ATV Carb Help, please!

TheCougar

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Hey fellas, I need some help with my ATV engine. Here is what I know:
  • Yamaha Bruin 350 (2004 yfm350ba)
  • She’s a beast - never had issues. Been sitting in a shed for most of the past 3 years and rarely used, but still ran well. After pulling out of storage and getting ready for an upcoming hunt, it had a high idle and a slow-to-decrease rpm after throttle with the choke ON (rich). No big deal, still worked like a champ once the choke was OFF and she leaned out. Spark plug was black, showing it was burning rich. Otherwise, she ran great.
  • I decided to do some MX - clean the carb, new air and oil filter, new spark plug, clean the fuel filter, etc.
  • I’ve been having issues I can’t solve after the carb cleaning. Here is what I’ve done: cleaned carb twice, new carb (non-OEM Carb), OEM carb rebuild kit on the old carb. I’ve adjusted the fuel/air mixture screw all the way in and out with no luck. No matter what I do - new carb, old carb, old carb with rebuild parts, the result is the same: engine bogs down under throttle (lean symptom). New spark plug is black again (rich symptom). I can get it to idle with the aftermarket carb, both choke on and off, but running at greater than 50 throttle she sputters and starts to die. This is new - it was not this way before I tried to “fix” the high idle.
  • In spite of the spark plug showing black (too rich), I sure feel like the motor is running hotter than normal. And the symptoms are consistently lean.
    • If I put my hand over the intake box tube and manually restrict nearly all airflow into the carb, she runs great at full throttle. That is totally bizarre to me that cutting off 75% of the airflow is required to get her to run at full throttle. I really am cutting off most of the air with my hand - it isn’t insignificant.
  • I’m convinced that I have missed something major that is feeding too much air or not enough fuel into the carb, but I don’t know what. As far as I know, all the tubes are hooked up properly. None of the air or fuel tubes are blocked - I’ve checked every line, put ethanol free fuel in the tank, and cleaned out every air and fuel filter. The air filter is new (same brand as before). I’ve tested for leaks all around the carb and intake boots to see if there is a leak, with no success. I’ve got another carb rebuild kit on the way along with new intake boots in case there is a leak that I haven’t found. There is a good vacuum at the intake box - I can immediately shut the motor off by completely covering the intake port, so I don’t feel like there is a leak.
    • The fact that I have the same problem with both carbs and rebuild kits points to something beyond a dirty jet. I’ve got a major air leak or a fuel restriction somewhere, or I’ve hooked something up wrong, or something stupid like that. I’ve been chasing down a phantom air leak this entire time, and maybe it’s a fuel supply issue that I should be searching for, but again she runs great at full throttle when I restrict airflow at the intake, so if it was a fuel supply issue I would think it would sputter and die from fuel starvation anyway. I’m out of ideas at this point, besides taking it to a professional. It is taunting me from the garage right now…. I want to figure this out before I have to go spend more money on a mechanic or waste more time on it.

That’s all the pertinent information I can think off. I’ve searched the depths of the internet and my very rudimentary carb skills and come up empty. Does anyone here have any ideas?
 
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505Wapiti

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What have you checked in line/tank prior to fuel getting to the carb. I’m sure you’ve already done it, but just a thought because I’ve seen it happen before. I made the same mistake first time I rebuilt a motorcycle and three wheeler carb as a kid. I gunked up the rebuild because I had a line/tank issue and crap gas and crap in the tank. Even past the filter the inside of old rubber fuel line was breaking down and had crap coming loose and getting in the carb.
 

505Wapiti

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Bad fuel is a pain in the butt, it never hurts to put some STA-BIL in the tank if it is going to sit for extended periods without use.
 
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Is there a petcock, aka metal valve on the bottom of gas tank etc to change to reserve tank?
Almost every quad ever been around has some sort of screen at the sump, fuel pickup, petcock.

Another thing to try would be make a gravity feed gas tank out of a gear oil bottle or something and run the line straight to the carb.
 
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TheCougar

TheCougar

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What have you checked in line/tank prior to fuel getting to the carb. I’m sure you’ve already done it, but just a thought because I’ve seen it happen before. I made the same mistake first time I rebuilt a motorcycle and three wheeler carb as a kid. I gunked up the rebuild because I had a line/tank issue and crap gas and crap in the tank. Even past the filter the inside of old rubber fuel line was breaking down and had crap coming loose and getting in the carb.
I’ve checked the inside of the tank, checked the petcock valve, fresh ethanol free fuel, and checked the inside of the fuel hose. If I’m missing something, I don’t know about it. Fuel flows freely when I open the valve. But I’ll check again.
 

505Wapiti

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I’ve checked the inside of the tank, checked the petcock valve, fresh ethanol free fuel, and checked the inside of the fuel hose. If I’m missing something, I don’t know about it. Fuel flows freely when I open the valve. But I’ll check again.
Figured you had all that covered, I just start with the simple and work through it. Head scratcher for sure.
 
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TheCougar

TheCougar

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Figured you had all that covered, I just start with the simple and work through it. Head scratcher for sure.
There are no stupid questions at this point. I didn’t have this problem before. Now it happens with both carbs. That tells me that I’m the common denominator.
 
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I had the same problem with a 98 kodiak. Turns out that after I rebuilt the carb, I put a small tear in the rubber boot that the carb sits on. They are quite brittle when they get older and if you have a hard time pulling or installing the carb, they are easy to tear.
 

cnelk

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Your spark plug color tells you lean or rich.

You want a cardboard color. If it’s white it’s way too hot.

If the plug isn’t a white color, you’re not running hot.

Did you replace all of the parts in the rebuild kit?
Sometimes existing parts look good and don’t get replaced
 
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Yes

Maybe check the intake boot as another suggested.

The fact when you put your had over the inlet and take air way says you ether have way to much air or not enough fuel.
That’s correct. He is more than likely pulling air through a crack in the rubber boot between the carb and head.
 
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TheCougar

TheCougar

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Yes

Maybe check the intake boot as another suggested.

The fact when you put your had over the inlet and take air way says you ether have way to much air or not enough fuel.
Yup! This is what I've concluded also - too lean mixture, either from too much air or too little fuel. I'm not sure how to identify which one it is. When I get home tomorrow night, I'll install the new boots and O-rings, as well as rig a direct-feed bottle for fuel to the carb. I'll report back. I appreciate all the input.
 
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