchased, so maybe the plunger in the kit I purchased had been substituted in place of a VM15SC4/85. So, I found a Mikuni kit MK-413 on Ebay marketed as made for the VM20/24 carburetors that had an assortment of other fittings slightly different than the MK-412 (threaded main fitting was shorter, had an elbow with thread-in adjuster, slightly less-stiff spring, and a plunger of same 18mm length and 8mm diameter on the plunger portion was turned down to a smaller diameter such that the small end of the plunger could slide into the brass fitting all the way to the plunger larger diameter section.

I ended up using all the MK-412 parts except for the MK-413 less-stiff spring and plunger. No matter which spring I tried, the length of coils kept the plunger from being drawn into the brass threaded fitting more than say 2mm. But that was enough to make a big difference compared to the MK-413 plunger when assembled and therefore had an overall length from base of threaded fitting to end of plunger when retracted within 1mm of the original pull-out plunger. I did bevel the edge of the fitting that the plunger slides into, to ease the plunger slide-in into the fitting when operating.  It did help as the stock square edge did hang up a few times when testing, but not at all after beveling the edge

I did use a thin aluminum washer under the main brass fitting that threads into the carb, in order to have more room for the plunger to move more outward when cable pulled. I then adjusted the cable free-play so that a pull of the cable knob so at the 1st and 2nd detent of the knob would stay open. Since installation the choke has worked perfectly.

Idle Speed: I found three different idle speeds in manuals: 1500, 1550, and 1600. The "Condensed Service Manual" calls for 1550 so I set it at that. My idle varies around 1550-1600 after all settings made, and I use an aftermarket tach with wire that wraps around the spark plug, with setting on the tach as "2 cycle, 2 cylinder" to get the proper idle speed readout for my tach. It takes awhile for the motor to warm up, so patience until setting the idle speed, which pays in that it will start from cold with the choke and idles at approx 2500 to 3000 as it slowly warms up on choke.

Carb model: The Condensed Service Manual states VM20SS. However as noted above my idle air screw looked similar to a VM24 type (from YT175) instead of stock VM20, and there was no metal plate inside the carb below float needle or main jet. Also, the jet needle hold down was like a VM24 with a small screw holding a plate over the needle clip. I wanted to replace my float needle and seat and found that the thread diameter for the brass seat for my carb was 10mm compared to an original 3X3 part numbered needle and seat which was about 7-8mm and finer threads. The needle seat orifice was still just 1.5 for mine which was the same as the original 3X3 part I tried (I tried a couple of sources and each 3X3 was too small). Also, the fuel inlet on my carb was straight down, instead of a 90 degree elbow found on the stock carb.

Float Height: See above discussion regarding challenges determining float height specs. I measured from base of carb bowl. There is a cutout in the side of the bowl at the base to enable using a measuring tool. My float height was 23mm when I initially opened it up and checked float height. I tested both 21mm and 25mm and returned to the base 23mm float height I initially found and it seemed to run great there. (but not bad at the others although there was overflow a bit at 21mm).

Air Filter: Mine has the air box removed and a Uni foam filter attached at the carb inlet bell.

My exhaust pipe is stock, as is spark plug heat range. With carb cleaned and settings made, the YT runs fantastic! It really rips. I'm really surprised how fast it is, yet seems very torquey and tractable at putt-around on technical trails too.

I am at sea level, temps for setup and testing ranged from 50 to 75 degrees, with 50-90% humidity. I tested over a several week period after arriving at settings (and left them alone after making final settings) to see how it started/ran. Generally now it just takes longer to warm up when cooler out than if warm. Once with warm out (70-75) I didn't need the choke after having on for a few pulls, but flicked the throttle lever instead. Usually will start on 2nd pull with the choke on from cold, and with 1 pull when warm and maybe a flick of throttle.