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..when I got it, my YT125 was (and still is) fitted with a replacement Mikuni carburetor and not the original VM20. The only markings on left side of throttle valve body says "Mikuni" with small "2Nz" stamped near air bleed screw on a flat spot, and large "MIC" on right side of throttle valve body. Apparently the MIC version was made at a certain Mikuni factory for maybe 10-15 years. The dimensions are 24mm bore, with 35mm spigot and 43mm air inlet outside diameters. It appears to be a Mikuni VM24, with idle air screw in same position as found on the YT125 VM20 and YT175 VM24, and other VM24's I've looked up from the 1980's era. Determining Float Height has been a challenge, as the YT125/YT175 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. Mine was 23mm when first opened and inspected. The VM24 found on the YT175 is different than "normal" VM24's in that it has an internal plate below the float needle seat that extends to below the main jet, which has a hood on it too. Those extra pieces may account for the increased float height, as the extra plate and an extra fiber washer (per parts manuals) would elevate the needle and needle seat. I have been searching every other 1980's service manual that I can for motos that had the VM24 installed, and so far all of them show a 21mm float height, except a Kawasaki KZ model that showed 23mm.

Initial Air Screw setting - 1 1/2 turns out, based on a lot of 1980's owners and service manuals of motos that had the VM24's fitted. While the "Initial Setting" is 1 1/2, I am still working on final adjustment and am currently at 2 1/2 turns out. (I couldn't read the pilot jet size clearly, perhaps a #35, which could explain the need for air screw to be further out in this application). My current setting at 2 1/2 turns out is indicating a too-large pilot jet. Of note, I did replace the stock idle air screw that I found in my VM24, with a modern brand new one from Mikuni, with new o-ring and spring. Mine was missing the o-ring and spring. The shape of the new idle air screw "looked" identical to the old stock one, but it could have been slightly different taper and/or thickness by a few thousandths, which would require a setting further out than the stock/worn idle air screw I found on the carb. I may go back and look up the various pilot jet sizes on all the VM24's fitted in early '80's but depending on cc capacity/use (enduro vs moto tune/design) I may find too much variation to consider an initial new pilot jet size easily, it may be worth it though.

Main Jet : 110 is stock on a YT125 VM20, while my VM24 is fitted with a 115 and runs great at 3/4 to full throttle.

Slide Jet Needle: Clip in 3rd groove is the stock position, while mine runs 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 various manuals: 1500, 1550, and 1600 rpm. The "Condensed Service Manual" calls for 1550 rpm, while owners manuals state 1600 rpm. I am still determining the best idle rpm as a compromise for starting and running, because if I use an idle rpm of 1550-1650 rpm when fully warmed up, there does not appear to be enough velocity through the venturi upon cold starting to pull mixture through the pilot/start circuits. When warmed up it is fine, it will start with one pull. But, when totally cold, a 1600 rpm is just too low for motor to fire and stay running more than a few seconds. If I raise the idle slightly for starting it works fine, and then idle is approx. 1800 rpm when warmed up.  So, it just may be that to attain proper starting venturi velocity for my 125cc with VM24 carb, a slightly higher idle rpm setting is needed. And, the too-large pilot jet and more turns out needed on the idle air screw factor in as well.  I spent a lot of time chasing down possible air leaks through the throttle cable, carb cap (the rubber gasket/o-ring and circlip were missing), vent ports (one did not have a vacuum sealing cap, the other one was cracked), and intake boot was also checked with no issues at all. I used an aftermarket digital tach with wire that wraps around the spark plug to measure RPMs, with the 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 is needed until setting the idle speed.

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 slide jet needle hold down on my carb was like a VM24 with a small screw holding a plate over the needle clip. 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 have tested 21mm and 25mm and 23mm float height that I initially found, and I ended up going with the 21mm I found in the majority of other early 1980s service manuals. I put it back to 23mm as I was getting a little overflow at 21mm but I traced that to a weak float needle spring so I replaced the needle and seat and so I have gone to a final 21mm float height again. The 21mm is the float height the vast majority of early 1980's service manuals of motos fitted with a VM24 are stating. Just a couple of manuals (like two-three) where different, from 17mm to 23mm, with the 17mm's from mid-1970's manuals, the 23mm from an early '80's KZ Clymer manual.

The float needle and seat: Sudco lists the 786-1500-2.5 viton-tipped float needle as stock in the VM24-512. However, in the Mikuni USA catalog, they list a VM26/26-2.0 steel float needle as stock in the VM24-512. Niche Cycle Supply show the VM24-512 they stock as having the VM26/26 float needle and seat version (steel, not Viton tipped needle). The VM26/26 float needle and seat is listed as fitting a wide range of carbs from VM22 on up so I went with the VM26/26-2.5 as it matched the dimensions of the float needle and seat in my VM24 (9mm thread diameter and 10mm head). The float needle seat in my VM24 had  2.5 stamped in it, with the lower part of the "5" barely visible. The Sudco catalog showed a 2.5 orifice for the VM24, and the VM26 has a 2.5 in both catalogs, so just to be safe I went with a 2.5 float needle seat orifice. And, both Sudco and Mikuni recommend using at least 2.0 to 2.5 as a minimum for a gravity fed fuel supply (and 1.5 or smaller for fuel pump fed), and since the fuel tank and fuel line on the YT125 sit relatively low on the frame compared to the fuel inlet on the carb I thought it prudent to go with the 2.5 orifice.

Air Filter: Mine has the air box removed and a 5" long Uni foam filter attached at the carb inlet bell.

Spark plug NGK BP6ES with .028" gap.

My exhaust pipe is stock. 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.

Parts sources:

Ebay

Partszilla

CMSL worked well to confirm part numbers, both NOS and superseded.