Old trx 300 missfire troubles

[lloydguy]

As per title I have recently bought a 95 trx 300 fw.Not running when purchased(stupid me)
Any way,starts and idles fine(after new carb installed) run's great till about 2500 rpm,start's
to studer and misfire a little ,3000 rpm misfires more,3500 will start to pop and if you don't back off on the rpm's it will backfire
New spark plug,cdi,coil,Carb. problem still remain's.I checked the timing at idle as per manual
F mark on rotor is dead on the crank case mark as it should be. At my wit's end as to what else it could be.
Compression is 160,valve lash is good,rectifier/regulator seems to be fine(charges properly)
Pulse coil or generator is in factory specs.
I'm STUMPED................. PLEASE HELP........
Thank's

 

[indyguy]

I had a similar problem with a different atv -backfired constantly- found out there was water in the fuel - drained carb and drained fuel tank and put in fresh fuel and it was like new again. Found out afterwards that the kids were seeing how far they could drive into a lake.

 

[X-it]

Aftermarket Chinese carburetor? Does adding choke help? Plug Color?

 

[lloydguy]

OEM carb, I haven't tried to add a little choke while it's running.It started yesterday at zero with no
choke and had no problem's at all.I have only ran it a few minutes on the new plug,I haven't checked
for color either. One green wire was burnt (melted shielding) from main harness to frame ground on coil
bolt,it was also melted at the other end by battery.I cut open the main harness and replaced the wire.
Tank is off right now for a 2 day apple cider soak.so i'm just feeding the carb gas using a 1 litre bottle
gravity draining to the carb.

 

[jonyblade]

i've done the gravity tank also,, works great,, try get it up to 2500 rpm, when it starts to sputter, get a syringe and spray some fuel in, see if it balances and runs good,, do a intake leak test also, this will tell u if ur main jet is working or not, sounds like u may have a grain of sand or something, cause its not getting enough of something,, just need to narrow it down more,,, if no difference,, turn fuel off after running and get to 2500 rpm,, let the bowl run out, you will have a window of time, and narrow at that to lean it and see if air is the problem,, how the plugs look?if timing and valves checked out, i would say carb for sure, and remember, just cause a part may be new, does not mean it works

 

[lloydguy]

Exact same problem with 2 carb's,I bought a new one cause a whole new carb was the same price as just buying
a new primer pump from honda.(primer pump was toast on original carb) Because of the burnt ground wire I found
just after purchasing this POS I have been stuck in the mindset that the problem is in the electrical somewhere.
I could be wrong for sure.3 yellow wires leaving rectifier all have continuity with each other and none with ground,
so everything seems OK there. Pulser coil is also within OEM spec's. 1500 rpm timing is bang on the F mark on the
crank case,I just can't really test the timing at 3500 as per manual cause 1 I don't have a tach and 2 timing light cut's
out approx 3500 rpm.
All of plug seems light brown except the end or tip of the ground strap(whatever it's called) is grey'ish .
I'm pretty much at the end of my rope with this thing,duno what to even try next.
Any and all help is appreciated.

 

[Nytroman]

Sounds like fuel/air issues still to me. Are you trying it with the air cleaner and box bolted up? Had a buddys kids quad doing the same thing. They took it to a shop and after a few hundred bucks later was the same. I spent an hr playing with jetting and problem worked its self away. Make sure when jetting you try it with all components in place and dont go by factory specs on jetting, go with what works if you have to. Also recently had neighbor having issues with his kids bike. Hrs of him starting it and reving it, sounded like fuel issue. Grabbed some beers and headed over. Pulled carb and main jet was not even drilled out from factory on a new bike from crappy tire.

 

[X-it]

Again.. what happens when you add choke at 3500 rpm? This could help to tell weather it is fuel or electrical. Unless your valve springs are bagged out.

 

[lloydguy]

Again.. what happens when you add choke at 3500 rpm? This could help to tell weather it is fuel or electrical. Unless your valve springs are bagged out.

I will try that,It needed full choke this AM to start it,but after running only a few seconds it ran better with choke off.
The only thing I tried regarding the choke today is while it was warm and at idle I turned the choke on and it died right away.Should adding choke kill it or just make it rich and studder ?
A mechanic friend of mine also thought valve springs could be involved in the problem.Could be they are strong enough to close valves at low speed,but at higher rpm they are not.
Just for peace of mind I will pop this new carb off tomorrow and inspect it,I just never thought of it because it is doing the same thing with 2 carb's.I will also add choke and or spray in gas and see what that does as well.
I ran it some more today at an rpm just under where I knew it would backfire,checked the plug and the
tip of the ground strap is borderline white.

