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 (1980s-1990s). 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 location on carb body 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 almost all of them (Yamaha manuals) show a 21mm +/- 2.5mm float height, except manuals for Kawasaki KZ models and Yamaha XS models that showed 23mm float heights.

Initial Air Screw setting - 1 1/4 turns out may be the "initial setting", 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 1/4, I am still working on final adjustment and am currently at 1 turn out. 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, 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. All the service manuals noted above (Yamaha, Kawasaki) for motos with VM24's in early 1980's show initial air screw settings of 1 turn to 1 1/2 turns out, with most having a "+/- 1/8" turn or "+/- 1/4" turn added to the specified setting. So, my 1 turn out isn't far out of the norm, and seems within accepted range per the various manuals. I'm thinking also that with less vacuum from the YT125 (normally fitted with a VM20) compared to the YT175 (normally fitted with a VM24), that a air screw setting of 1 turn out may suit the 125cc better than using more turns out. My YT125 seems to be sensitive to 1/4 turn differences in the air screw, and has taken a bit of fine tuning to balance cold start needs when starter/enrichener circuit is engaged (see below).

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 indicated stock position in manuals, while mine runs best with the clip in the 4th groove down from top (slightly richer than stock).

Choke Plunger: On mine the knob broken off and I had to replace it the plunger/starter valve assembly. 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 the plunger would stay open. Since installation the choke has worked well. BUT, that was after having trouble starting from cold. Warm starts never a problem, just cold starts, and again I looked into pilot air setting, throttle valve idle height setting, air leaks, crank seals, etc, but ultimately it came down to set up of the choke cable free play so the plunger opened properly. I set the plunger (by way of cable free play) so that 1/2 the enrichment port (air/fuel port) in the intake side of the carb venturi was obscured by the plunger - easy to see with the carb off, plunger setup in place, and cable operating. In order to get the plunger to fully open/clear the port I needed that aluminum washer upon brass fitting install to enable the plunger to be pulled further outward at the 2nd detent of the "choke" knob on the dash with cable. Now, on the first detent, the plunger comes to 1/2 way into the port (leaving the port 1/2 open 1/2 closed), and at the 2nd detent the port is fully open and plunger just out of the way and barely visible. There is no "spike" or needle/pin on the plunger, but normally it appears for the cable operated systems that have two detents the needle is there to limit fuel flow at first detent. My setup of obscuring the port by 1/2 at the first detent appears to have a similar effect of limiting fuel (but also limits air while the needle/pin plunger type may not limit air as much).

The Mikuni uses a separate starter enrichment circuit so the plunger opening amount does effect how much fuel/air enters the intake depending on plunger amount open, and just like other jet/air settings to get to the proper mixture, taking time to set up the Enrichment circuit certainly seems to help. BUT, the pilot circuit still operates as if at normal idle too, so setting pilot at "correct" setting to enable combined fuel/air at cold startup seems more critical than normal setting of pilot to highest idle speed then adjusting idle to correct rpm. When I did that, the pilot was at 1 1/2 turns out but may have been a touch too lean when combined with the starter/enrichment circuit at cold startup and took many pulls of starter rope to get it to stay running more than a few seconds at cold startup, until finally stumbling to life and staying running. Now at 1 turn out of the pilot, when fully warmed up and returning to idle, it does go "low" idle speed by 50-100 rpm before coming back up to "normal" rpm setting (indicating a rich mixture for pilot setting), while at 1 1/2 turns out no such drop occurred, more of a "hanging" idle condition was occurring (barely) indicating a slightly lean pilot setting. I tried 1 1/4 turns out and that was about right, with a little bit of hanging idle, and at 1 turn out that went away with a quick return to idle under most conditions. I would rather trade off good cold start ability over a slightly rich pilot setting when fully warm. Of note, when warm there is no bog at all when flicking the throttle from full closed, so my 1 turn out does not seem overly rich, and only the slight idle rpm drop at times seems to indicate a slightly rich pilot setting when fully warm/hot (and air temp 70+)

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 1500 rpm for the YT125 and YT175. Clymer shows 1500 +/- 50 for the YT125 and 1600 +/- 50 for the YT175, I am still determining the best idle rpm but so far it is approx. 1500 rpm (floating/hovering around 1500 rpm). 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.

When totally cold and on 1st/half-out detent of enrichment circuit it starts with one or two pulls, and the goal is to have it start with one or two pulls under every possible ambient air temp and condition. From all the owners manuals I have found, the all-the-way-out 2nd detent of the "choke" knob/lever is for cold engine start- ambient temp "below 40 degrees F" conditions, while the 1st detent is for cold engine start - ambient air temps ranging from "30-90 degrees F" and also for "warming up position", and choke knob off (all the down) is for cold engine start - ambient air temp "above 80 degrees F" and also for "warm engine start position". The manuals state: After the cold engine started at the 1 position (full cold below 40 degrees 1st detent), the starter knob/lever is returned to the half open/2nd detent position for warming up the engine.If the cold engine started at the half open/2nd detent position, keep this position (2nd detent) for warming up the engine, Continue warming up the engine until it idles smoothly and return the starter knob/lever to the original closed position. Right now, at about 70 degrees F ambient air temp, with the choke knob at just under the first/half-out detent (about 1/3-3/8 or so) it starts with one pull on cold startup, and I then slowly move/pull the knob to the first detent to bring the idle up for warmup (although I could leave the idle rpm at about 1500 rpm where it started, I don't want to risk fouling or stalling), and at that first detent it then idles about 2000 rpm). As weather cools (or heats up) I will test other knob positions as conditions may require for smooth easy cold startup. Warm startup is no issue - one pull every time.

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 23mm. The horizontal line on the side of the float is level with carb bowl base at 23mm, while 21mm and 25mm the floats appeared way "off" level-wise. 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 +/- 2.5mm is the float height the vast majority of early 1980's Yamaha service manuals of motos fitted with a VM24 are stating. One of the manuals stated 21mm +/- 1mm, but all the other Yamaha service manuals stated 21mm +/- 2.5mm. Kawasaki KZ 650 manuals state 23mm, as do Yamaha XS 650 manuals that I found. Of note - the Starter Jet tube has emulsion holes on the tube sticking down into the float bowl, and I am thinking that if float height is too-high then fuel may cover the emulsion holes on the tube, preventing forming of fuel droplets needed for further mixture at the plunger chamber, but would pull up only fuel instead creating an overly rich fuel-only state at the plunger chamber and could lead to fouling or incorrect starter/enrichment circuit mixture. So far the 23mm appears to work well on my carb with the Starter, Pilot, and Main Jet circuits, with no apparent overflow or lean conditions noted so far.

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 (with rubber grommet) 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, and temps for setup and testing ranged from 50 to 75 degrees, with 50-90% humidity.

Carb settings for my VM24: Air Screw 1 1/2 turns out, Idle RPM 1550, Float Height 21mm, Main Jet 115, Jet Needle clip is at 4th groove down from top. Throttle cable play 3mm, Choke/Enrichment cable play 3mm.

Parts sources:

Ebay

Partszilla

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