Fuel and Air
The carby is 31 years old so is going to be worn. A carby kit probably would not cut the mustard so i new one will be the order of the day. Luckily, new Mikuni carburetors are quite cheap. The Yamaha Race Tech sheet recommended an IT250J carb as the hot setup for the YZ125K. This is a 36mm VM as apposed to the original 34mm. They are still readily available brand new for only $150 in 2014. The only problem is getting some of the original spec jets.
I have also sourced an air cleaner for the bike.
NU-2263ST UNI FIL YZ125/250/490 (Parts Unlimited) $23.32
Below is a list of the carby mods that need to be made if you want to do the Yamaha Tech mod.
I have also sourced an air cleaner for the bike.
NU-2263ST UNI FIL YZ125/250/490 (Parts Unlimited) $23.32
Below is a list of the carby mods that need to be made if you want to do the Yamaha Tech mod.
The VM 36 is cheap and easy to get but some of the jets are a bit pricey. A standard new Mikuni comes with a 2.5 slide, so you need to get the 2.0 which isnt cheap. The hardest part is getting a 6F34 jet needle. They don't make them anymore and are rare as hell. I am currently trying to source one and also trying to get someone to measure up an existing one to see what other needles are very close in measurements and also still available.
OK, finally got the measurements for the 6f34 needle as recommended by the Yamaha Tech Report. A bloke on the Yamahait.com.au forum had a new old stock one and he was able to get reasonably accurate measurements using the correct technique. Below is a graph i have made up with needles that are available and with roughly the same dimensions.
OK, finally got the measurements for the 6f34 needle as recommended by the Yamaha Tech Report. A bloke on the Yamahait.com.au forum had a new old stock one and he was able to get reasonably accurate measurements using the correct technique. Below is a graph i have made up with needles that are available and with roughly the same dimensions.
Because i cant get a 6f34 as per the Yamaha report and a 6f4 is the closest available but still a little lean, i decided to take get the measurements of the jet needle venturi and calculate the restriction that is created between the needle and venturi at a given point. This will tell me how rich or lean i need to be at a given point from 1/4 to 3/4 throttle and hopefully provide solutions. Below is a matrix which shows the P-0 jet (recommended by Yamaha) and the Q-5 (standard on new 36VM's). Using a cross section area of the jet (its the same measurement its entire length) and subtracting the area of a needle at a given point, we arrive at the restriction surface area in square millimeters at each 10mm interval along the length of the needle.
The conclusion i have made it that a 6f4 needle together with the standard Q5 needle jet venturi has almost the same characteristics throughout the throttle range as the Yamaha race report specs for the 6f34 and P-0 combination. Whew... well, i am now going with the 6f4 and Q-5 set up as a baseline.
I got Tom at MikuniOz to make a carb with the following specs to start off with. The 6f4 is the needle i have chosen to replace the 6f34. It is a little leaner but i will start off with the needle clip lowered an extra notch and that should help richen it up.
Mikuni carb VM36 VM36-4 001-035
Genuine Mikuni jet size 50 (SKU-VM22/210)
Genuine Mikuni main jet size 270 HEX (SKU-MH 4/042)
Genuine Mikuni main jet size 280 HEX (SKU-MH 4/042)
2.0 throttle valve slide VM36/36 003-125
Genuine Mikuni needle jet 159-P-0 (SKU 003-208)
Genuine Mikuni needle 6F4 002-312
Mikuni carb VM36 VM36-4 001-035
Genuine Mikuni jet size 50 (SKU-VM22/210)
Genuine Mikuni main jet size 270 HEX (SKU-MH 4/042)
Genuine Mikuni main jet size 280 HEX (SKU-MH 4/042)
2.0 throttle valve slide VM36/36 003-125
Genuine Mikuni needle jet 159-P-0 (SKU 003-208)
Genuine Mikuni needle 6F4 002-312
The carb was a near impossible fit on the original boots. To make it easier, i cut about 20mm off the large intake end of the carb. You can do this in various ways but i just taped and wrapped it all up and then used a grinder with a fine cutting disc. I then beveled the cut and cleaned it all up. I also beveled the inside of the large airbox boot. I used a dremmel to grind the inside of the rubber. The front was just a case of heating the rubber with an airgun and forcing the small end of the carb onto the front rubber boot. Now, the carb goes on relatively easily. I have had to use an OEM YZ250K throttle cable that has the rubber boot on it. Also, i purchased a Mikuni 45 degree elbow that fits on the carby cap and which the new cable screws into. This is part number I-M22-10-3.
For the airbox and to complete the Yamaha Tech Report mod, i fitted four UNI AIR FILTER Round 1 inch air filters for air boxes. These are plastic snap in round vents that have filter elements in them. PART#: UFV-6. I also fitted an aftermarket fuel tap which was a perfect replica and new fuel hose and clips.
Below is a pic of the new 36VM fitted up and the airbox mods... all per the Tech Report.
For the airbox and to complete the Yamaha Tech Report mod, i fitted four UNI AIR FILTER Round 1 inch air filters for air boxes. These are plastic snap in round vents that have filter elements in them. PART#: UFV-6. I also fitted an aftermarket fuel tap which was a perfect replica and new fuel hose and clips.
Below is a pic of the new 36VM fitted up and the airbox mods... all per the Tech Report.