01' Yamaha Big Bear 400

[Nromes]

My big Bear sputters at half throttle only. Does fine at idle and full throttle. I rebuilt carb and cleaned countless times. I check valve clearance, timing, spark, fuel flow to carb. Also checked boot from carb to cylinder. I also have used fuel treatment. I am getting tired of it just sitting. Am I just not thorough enough when I clean the carb or is there something else I'm missing? Any help would be more than appreciated.

 

[Jimmy Joe]

I would try taking a propane torch unlit crack the valve and move it around the carb body and carb boots while the machine is running to see if there is any change in the rpms.
You may have a cracked boot or air getting past throttle plate pivot.

 

[kwbretired]

When you clean the carb,mod you just blow it out with carb cleaner? I've always used carb cleaner, compressed air, and wires to punch through all the little holes that just won't blow out clean. Make sure you pull the jet(s) and float needle valve to thoroughly clean. Also when using compressed air be careful because some carbs have rubber plugs in some none used passage that can blow out, did this on one of my carbs. Only reason i knew I blew the plug out is it hit me on the cheek.

 

[Nromes]

Yes, I use carb cleaner. I used a long piece of wire to poke through some passages . Pretty sure I did not do all the passages. I don't have an air compressor.
I may need to pull it back off and penetrate every passage with wire [that sounds terrible].
Now I need to find the time to try this all out. I like the idea of the propane torch. I'm definitely going to check.
Thanks for all the help.

 

[kwbretired]

You can use gas cutting torch wire tip cleaner, they are cheap with lots do different size of wires. You can use about anything to spray around the carb and boot, all you are doing is displacing any air from getting into the carb other than the normal route. The rpm's will change if you have air leaking in where it shouldn't be.

 

[pmsmechanic]

I use a piece of stranded copper wire to push through all of the passages that I can get too. I also use brake clean or WD-40 to spray through all the passages making sure that it comes out at the other end. You can usually see by how the passages are drilled where the spray is supposed to come out.

I once had a carb rebuilt by a professional and he didn't cure the problem. What I had to do was remove one of the ball plugs on the outside of the carb and clean the passage in behind. I closed up the passage by pushing the old ball plug back in and staking the hole. A bit of silicone on the ball sealed the hole.