I have a 99 big bear. The carb is brand new. I put a big bore kit on and tuned the carb with a 150 main jet. Ran great. Now two weeks later, I started it up and it bogs as soon as I hit the throttle. I’ve tried several jets going richer and leaner from my 150 jet. All of them speed up idle when backing out from my 2.5 start point. Doesn’t make sense to me. Vacuum leak?

 

Could be a vac leak so check, but the idle set up is different to the main jet, it could need more fuel on idle, so runs faster when you give it more, the 2.5 turns is only a starting point. To be honest I have been having trouble getting Yam carbs to run right lately, they don't seem to follow the rules like other makes. I have an 80 at the moment where adjusting the idle speed screw doesn't seem to make any difference to idle speed.

 

I attached a video of the idle to throttle response. I’ve tried everything I can think of with the carb. Sprayed carb cleaner around everything to check for leaks and nothing. Compression is 150 ish. Timing is set valve lash set. Spark seems good and deep blue color. Seems like no matter what I do it’s starving for more gas. I lifted the carb needle and tried every jet I have from 140-185 I’m kind of lost at this point. It was working fine. I parked it for two days then this.

 

I don't want to send you on a wild goose chase, but that sounds rather like a rev limiter, so spark related. You wrote that the carb is new, I would be cleaning out the old one and putting it back on to see if the fault has gone, or changed. If you get exactly the same fault with a different carb, it would point to the problem being ignition. If it is, getting the workshop manual and doing the ignition circuit tests would be the way forwards.

 

Thank you for your help in this. I will keep you posted what I find next. I’m waiting on a custom jetting kit and a rebuild kit for the Mikuni. See if it’s the cheap carb I bought. Otherwise my thought would be CDI related. When I took the entire quad apart for rebuild I replaced every ignition component except the CDI. I figured it’s apart why not do it all now. Is the Rev limiter controlled through the CDI on these?

 

Answered this yesterday but it seems to have been consigned to cyberspace. Anyway, many bikes have a lower rev limit when in reverse so the reverse warning light circuit is fed to the CDI, this makes the CDI lower the limit.

One thing I would check is the charge rate at the battery, although theoretically separate from the ignition circuit, because things like the neutral and reverse warning light circuit are often fed into the CDI, an overcharging regulator (14.5v max) can affect the spark. I would need to be very sure that your problem is the CDI before buying a new one, as they can be expensive.

 

Easy enough to check. If I do need a CDI how important is it that I get the exact model/part number for my quad? Reason being this bike was sold to me as a 93. It is most definitely not. No VIN on the frame anywhere. I traced it down to being a 99ish. It has round headlights and the stator is slightly smaller and has two pickup coils. Pretty different from other big bear models so I’m having trouble pinning down model.

 

The year would be important to get the right CDI. Our local Yam dealer always asked us to provide information from a white sticker on the frame, somewhere under the seat, I've no idea what the letters and numbers mean but he could pin down the exact model from them.

 

Well I figured it out. There is two pressure switches on the right handlebar controls. The one is for parking brake which applies the rev limiter through the CDI. It was stuck. Thanks for the help my guy. This was a brain scratcher for me. Apparently my quad is a one off unit before they switched to the 400s. My manual doesn’t even have the schematic for it.

 

Glad you found it, and that my long distance diagnosis of a rev limiter, was not sending you on a wild goose chase.