300ex stumbling miss on low end

[riley1374]

I swore that my problem with my low end stumble was jetting, but i am finally starting to believe that it is electrical. The bike starts up and idles beautifully and runs great when the bike is cold, but after it starts warming up, i develope this miss right after you open up the throttle to about mid throttle and then it runs like a raped ape on the top end. i have played with the needle and the pilot and its still the same. Does this sound like it could be related to bad CDI or coil? I noticed that the insulation around the coil was warn on the one side, but not sure it would cause this. What are some of the symptoms of a bad CDI or coil?

[riley1374]

TTT

[riley1374]

Anyone? BWAMOS help!!!!

[Mx_523]

Well if you want to know if its a jetting problem, just pull you spark plug out. If its black your burning rich, if its grey your to lean and if its tan your jetting is perfect.

[bwamos]

Originally posted by riley1374
I swore that my problem with my low end stumble was jetting, but i am finally starting to believe that it is electrical. The bike starts up and idles beautifully and runs great when the bike is cold, but after it starts warming up, i develope this miss right after you open up the throttle to about mid throttle and then it runs like a raped ape on the top end. i have played with the needle and the pilot and its still the same. Does this sound like it could be related to bad CDI or coil? I noticed that the insulation around the coil was warn on the one side, but not sure it would cause this. What are some of the symptoms of a bad CDI or coil?

Usually if you have an elecrical problem, such as corroded connections, etc.. the problem shows up more as rpms increase. More energy needed.

I'm not going to be a ton of help on this.. but a couple things you could look for beyond jetting would be..

1) The #1 thing I'd check is to make sure your Accelerator pump pin didn't slip off of the compression arm. It will be under the black plastic cover on the right side of the carb. Lower rear area. It sounds like your acclerator pump may not be working correctly. If its on the arm correctly, give it a pump to make sure it squirts. Don't look into the carb while you do this.. lol.

2) Did you replace the pilot jet? If so, did you replace it with a long or short? If it's the right style pilot the bottom will be pretty flush with the collar/tube if it is recessed like a 1/4 inch it the short series (wrong one). When the fuel in the bowl sloshes around (usually when riding slow in rought terrain) you can get bubbles trapped in that tube causing the pilot to suck air for a second giving you that hesistation.

3) Have you done any stator work? My riding partner had one of the bolts on the stator start to back out, and it acted similarly because it was loosing it's ground at certian rpms.

[riley1374]

1)
I havent checked the accelerator pump, but i do not think this is the problem as i have swapped out two different carbs to see if that is what caused the problem.

2)
When i put the other carb on, it had a 45 pilot jet in it, so i replaced it with my 42 but same effect with both jets.

3)
I have not done any stator work, but i will check this out as well.

This is driving me nuts, i cant get this bike to run right. So you dont think this is coil or cdi related bwamos?

Thanks for your help!

[bwamos]

Anything is possible.. but I doubt it.. as a CDI or Coil problem should have more problems as the rpms increase.. not less.

I've seen so many people swaer their problems are CDI or Coil related.. and they rarely are. Usually corroded wires, jetting, loose connections on the switches, etc..

What mods do you have?

I'm still stuck with carb issues. Only other thing I can think of right off would be cam chain timing that would cause what you are describing.

CDI isn't out of the question.. but very unlikely.

Give us a list of your engine, intake (filter and lid) and exaust mods (including quiet core, spark arrestor, etc). Then give us what your carb settings are. Main, pilot, air screw, and needle.

I just want to make sure nothing is standing out.

Also what was changed before the stumble started. Or did it start doing it all on its own.

[Sjorge450R]

Originally posted by bwamos

1)
If its on the arm correctly, give it a pump to make sure it squirts. Don't look into the carb while you do this.. lol.



Been there, done that.

[riley1374]

Bwamos,
I bought the motor and wiring from from the same guy, and since i have installed this i have had the problem. Now the guy swears that the bike ran beautiful and no problems what so ever, and i have to believe as all parts were delivered in the condition described, and he even helped me out installing everything. I am using a 250x frame, and modified it for the 300ex battery and all electricals. I painted the frame and made grounds, i am wondering if that would have something to do with it, but like i said it runs great on top, if it was a bad ground or corroded wire wouldnt it be running crappy all over the place. The motor is modded with large cam, titanium valve springs, port and polish, oversized valves, 11:1, stock air filter, and quiet core FMF fully system. The only difference setup wise that i have that the previous owner did not, was he ran a 400ex carb and i am running the stock 300 carb. He swears that is my problem and the 300 doesnt have enough to feed that motor. I disagree only because it runs great up top. Really stumped on this one and any help is appreciated.