 

[X-it]

Start reving it up to where it starts backfiring and apply ...some choke...not full. Carb boot not full of holes?

 

[lloydguy]

Just came in from garage,took carb apart today and inspected.All seemed ok. Added choke at approx 3000 rpm
just makes it studder a bit,also while holding it steady at that rpm I lifted the carb slide to add more fuel,that as
well just made it studder. rpm stayed about the same it just sounded boggy when I did either of the 2 thing's.
I don't know if this is a clue or not but if I go up to about 3500 for over 2 seconds,gas mist spray's out the back
of the carb just before it start's to pop and miss,if I don't back off a bit at that point is when it will backfire.
Carb boot look's 100% not even small dry rod surface crack's.

 

[Nytroman]

Have you have the factory airbox on the whole time?. Have you done a leak down test?

 

[lloydguy]

All tests I have done up untill today,the airbox and filter were in place.The testing I did awhile ago was with no filter
or airbox.I wanted to make sure the suction from the engine was raising the carb. slide.No, I have not done a leak
down test.I will have to do a search on here to see what all is involved with that.
Should there be pulses of air coming out of the timing hole when I have the plug removed?Mine does,I checked my
uncles trx 250 to see if it did and it does.His machine run's perfect,so I assume this is normal?
Do you mean a compression leak down test or something to do with the air intake?
Thank's

 

[X-it]

It would be interesting to see how many thousands your intake and exhaust is set at? Also adjust you cam chain tensioner both of these cost you nothing, they should be adjusted anyway.

 

[lloydguy]

I set both intake and exhaust to .006 ,they were tighter than that before I reset them.
Maybe .003 or .004.How do you adjust the cam chain tensioner?I thought it was a non
serviceable spring loaded thing.Would it be possible that the cam chain was loose,slipped
a cog or 2 and the guy tightened the chain before I bought it?In my mind that is not likely
cause the only time I have freeplay in the rocker arm's is when it's at TDC.

 

[X-it]

I see yours is none adjustable, just going off the older 200 from memory.

 

[X-it]

i wonder what happens if your exhaust if full of rodent stuff?

 

[jonyblade]

so what happened u try it?

 

[lloydguy]

I farted around with it a little yesterday,but it was too late at night to rev it up to see if what I did made a difference.
Factory spec of fuel screw is 2.25 turn's , I turned mine in until the engine rpm's started to lower( the screw almost
had to be seated before rpm's would lower)SO, then I went back to 2.25 turns and kept turning out untill rpm's
started to drop Once I got from 2.25 to 2.5 turns rpm's went from 1400 to 1600 ish. They stayed at that rpm untill
I was at 6 turns where they started to drop again.So the way I understand what is happening is 2.25 turn's out
is WAY lean . 2.5 is my lean spot 6 is my rich so unless I have fried my brain watching too many carb tuning video's
I should be approx 1/2 way between those 2 positions.
Someone correct me if I'm wrong please.
6 - 2.5 = 3.5 (turn's) /2(half way)= 1.75 So, 2.5 turn's + 1.75 turn's = 4.25 turn's
unless the engine stumbles when I hit the throttle ,then I should turn towards lean untill the studder or stumble
goes away.
I have it set to 4 turn's out right now,( that sound's like WAY too much.) if the math didn't tell me otherwise.
AGAIN SOMEONE PLEASE TELL ME IF IM NOT LOOKING AT THE FUEL SCREW TUNING PROPERLY.
THANK'S

 

[lloydguy]

With the fuel screw set anywhere between 3.5 and 4 turns out it idles good,has good throttle response
but still spit's back through the carb around 3000 rpm. Mechanic I know said cam timing,I took the valve cover
off last night and (i think) it was off by 1 tooth.I should have moved it over 1 tooth to see if the dot's lined up
better but I took the head off since I was half way into it already.Piston and jug look great,rubber on chain guide
is flaking/decomposing so at this point I don't know if I should waste a top end gasket set by putting it back
together as is.The automatic chain tensioner was screwed all the way out when I removed it,I thought my chain
must be stretched untill I read that when you remove them the spring inside spin's all the way out when there
is nothing pushing against it.(it's kinda worded that way)
I measured the cam and all measurements are right at the wear limit's in the oem book.Cam,chain,and guides are
not too expensive(ebay)so I don't know weather to put it back together for a test run after I move the cam timing
over 1 tooth or buy more parts,not knowing if I've found the source of the problem?