i know this might sound crazy but check your coil as i believe is under the gas tank and in front of your carb maybe loose connection there might not be a problem but worth the check,

make sure your slide needle is properly installed on my 99 400 the spring got dislodged one time making it run terrible and im not sure on the 300's carb but i had a small jet on the top of the carb under the diaphragm that was loose and it also cause similar problems not saying that's gonna fix it but there simple things to check hope it helps ya

 

i guess this is my next step in the limitation process is electrical, i got this used with this problem thinking easy fix boy was i wrong.... but there is still somthing up with the carb. it got to be

 

I have run into this one before (I think). that model should have a vacuum operated fuel shut of or petcock on it, mounted where the fuel comes out of the tank.. if it does try pinching off the vacuum line on the petcock. when the diaphragm in the petcock goes bad it will suck fuel through that line and dump it in the carb. also it won't shut the fuel off when you shut the motor down. that is why it fills the crankcase with oil. if it runs better with the line pinched you have found the problem and you need to replace the petcock. hope this helps.

 

have seen no vacum line but the shut off dont work right or shut all the way off. the fuel line was short and my filter had a air bubble in it, added about 3 inches to the line got rid of the bubble, doing good on not flooding as of now....checked all the electical had to replace or bypass a plug on the charging system had croded were the battery looks like it cracked at one time getting acid on it....it is running like a charm with out the rubber tube going from the carb. to the air box....put it on and runs like crapI dunnoI dunno

 

the rubber hose from the top right of the carb goes to the air box? is that what your saying ? i did that to mine in hopes of waterproofing with out buying hose but it did that, the air box is a partial vac. trying to suck air from the carb! i hope thats what your talking about because this will solve your problem.

 

Believe he is referring to the sleeve that runs from the air box to the mouth of the carburetor to supply it with clean air from the air filter

 

macgyvr i see what he is saying now hahah i didnt read it slow enough....mudwize i have an 02' 300 and between the airfilter and the boot that goes to the carb there is a layer or two of mesh i dont know if the 98's have it but its worth look at? idk i thought i knew what i was talking about

 

If he is talking about the main inlet air tube from the airbox to the carburetor and off it runs but on it runs like crap, then there must be either a very dirty or clogged air filter or an obstruction in the air inlet piping supplying outside to the air box, then through the air filter and on into the engine. Such as a mouse nest or crushed air inlet tube. Removing that tube would overcome a overly rich condition. Overly rich could arise from mistakes in carburetor reassembly, parts left out, Float level set too high or a stuck choke, AKA (enrichment plunger)

I highly recommend you get the fuel shut off valve repaired or replaced. Since the fuel if gravity fed in to the float bowl and any amount leakage past the float needle will fill up the cylinder with gasoline. Especially from vibration during transport in the back of the truck on on a trailer. That is why it has the fuel shut off valve under the tank so fuel can be shut off during storage and transportation to prevent fuel flowing into the cylinder. A damaged float needle, damaged float needle port in the carb body or crud stuck in the port of the carb body that the float needle seals off when the carburetor bowl is full or a improperly set float fuel level will also cause the same problem.

 

mine is got the wire mesh in the air box also, its been blowed out and cleaned, new air filter, with the air filter out and the lid left off of the filter box with hose on still not running right but off will run like a charmI dunno it just dont make no seince at all... my tube is hard getting on and off looks like the wrong bend in it could it be possable wrong tube, well that should not make that big of a differnce after typing it out and thinking about it....i think it is missing part of the breather, should there be somthing else contected to it were it gets the air to the filter box, like mounted to the tank but still under the seat? the guy that had this b-4 me had a screw in were the choke goes in to the carb to plug it off i guess, cause the cable had broke, fixed that (new cable) almost 70 bucks with the choke end also... to turn on the choke it goes down right? making the plunger come out and push it up to turn off right? cause dont need to choke it on starting.................to all THANKS for all the info

 

how well does the slide move when the carbs off but all together? did you ever clip the spring or drill the slide?

 

moves freely..what do you mean on clipping and drilling ? i can tell a differnce with the choke on and off and have blowed through it with air compressor every time i have cleaned it, but dont mean that it might have a bur blocking it some from the screw being put in causing one ...now if the jets was to big would this cause the same problem? cause i dont know if some one has put a jet kit in it or not, far as i know its stock

 

to turn on the choke it goes down right? making the plunger come out and push it up to turn off right? cause dont need to choke it on starting.................to all THANKS for all the info

That is correct! Now you make it sound like there could be a problem with the choke not working properly, seeing as how the last guy just stuck a bolt in it. If its always choking itself then it would run with the air box off and not with it on. I would take the carb out and blast through the choke port with some compressed air as well as the other passageways and ports i would be suspicious of something being clogged up.

I am open to be corrected if I am wrong on that theory. Although these bikes are designed to run with the proper supply of air with the air box off its getting more air then it needs which would let it run with the choke on. and then putting the air box back on its then chocking properly causing the bike to shut off.

 

carb. air screw results
1/2 turn out wont idle smokes sputters out at about 1/4 throttle
1 turn wants to idle then the same
2 to 2 1/2 turns out good idle no smoke sputters out at little over 3/4 throttle loses rpm
3 turns and more smokes no power and sputters after idle
.................................................................................................
now with it being set at 2 turns out good idle no smoke i take the rubber tube off going from the carb. to the filter box it picks up just a little bit of idle(around 100 rpm guessing not much) got full throttle runs perfect.....real peppy
.................................................................................................
all runs made with motor warmed up..with new air filter in and out with no change with air filter in or with out filter or filter box lid even being removed
..............................................................................................
I dunnoI dunno would going to a smaller jets sovle this i hate to run with out the filter

 

i doubt that a jet kit would be that rich and i doubt some one would change jet size enought to do this to it it does seem that it very well could be choke related Macgyvr. drilling and clipping is just a mod done to the slide and the spring above it the more i think about the choke theory the more i think thats it ill be tearing my 300s carb down very soon ill pay close atention to that area to see how it works and ill have to play with my choke and the boot to the airbox to see how mine reacts maybe the same? ill report back with my findings good luck!

 

thats fricken wierd i was typing when you posted with that info the game is changed im stumped....

 

its got me, i run into a problem like this before i was working on a guys honda 110 3wheeler it was starving for air,some one put a differnt carb. on it dont know what from,but drilled a few holes in the air box and put plugs in and sovld it.......wish this was that e-z

 

does it have a stock exhaust on it?

 

yes stock exhaust..i am thinking about going down a size on the main jet, took it out riding last night found some good thick mud, when had to nail the gas to full and hold it there for a bit was getting some gas smoke it was smelling rich but no cutting out(with out tube) this was the first time bike has been out of the yard...

 

do some plug chops best way to check you jetting refer to
Jetting 101

 

what is the stock main jet size? i have read on some forms 135 and went to a web site were i have ordered parts and looked at the digram and all it gives me is 2 part numbers 4505-060 and 4505-019 what is the size of the jet? mine is got 137.5 jet and thinking it little big

 

4505-060=137.5

4505-019=142.5

 

gutted exhaust had carbon build up but not that bad......started smoking, oil had gas smell to it so tore the jug off for a top end rebuild pistion looks good,wall real good,no ring grove,rings looked good but one is thicker than the other on the compression rings but my oil rings was tight on the pistion.....had some oil on top of pistion when got to it maybe run off from the top? had about half of tea spoon oil on top...now the vales was carbon up good the exhaust has carbon baked on there pretty good.....so just going to hone it and try new rings bike only got 2200 miles on it. then try to jet from there

 

up date

parts came in today, re ringed it, running like a champ with air box and filter on Rockerdude but dont think the rings solved the problem thinking it was the timming chain was off about 3 to 4 teeth dont see how it run as good as it didI dunno......really no power gain in low end but got top end like it should with box and filter in...

 

very nice good to hear you got it goin man! Rockerdude

 

Good DIY man

 

thanks all.. also put a 130 main jet in

 

congrats man... now go back to the atv and tell it, wide mouth open: "you're mine!" you may want to add a thrusting gesture.

 

Just an FYI for anyone who is reading through this having the same issue. I’ve got an 98 AC 300 4x4 that would run totally fine with the boot (black tube) from the air box to the carb off. But you put that boot on and even with the lid and air filter off it’d start running rough and eventually die. I noticed little specs of fuel coming out of the intake side of the carb when turning over and engine running. So when it was running with the boot on it was then sucking the fuel into the carb and flooding itself out! If you are having the same try adjusting your intake valve, you want about 3/1000 of play. Mine was way to tight and restricting the flow of fuel! She runs top notch now!