high speed bogging

[Dave223]

I have a problem thats driving me nuts trying to figure out: When Im running my kids 300, it takes of fine. Lots of power, smooth idle, but when i get into the upper rpms, about the time it starts pulling really good, it "sputters" or starts to cut out really bad.it does this throughout all the gears and will cut out until i shift. It doesnt do it to the point that it shuts down, it just makes the bike jerk around and lurch. at wot it will do it to the point that the bike will slow to about 35-40, but if i back off the throttle to say 3/4 it will take off again .

So far i have reset the carb to factory specs and cleaned it, replaced the spark plug, checked the wiring,ran it with and without the k&n air filter and air box lid, made sure the exhaust was clear and free flowing, now what???:mad:

[terko440]

I was gonna say your were probably running rich Dave but you said you put your carb back to stock settings. I would still think it would be carb problems

timing chain

Jesse

[400exrider707]

Any mods done to the bike? Sounds like it could be jetting. DirtMerchant what makes you think its the timing chain out of curiosity?

[j753301]

i thought the same thing when i first got my 400ex,turns out i was hitting the rev limiter.just a thought

[Dave223]

ok now i have some leads.....first off, out of curiosity, why the timing chain? maybe a lil out of time?

rev limiter..hmmm didnt realize it had one, thanks, and how would i check it to see if its working properly?

jets, the jets are stock, 122-38 i think thats the numbers, whatever the numbers are, its the same as shown as stock in the manual.

as for mods, the only thing I know it has is a DG MegaMax? exhaust.

I have 2 carbs, so i rebuilt the "extra" and the same thing happened, so i put the original back.

[Chino]

Originally posted by Dave223
ok now i have some leads.....first off, out of curiosity, why the timing chain? maybe a lil out of time?

rev limiter..hmmm didnt realize it had one, thanks, and how would i check it to see if its working properly?

jets, the jets are stock, 122-38 i think thats the numbers, whatever the numbers are, its the same as shown as stock in the manual.

as for mods, the only thing I know it has is a DG MegaMax? exhaust.

I have 2 carbs, so i rebuilt the "extra" and the same thing happened, so i put the original back.

give the bike full throttle while it's in neutral and hold it for a few seconds, you should hear it bounce off of the rev limiter it will be smooth sounding then you hit the rev limiter and it makes a ba ba ba ba ba, dont hold it wfo for to long of a time, i am not sure how you tell if one is working properly, or if one can actually fail, sorry..

[hondardr4life]

I bet that you are hitting the rev limiter.

[Dave223]

im beginning to think the same thing...so who makes an aftermarket rev limiter?

[gojk]

Pull the spark plug and see if it is really black. It could be fouled. Also, is the air filter really dirty. If it is really clogged it will make the quad rich and it will studder up top like that. If you just stab the throttle from idle while it is neutral will it just studder then start reving, studder then die or does it rev up fine

[Dave223]

from an idle, if i stab it open, it stalls kinda hard then starts revving.

the plug is new an a golden brown color, Ngk BR8ES-L air filter is k&n and is clean as is the outerwear. i tried opening and closing the one jet on the exterior of the carb(i forget the name) but it is set at 2 1/4 turns as the manual says it should.

[gojk]

Try leaning the idle circuit a little(that is the exterior adj that you made), and also drop the main jet a size or 2 and see if it gets better. It really seems like jetting to me. If the adjusting the idle circuit doesn't help with it studdering you may need to lean the needle out 1 notch.

[prepracing]

its either the rev limiter or valve clearance, better be careful if you put an aftermarket rev limiter on a 300ex ( trust me they won't handle much for very long unless u have some beefier internal mods). Try gearing it a little taller so it won't hit the rev limiter so quick. For 300ex not really sure who makes aftermarket boxes besides white bros.

[2004exrider]

id have to go with dirt merchant, there was another guy on here that just happened to and he was showing the same probs.

[honda250xrider]

if it was the rev limiter it wouldn't slow down to the point to where you would notice it to much it would pretty much just stutter and pick back up and stutter. it sounds like you are to rich, try going down on your main jet a size of two. Or it could be your timing sliped a tooth and you are a little off timing i have had the same thing happen. But i would try the cheap fix first and go for the main jet.

[gojk]

Originally posted by honda250xrider
if it was the rev limiter it wouldn't slow down to the point to where you would notice it to much it would pretty much just stutter and pick back up and stutter. it sounds like you are to rich, try going down on your main jet a size of two. Or it could be your timing sliped a tooth and you are a little off timing i have had the same thing happen. But i would try the cheap fix first and go for the main jet.

This was my thoughts too. It really doesn't seem like the rev limiter, but I haven't ever had a 300ex either. I know the 400 won't stumble that bad when you hit the rev limiter.

[prepracing]

it may not be the rev limiter, but I had a couple 300ex's and they did the exact same thing as your describing and it was the rev limiter. But, yes I would definately eliminate simplest things first. Play around with the jetting and air/fuel mixture screw, and check clearance on your valve's I did have a problem on one of mine where it acted similar to your description and I ended up having a rocker arm that was wore out.

[honda250xrider]

a simple way to tell if it is the rev limiter is to listen to the way it sounds. if it was the rev limiter you would hear it somewhat bouncing on and off... and would sound somewhat like dadadadadadada but you should not speed at least not more than 2 mph, I'm not sure but i imagine it only knocks the rpm's back about 500 or so. the rev limiter will only allow the rpms to stop reving any further so you should maintain the same speed it may feel like you lost some power when you reach that and actually you have i think the power drops off about a grand or so before the rev limiter will actually kick in. I would say it is either your carb, or your timing. Have you ever taken the head off? I have had the timing just a tad bit off and it does the exact same thing as your describing and it feels like it stops pulling in the upper rpm range. But first try to go down to a 120 or 118 main jet. The only other thing i would say is that it could be your Stator assembly, It sounds like a weird problem but i have had 2 250x's to where it would act like it would lose power only in the upper rpm range but it would run fine down low, And no matter what jetting it would never correct the problem. If it is the Stator it will only grow worse. Just take it one step at a time.

[Dave223]

hey,
thanks for the ideas, now i have somewhere to start. I did try the other carb with a 118 jet , but it did the same thing. The power is awesome down low an in the mid rpms. But when it gets to the point that its pulling the hardest, then it begins to cut out. if i run it hard on a long stretch, like a drag race, i have to shift quickly through the gears in order to stop it from cutting out. Thanks again, dave

[bwamos]

The top end sounds like off time (1 tooth late on the valves, early on the spark) if your still running golden brown, or your coil is going bad. Sounds more like the timing though.

Regarding the bottom end stall.
Take the Black Plastic sidecover off of the carb and make sure the Accelerator Pump shaft didn't slip past the little lever arm that pushes it. That part wasn't lined up good in the factory, they will slip off the lever occasionally making your accelerator pump not do anything.