When I had the carb apart I didn't check the float level, as far as measuring it or anything, but it did at least seem to operate freely. I got a notification that my new CDI box for delivered earlier today, so when I get home from work I'm going to toss that on and see if that does anything. If not I'm going to try a coil off another Yamaha I have, just to rule that out. And if that fails, I guess I'll pull the carb off again and double check the float level measurement. If all else fails, I'll just replace the carb I guess...

I'm really hoping for it to be the CDI box. The more I think about it, it seems more RPM related than throttle position, which leads me to believe it's related to spark control and not fuel delivery.

 

Double post...

 

I'm dealing with the exact same issues with a Kodiak 400 I replaced carb and didn't help I have a new cdi and coil to put on next I'm gonna follow and see if we can figure it out

 

Well it wasn't the CDI box in my case... Installed the new one this evening and there was absolutely no change, for better or worse.

I still have the coil to swap over and try, so I'll give that a shot tomorrow.

While ripping around this afternoon though, I did notice that it now runs better, a little more "peppy" if you will, and actually seems to rev just a little higher with the choke on and restricting airflow, vs with the choke off... So it now again has me leaning back towards a fuel delivery and carb issue. I might end up just changing the carb out completely for a new one... Don't know if there's something internal that I just can't get cleaned out, or if someone in the past tried to clean it and damaged something in it, I just don't know.

I've also noticed what seems like a valve tap... Will out of adjustment valves cause this sort of issue? I would think that if the valves are that far out of adjustment that the issue would persist through the entire RPM range, like it wouldn't idle or anything, or have any sort of throttle response. Either way I'll probably still check the lash, since I can hear them they're probably out of spec.

 

If the valves are adjusted too tight, that could cause them to stick a bit, and, this is just a thought, maybe at lower RPMs it has time to "unstick" and close, but at higher rpm it doesn't?

 

"actually seems to rev just a little higher with the choke on and restricting airflow, vs with the choke off."

Bruins, like most ATVs use an excess fuel device instead of restricting airfow to provide a "choke" effect. I would be leaning towards it being a CD diaphragm problem, but the diaphragm can swell and refuse to fit back in the carb properly on Kehins, rumour has it that, wait two weeks for the fuel in it to evaporate and the rubber shrinks back to size.

"If the valves are adjusted too tight, that could cause them to stick a bit,"

No it doesn't, it causes the valve to be off it's seat all the time, so loosing compression.

 

Yeah I discovered that after a little research. I thought the choke mechanism was a little odd when I dug into the carb, at least compared to automotive carbs I've played with before lol.


So the fact that turning the choke on and introducing more fuel make it run a little better, tells me it's running lean either by either getting air in after the carb before it enters the engine (which I checked for by spraying starting fluid around the intake piping, but who knows maybe I didn't hit the right spot, I suppose I could try again), or something is wrong inside the carb itself (either a blocked passage that I didn't quite get cleared out or an actual defect in the carb).

The carb is an aftermarket cheap knockoff that the previous owner put on it, looks like it's been pulled apart at least once before, so I have no way of telling really if it was reassembled right then...

I'm going to attempt pulling and cleaning the carb one last time, but if that does nothing I may end up just replacing the carb completely to rule it out at this point.

 

Well another carb cleaning has done nothing... I think the previous owner either damaged something internally it resembled it incorrectly some time in the past possibly.

Either way I decided to start fresh. I​​​​ just ordered another cheap carb off Amazon for $25 and I'll see where that leaves me. Hopefully it makes some sort of change. With my luck with the new cheap carb, it'll have full throttle but won't idle lol. We'll see. It's too cheap to not give it a try.

 

I was actually having the same problem with an Eiger, not exactly as it ran OK, just ran out of puff at about 3000rpm, no coughing, missing etc. Anyway, had rebuilt the top end including re-bore and could get it to do nothing except idle, took carb apart and found the diaphragm soaked in fuel, this had swollen it and I had a hell of a job getting it back in. Anyway fault turned out to be plugged silencer, no idea why, but the bike had stood outside for a couple of years waiting for the engine work, maybe rodents, or rusty baffles. Anyway after all that it still wouldn't rev over 3000. No cheap carbs for Eigers and cleaning the float chamber/jets didn't work, so reluctantly I took the diaphragm out again yesterday, and found it hadn't been seated properly. The rubber has dried out, presumably spitting back from the plugged exhaust must have soaked it previously, so I put it back carefully, and it now revs out fine.

 

Well it ended up being a carb issue. Tossed the new $25 carb from Amazon on it this afternoon, it fired right up and ran like a raped ape for the first time I've owned it!

Before I installed it I did a little investigating. I pulled the bowl off the old one and the new one so I could compare the jets. Old one on the left (also a cheap Chinese unit) and the new China made carb on the right.






Aside from having what I can only assume is the idle mixture screw accessible on the new one whereas it's capped off with a brass insert on the old one, everything else looks identical at first glance.





So I pulled the jets out to compare. Take a look at these main jets... Old one on the left, new one on the right. The left one is CLEAN, it's just that small... So at some point the previous owner put the wrong jet in it during a rebuild, so it was starving for fuel when I punched the throttle, which is exactly how I was acting.





So in the end, it was all because of an incorrectly rebuilt carb... And I can't believe I didn't catch that the first time I pulled the carb apart and cleaned it lol, but oh well. All seems to be well now Thank you for everyone's input and suggestions!