[bwamos]

Aye, not true. I am running a 300ex carb on my 330. A 400 would be better.. but I haven't upgraded yet.

Definatly revisit the grounds. Can't hurt.

If you have a friend with a 300ex.. you could borrow his CDI and Coil just for a quick swap-out test.

Did you test the accelerator pump yet. Only takes a minute.

Also, what are your current jetting settings.

With your setup (assuming you are runngin no lid and are stock bore) you shoud be running somewhere around a 135 Keihin Main, 40 Pilot, Needle 3rd-4th slot, 2.5-3 turns out on screw.

If anything is far off from that.. you may want to just try that as a test.

You could also be getting a weak spark until the engine rpms increase enough to get enough energy to get a full spark.

Turn off the fuel.. pull the plug and test the plug up against the cooling fins while hitting the starter. If the spark is blue you're ok. of its yellowish you're getting a weak spark.

Asking the obvious here also.. but have you changed the spark plug out?

[bwamos]

Originally posted by riley1374
Bwamos,
The motor is modded with large cam, titanium valve springs, port and polish, oversized valves, 11:1, stock air filter, and quiet core FMF fully system.

If none of the previous helps you out.. pull off the head cover.. and double check your cam timing.

If your cam timing is off 1-2 teeth.. it still runs but acts like a really screwy jetting problem. It acts lean, but reads rich. Personal experience.. lol.

[riley1374]

Originally posted by bwamos
Aye, not true. I am running a 300ex carb on my 330. A 400 would be better.. but I haven't upgraded yet.

If you have a friend with a 300ex.. you could borrow his CDI and Coil just for a quick swap-out test.

Did you test the accelerator pump yet. Only takes a minute.

Also, what are your current jetting settings.

With your setup (assuming you are runngin no lid and are stock bore) you shoud be running somewhere around a 135 Keihin Main, 40 Pilot, Needle 3rd-4th slot, 2.5-3 turns out on screw.

If anything is far off from that.. you may want to just try that as a test.

You could also be getting a weak spark until the engine rpms increase enough to get enough energy to get a full spark.

Turn off the fuel.. pull the plug and test the plug up against the cooling fins while hitting the starter. If the spark is blue you're ok. of its yellowish you're getting a weak spark.

Asking the obvious here also.. but have you changed the spark plug out?


Ok, at least i can rule out the stock carb. LOL

I have the 250x coil and cdi i could test, are they the same as the 300ex?

I havent gotten a chance to check the accelerator pump but i will.

Current jetting is 140 main, Needle clip 4th from top, and 42 pilot.

Thanks again!

[riley1374]

Oh yeah, one other thing, i havent changed my spark plug in sometime since i have been messing with the jetting. Maybe that would help. Hmmmmm

[bwamos]

Well.. you may be a little fat on the main.. but that wont hurt you at the bottom. Just loose some power on top. Hard to tell depending on the porting and cam. You're probably ok though.

I think your a tad rich on the pilot.. but still shouldn't cause much of a problem.

Once you verify that the accelerator pump is squirting , a stream like it should, and the spark plug is good.. I'd verify the cam timing.

It very well could be the plug. If you're gap has gotten bigger.. it will take more energy to make a good spark. In other words at lower rpms you may not be getting as good of a spark until you get more energy from the stator at higher RPM's.

No the 250x CDI wont work. Different wiring patterns. You could make it work, I'd imagine, but not a direct swap.

250x coil should work fine though.

[riley1374]

Ok update on bike.

CHecked the accelerator pump and it is working fine.

Checked the plug, changed that as it had quite a bit of carbon built up on it.

Fired the bike up and it ran awesome while in neutral, but as soon as i put the bike in gear and start letting out the clutch, it starts to miss. Put it back in neutral and it runs awesome again. Could this be a clutch related issue? Once it gets pass that low end miss it runs great.

[bwamos]

If you have it.. toss in your stock pilot (38) to see if that helps.

I'm thinking you're fat on the pilot.

I only run a 42 on my 330.. and I should be running a 40.

[riley1374]

You really think its jetting still? Wouldnt it miss at idle if it was the pilot?

Thanks