Hey fellas, I need some help with my ATV engine. Here is what I know:
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?
- 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?
Last edited:
