TRX300 Carb Issues driving me nuts...

[kb0nly]

Got a 88 TRX300FW last week, started digging into it's issues, the worst one is how it's running.

Pulled the carb, cleaned it and I'm no stranger to cleaning carbs, it's spic and span clean, put a carb kit in all new parts, got it back on and the same issue. It will start just fine but it's running super rich, like starts to burn your eyes running it in the garage with the doors open rich. Mixture screw doesn't seem to affect it, cranking out the idle stop all the way don't change it, if you try and tap the throttle it just dies.

My first thought is of course carb issues obviously but I can't narrow down what it is. The choke plunger wasn't stuck and I put a new seal on the plunger but it acts like the choke is always on, and turning the choke from off to on and vice versa don't seem to make any difference. I pulled the carb again, checked everything again, it's super clean and fresh gas no issues.

It's been about fifteen years since I worked on a carb with the choke slider setup and those were 250ES's but this carb is very similar. I ordered a new choke plunger set, mainly cause after tearing down the carb and testing again five times I was at my wits end and don't know what else to do.

What would cause this thing to run this rich? Jets all stock sizes, carb has been cleaned five times and you couldn't find dirt or blockages if you tried. I'm still thinking it's a choke issue since turning it on and off don't make any change but I can't figure out what that issue is?

Suggestions?

Got it on a trade and fixing it up, pulled the front end apart to replace seals on the front diff and both cv axles today, got the leaks fixed and everything else is good other then brakes which I'm waiting on parts for, just can't get it blessed thing running right.

The previous owner punted it because he had the same issue despite cleaning the carb twice. Guessing we both missing the same thing but no idea what that would be.

IMG_20220520_174617135.jpg

[shortline10]

Make sure your not missing the rubber plug that seals off the bottom of the choke Circuit .
Comes in the factory rubber kit .
https://www.partzilla.com/product/ho...70f4c41d982468

Also make sure the air box lid and snorkel are connected , this model is very picky and needs the entire intake sealed and tight to run properly .

[kb0nly]

It's there and tightly installed and the entire intake system was installed each time I tested it also. Just took it apart again to check the sixth time.

Still stumped..

[shortline10]

If you get no difference when adjusting the air fuel screw or pulling the choke then is probably time to call it quits on that carburetor . Are you 100 percent sure the choke plunger is seating all the way down ?

[kb0nly]

Took it apart again, measure the choke plunger and the depth of the bore it goes down into figured out where it should roughly be sitting, it was close enough, but i decided to clean the choke bore again, small brass gun cleaning brush is how i clean them out. Ran that through again and then a little bit of WD-40 on the plunger just cause it felt sticky, and everything is smooth as can be and its definitely going all the way down. Put it back on and now the choke seems to affect it, but theres all sorts of other issues now.

Can't set idle, its running too fast or not running at all, theres an 1/8th of a turn on the idle stop knob and it will be fast idle or die, no inbetween, tried the mixture screw from all the way in to 2-1/2 turns out at 1/2 turn intervals, still can't set idle, pops when revving, and it won't settle back down to idle after a quick stab of the throttle, just revs up then hangs, then jumps down a bit, then down a bit, then down a bit, slowly until its at where it should be for idle then dies.

I've rebuilt over a hundred carbs on ATV's and this one is a real nightmare, its not giving me any feedback to figure out whats wrong, it runs or it don't, idles one second then don't the next. And now its only starting with the choke but runs fine after turning off the choke, which leads me to believe its lean but i can't get any adjustment to affect it. Letting the motor cool off now i can go back and check things again later, getting to the settings on the carb on these is a real adventure, burning your hand on everything while trying to turn the dang screw is fun.

So i don't know what to do now... I guess look for a new carb would be my next step. I do have a new choke plunger coming from Dennis Kirk that will be here today, i will put that in as a last ditch effort just to make sure its still not that, but otherwise i am beyond stumped.

[kb0nly]

Oh and i checked for leaks, new o-ring on the intake boot, made sure its tight, checked for cracks and its perfect, sprayed a little carb cleaner around the intake no change while running, so its not a leak causing it to go lean either.

Took it apart again, rubber plug still there...

[shortline10]

The OEM carburetor is still available if you purchase one for the 2000 model trx300fw .
I just purchased one about a month ago .

[kb0nly]

Yeah i saw that, $400... Yeah no, i would rather push this thing off a cliff then spend that.

Right now it idles fine, will idle for hours, but it just won't rev up, hit the throttle it dies, turn on the choke it dies, adjust the pilot screw and nothing changes, fully in or fully out, no difference. Never loses spark, i have had guys tell me its the stator, its the cdi, its the valves, its the timing, its this and that... Everything has been checked a dozen times, as far as i can tell its still a fuel issue but i can't get it working.

[shortline10]

Did you pull the valve cover and check cam timing ?
I had one that had me stumped for a bit , cam chain was so stretched that it was about a half tooth off and had the same symptoms as yours .
Customer was hard set that it was just a carburetor issue .

You could also spend 25 bucks on a knock off Carburetor to rule that out .

[kb0nly]

Checked cam timing and valve clearances. Chain seems pretty tight. Ordered another carb off Amazon just to try it, cheap enough to give it a shot and if it runs the same way I guess I light a match and run away.

[shortline10]

Hopefully the carburetor fixes it , another thing to check is the internal pick up coil plug , they get loose over time .

[kb0nly]

Well what's really weird is when we got it the darn thing ran, first day we drove it around the property a few times, my wife took it for a spin a few times, everything was fine. Shut it off put it away, next day i noticed the carb was leaking, like pouring gas down the side of the engine onto the floor. I forgot to turn off the petcock but still it shouldn't be doing that, the bowl gasket was shot, last guy in the carb re-used the old bowl gasket and looked like he added a dab of RTV to one corner where it was ripped to try and get by. So i ordered a rebuild kit for the carb, tore the carb down and put it back together, it ran like crap ever since then. I figured i must have done something wrong, but i have had it completely apart and back together 8 times now, meticulously checking each and every part, port, cooking the body in the ultrasonic cleaner, just can't figure out what changed from the first day when it was leaking yet working to not leaking and not working.

My first thought was it was leaking when we were riding it but didn't notice, and the leak was allowing extra fuel to dump off, so i checked the float and the needle valve, polished the needle valve seat clean with a q-tip and drill like i always do, its shiny clean, checked that its sealing and it is, and moved on. When it started running really rich i figured my float level was off, took it apart again, checked again, still good, tried again. And so it goes, keep taking it apart, putting it back together, over and over.

Currently it just idles great, doesn't seem to be running rich, exhaust seems normal, plug color is changing, used to be jet black rich now its pretty brown and decent looking. Can't give it throttle or it dies still, give it choke it dies, put the choke on it won't start, gotta start without choke, this leads me to believe its still running rich.

Turning the pilot screw in or out doesn't seem to do jack... Is it in for leaner and out for richer on these?? Its been so long i don't remember.

[kb0nly]

Oh and forgot to mention, spark is solid, never loses spark now. Replaced the CDI and that took away any ignition issues, its consistent. I put an inline spark tester on it and when you try to rev it theres still good spark until it dies, its just like you put your hand over the carb and choked it to death when you press the throttle, but you will still see a few more spark plug fires as the motor spins down if that makes sense, as long as the crank is spinning its firing, so i don't think i have an ignition or electrical problem.

[atc007]

Sounds like spark. Aftermarket CDI? Can't wait to hear how the china carb does. I think the same

[kb0nly]

Zero issues with spark, i bought a new CDI from Dennis Kirk for it, was expensive, $71.99 but i haven't had good luck with the cheap chinese ones on Amazon in the past so i just put the money into it. When we first got it, it did have a spark issue, the CDI was intermittent, which is a common issue. Anyway, i have had an inductive tach clamped on to it for all this testing along with an inline spark tester, and since i replaced the CDI i have had ZERO issues.

But!! ITS FIXED...

Got it fixed, wasn't what i was expecting thats for sure. Despite the choke plunger going down all the way it must not have been making a good enough seal, a stronger spring on it and a new plunger and seal took care of all the issues. I put a new seal on the old plunger and it seemed to be sealing ok, so it might have just been the spring, but after putting both in there it stays sealed now. Cold and rainy here today, gave it choke, popped right off instantly, gave it about 10-15 seconds and turned the choke off to see what it would do, dropped to about 2k on my tach and was running smooth, set it down to 15-1600k on the idle and let it warm up for a few minutes, throttle response is perfect, left the pilot screw at 1-3/4 turns as the manual shows. Took it out and rode it around a solid 15 minutes, varying throttle position and which gear i was in, just gave it a normal ride around and workout, everything was great, good throttle response, really takes off when you want it to, got covered in mud cause i don't have the front fenders on currently and of course i had to do some victory donuts... Came back from riding it around stopped and it idles perfectly, restarted it a few times, instantly starts hot so its not heat soaking anything and causing other issues. Now on to the rest of the repairs. I already rebuilt the front differential, two new front CV axles, and went through and changed all the fluids and flushed out the rear diff and the transfer case, everything looked good, oil a little dark but no water or anything out of the ordinary in anything. Next is brakes front and back, new tires, and then on to cosmetic work. I have all new fenders coming from Maier, a new seat cover, and a few other little cosmetic things. I might repaint the tank eventually but that probably won't be until this fall at least because i want to use it now!

Currently two tires are going flat, dry rotted and cracked, and not much tread on them, so thats a priority to get done along with repaint the wheels so they look better, someone ned them with this awful flat gray, maybe just primer, looks like crap.