Decades-Behind
Member
- Apr 21, 2023
- 1
- 0
Hello fellow off-road enthusiasts,
Though I'm fairly young (29), I love older bikes and have restored many. I own a few old Yamaha IT's and love riding my 175 in super technical terrain. I've been recently practicing slow wheelies, getting the bike vertical, and pivot turning on the back tire for hard enduro type riding. I'm probably in the minority of people using their old IT in this manner, but I found that whenever I had the bike completely vertical it would stall out, like it would run out of fuel. I searched and searched online and never found anyone with a solution, just people saying " carbs weren't meant to run at that angle". Even on newer carb bikes I found people asking about this issue and never any real solutions presented.
I'm a mechanical engineer and I love old bikes and carbs and I decided to tackle it myself. Here is what I discovered. My bike uses old tech, a Mikuni 36mm VM.The Mikuni VM has the pilot jet located towards the front of the carb on the side of the intake spigot (side that goes into engine). When the bike goes vertical the pilot jet becomes uncovered and the carb can no longer deliver fuel through the pilot circuit.
Solution: The pilot jet needs to somehow still receive a supply of fuel when vertical. I found some tube I had and extended the pilot "straw" by forcing the tube into the orfice. When the bowl is installed, the tube bends towards the back of the carb and now always receives fuel. The bike now stays running and idling when completely vertical.
Adding the tube has not changed how the bike runs at all that I have observed other than allowing vertical running. I cut the tube so it does not interfere with float operation, it stops just short of the float hanger. I can now practice my slow balancing wheelies and pivot turns on the 175, it will idle indefinitely in the vertical position. I believe this is the problem and will work for other VM series carbs. Again, I know i'm probably in the extreme minority of people riding their old IT like this, but I wanted to share incase anyone else needs their bike to run vertically. This can probably help if you have issues dying in extreme hill climbs too! And yes I did have my float adjusted to the correct height prior to this issues.
A lot of forums I visited tried to suggest bikes were dying do to flooding or float height issues, I'm sure that is possible on all the various carb designs out there, but for this mikuni, it was solely due to pilot being uncovered. For others suffering with different carbs, take a look, it may be a similar issue. When my old IT is completely vertical ( 12 o' clock position) I have no fuel running out of the vent tubes, nothing coming out of the overflow, and it'll just sit and idle or rev indefinitely.
Here's some pics of the old girl:
New plastics now:
Though I'm fairly young (29), I love older bikes and have restored many. I own a few old Yamaha IT's and love riding my 175 in super technical terrain. I've been recently practicing slow wheelies, getting the bike vertical, and pivot turning on the back tire for hard enduro type riding. I'm probably in the minority of people using their old IT in this manner, but I found that whenever I had the bike completely vertical it would stall out, like it would run out of fuel. I searched and searched online and never found anyone with a solution, just people saying " carbs weren't meant to run at that angle". Even on newer carb bikes I found people asking about this issue and never any real solutions presented.
I'm a mechanical engineer and I love old bikes and carbs and I decided to tackle it myself. Here is what I discovered. My bike uses old tech, a Mikuni 36mm VM.The Mikuni VM has the pilot jet located towards the front of the carb on the side of the intake spigot (side that goes into engine). When the bike goes vertical the pilot jet becomes uncovered and the carb can no longer deliver fuel through the pilot circuit.
Solution: The pilot jet needs to somehow still receive a supply of fuel when vertical. I found some tube I had and extended the pilot "straw" by forcing the tube into the orfice. When the bowl is installed, the tube bends towards the back of the carb and now always receives fuel. The bike now stays running and idling when completely vertical.
Adding the tube has not changed how the bike runs at all that I have observed other than allowing vertical running. I cut the tube so it does not interfere with float operation, it stops just short of the float hanger. I can now practice my slow balancing wheelies and pivot turns on the 175, it will idle indefinitely in the vertical position. I believe this is the problem and will work for other VM series carbs. Again, I know i'm probably in the extreme minority of people riding their old IT like this, but I wanted to share incase anyone else needs their bike to run vertically. This can probably help if you have issues dying in extreme hill climbs too! And yes I did have my float adjusted to the correct height prior to this issues.
A lot of forums I visited tried to suggest bikes were dying do to flooding or float height issues, I'm sure that is possible on all the various carb designs out there, but for this mikuni, it was solely due to pilot being uncovered. For others suffering with different carbs, take a look, it may be a similar issue. When my old IT is completely vertical ( 12 o' clock position) I have no fuel running out of the vent tubes, nothing coming out of the overflow, and it'll just sit and idle or rev indefinitely.
Here's some pics of the old girl:
New plastics now:
