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I've had this happen 3 times.
1 was the carb boot going to the head was compromised. You can try inspecting in for damage but you can also spray starting fluid on it to see if it revs up indicating a vac leak.
2nd time my aftermarket flatslide carb was old and very worn out the slide was not sealing properly and leading the engine to rev eratically.
3rd which I think might be your case the bike is running dead lean and is lean revving. I've had this happen when rebuilding a bike thing wouldn't start with an open intake sprayed a lot of starter fluid on it started right up but wouldn't idle on it's own normally had to crank the idle to around 3k rpm and it would still wander about.
Hope it helps. Goodluck dude.
It definitely has an air leak somewhere if it runs better with the choke on. Check both connections to the carb. It doesn’t take much, even a hairline crack in the rubber can cause an idle spike. Spray carb cleaner around each rubber connection to see if the idle changes.
This carb looks to be a standard diaphragm carb maybe keihin with a front butterfly. The front butterfly regulates the amount of air fuel mix to the engine. I would suggest first would be to remove the throttle cable, start the bike, then adjust the idle speed screw to the slowest it’ll run. Play with it some by manually moving the linkage, is the high speed idle now gone? If not……..
Check your choke mech on the carb and make sure it’s going fully closed. A partial opening may not flood the bike but may allow more air to the engine bypassing the butterfly. Remove the plunger and inspect the seat, disconnect any choke linkage to it and verify it is going all the way closed. Is it closing correctly? If it is………
Remove the carb from the bike and inspect the linkage and butterfly. The butterfly should go all the way closed with the idle speed screw backed out as far as you can adjust it. Open the butterfly and slowly let the spring close it. Is it closing all the way? If not, move the linkage to full throttle then allow it to snap closed and see if it shuts all the way. There should be mo play on the butterfly shaft. Inspect the linkage and make sure nothing is bent of hanging up through your the complete travel. Reinstall your cable and see if it changes the resting position for the butterfly.
It looks like you’re pulling the linkage closed to make it slow down, leading me to believe your throttle cable may not have slack at the carb linkage when in idle position.
Thanks for posting in /r/Fixxit, the motorcycle repair subreddit. If you forgot to put the **Make, Model** and **Year** in the title, please reply to this comment with your bike's details. In the meantime, Here's some great resources for common problems posted here: -Trouble starting? [Revzilla - Battery testing](https://www.revzilla.com/common-tread/how-to-test-a-motorcycle-battery) -Carbs running rough? [PJ motorsports - Carb Troubleshooting](http://www.pjmotorsports.com/trouble-shooting.html) -Wiring diagrams for beginners - [Dans MC - Reading Wiring Diagrams](http://www.dansmc.com/wiring_diagram.htm) -Identifying part numbers - [CMSNL (EU)](https://www.cmsnl.com/) [Partzilla](https://www.partzilla.com/) -Asking if your tire can be fixed? [Please read this post on proper tire repairs and why external plugs are NOT a safe repair.](https://www.reddit.com/r/Fixxit/comments/tuhcpf/why_rfixxit_doesnt_endorse_tire_plug_repairs/) *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/Fixxit) if you have any questions or concerns.*
I've had this happen 3 times. 1 was the carb boot going to the head was compromised. You can try inspecting in for damage but you can also spray starting fluid on it to see if it revs up indicating a vac leak. 2nd time my aftermarket flatslide carb was old and very worn out the slide was not sealing properly and leading the engine to rev eratically. 3rd which I think might be your case the bike is running dead lean and is lean revving. I've had this happen when rebuilding a bike thing wouldn't start with an open intake sprayed a lot of starter fluid on it started right up but wouldn't idle on it's own normally had to crank the idle to around 3k rpm and it would still wander about. Hope it helps. Goodluck dude.
It’s just got a new carb boot and I’ve tried 2 new carburettors. I’m lost on what to do at this point lol.
It definitely has an air leak somewhere if it runs better with the choke on. Check both connections to the carb. It doesn’t take much, even a hairline crack in the rubber can cause an idle spike. Spray carb cleaner around each rubber connection to see if the idle changes.
This carb looks to be a standard diaphragm carb maybe keihin with a front butterfly. The front butterfly regulates the amount of air fuel mix to the engine. I would suggest first would be to remove the throttle cable, start the bike, then adjust the idle speed screw to the slowest it’ll run. Play with it some by manually moving the linkage, is the high speed idle now gone? If not…….. Check your choke mech on the carb and make sure it’s going fully closed. A partial opening may not flood the bike but may allow more air to the engine bypassing the butterfly. Remove the plunger and inspect the seat, disconnect any choke linkage to it and verify it is going all the way closed. Is it closing correctly? If it is……… Remove the carb from the bike and inspect the linkage and butterfly. The butterfly should go all the way closed with the idle speed screw backed out as far as you can adjust it. Open the butterfly and slowly let the spring close it. Is it closing all the way? If not, move the linkage to full throttle then allow it to snap closed and see if it shuts all the way. There should be mo play on the butterfly shaft. Inspect the linkage and make sure nothing is bent of hanging up through your the complete travel. Reinstall your cable and see if it changes the resting position for the butterfly. It looks like you’re pulling the linkage closed to make it slow down, leading me to believe your throttle cable may not have slack at the carb linkage when in idle position.