HELP me diagnose this problem

[Honda 250r 001]

OK so ive had a problem with my 250r missing and sputtering in the midrange part of the powerband. When i start in about 4th gear and slow down to about 1/8 throttle, and pin it wide open, it starts speeding up and it sounds good until about the 1/4-1/3 range. Then it starts "4-stroking" and missing and sputtering. And it wont go any higher in the rpm unless you shift down or clutch it. So ive ruled out jetting on this becuase ive tried everything. Could it maybe be a bad CDI box not properly advancing the time? What else could it possibly be? It rips ***** in the top end, Its got good reeds, no air leaks, good compression, and it has good spark at when i pull the spark plug out and kick it over. So does anyone have any suggestions?

Motor is a
CT 310 first over bore national porting
PWK 39 carb, DGH clip on any position never changes the missing, 175 main, 45 pilot.
Vforce 2 reeds
ESR trx 5 pipe
ESR CR 250 ignition with adaptor plate set on the middle of the adjustible slot
ESR airbox eliminator

[Dave83]

What cylinder(bore),carb and current jetting do you have.Sounds like the needle may be too high to me.

[Honda 250r 001]

Ive tried a DGJ needle, CGH needle, and a DGH neede. And it runs best with the DGH. I also updated my post above

[slamdak8782]

Have you cleaned out the jets it has to be in the carb. It wouldnt run right on the top end if it was the ignition. Check the float bowl to see if it is cracked and properely clean the carb out. If that doesnt work I dont know.

[Honda 250r 001]

Carb is clean, its running on fresh fuel also. I cant seem to figure out this problem! Its driving me crazy

[latheboy]

I had weird problems, and it was my stator. It was running for several tanks of fuel, with good jetting. I took a break while riding, and when I started it up and took off, it seemed way fat on the pilot. It would sometimes clear out on the top end. It ran real strange whenever I demanded throttle. Turns out I checked spark, and it would only spark at the plug every other time at best. Changed to a known good stator, problem solved. I have no idea if your problem relates, but if it ran good on the current jetting in similar conditions, look elseware.

Also look at the pickup coil and the magnet on the flywheel, make sure there is no corrosion.

[slamdak8782]

try a spark tester and see whats going on you can get them at the auto store

[ckelly30]

your elevation may affect your jetting to some degree, but only when you start hitting 2000 ft above sea level, in which case you would need to lean out the main to get more air in your carb bc air is thinner at higher elevation.

being that your having problems in the mid range, if it is a jetting problem, which i think it is, look at your main and needle. those are the two biggest factors in the midrange within the carb. people easily overlook the needle.

check again for an air leak and compression.

if you have no problems starting, taking off in first gear, or idleing, your idle screw, air screw and pilot are fine as those components affect 1/8 - 1/4 throttle.

once you hit 1/4-3/4 you are looking at the main jet and needle.

the needle diameter has the most effect on 1/4 throttle you may have to go leaner on either the main OR needle(that would mean you need a thicker needle but dont lean both bc you may end up in the same spot you are in now bc the diameter and main, when altered, should be altered opposite of each other. ex: lean the diameter and keep the main or rich up the main. you just have to do a bit of R&D) if you are sputtering and gurgling that indicates that you may well in fact be too rich.

at the 1/2 range, your looking at the clip position aka needle lenght. The highest clip position is the leanest and the lowest clip position is the richest. usually you want to be in the 3rd or 4th clip position. check it.

the needle taper affects 3/4. a change in the taper is very subtle and may be difficult to feel the difference. while the taper does affect the jetting, it does so the least amount.

go to duncanracing.com. they have an excellent breakdown on jetting, which is what i think your problem truly is. Also try carb parts warehouse.

i cant think of anything else besides possibly an airleak or a bad internal carb part.

[wilkin250r]

There is a lot of mis-information flying around this thread.

Let me get this straight. You are saying, in a 4th gear or so, running just above idle, if you pin the throttle, it will start off good, but then sputter in the mid-rpm range, and doesn't have enough power to speed up past that?

Lots of people are talking about needle, but that's NOT your issue. The needle isn't "midrange" the same way. Needle is mid-THROTTLE. It has nothing to do with the rpm range of the engine.

We can help, but we really need to all be speaking the same language to diagnose this.

So let's re-phrase the question, or even do some more testing if needed. But I need the question to be in terms of THROTTLE position, and then RPM range (because it tells me how much air you're pulling through the carb).

*The actual gear you're in isn't relevant, except it does help me decipher your info a little better. You might be under-powered in the mid RPM range, but you might not know in 1st or 2nd gear, but you'd certainly feel it in 4th gear.

[Honda 250r 001]

Originally posted by wilkin250r
There is a lot of mis-information flying around this thread.

Let me get this straight. You are saying, in a 4th gear or so, running just above idle, if you pin the throttle, it will start off good, but then sputter in the mid-rpm range, and doesn't have enough power to speed up past that?

Lots of people are talking about needle, but that's NOT your issue. The needle isn't "midrange" the same way. Needle is mid-THROTTLE. It has nothing to do with the rpm range of the engine.

We can help, but we really need to all be speaking the same language to diagnose this.

So let's re-phrase the question, or even do some more testing if needed. But I need the question to be in terms of THROTTLE position, and then RPM range (because it tells me how much air you're pulling through the carb).

*The actual gear you're in isn't relevant, except it does help me decipher your info a little better. You might be under-powered in the mid RPM range, but you might not know in 1st or 2nd gear, but you'd certainly feel it in 4th gear.

YES! ive been wondering about this. Sounds like you for sure know what your talking about. Do you have any suggestions? If it sputters with the throttle wide open in the 1/2 range. Woudlnt that mean i need a leaner main?

[wilkin250r]

Originally posted by Honda 250r 001
YES! ive been wondering about this. Sounds like you for sure know what your talking about. Do you have any suggestions? If it sputters with the throttle wide open in the 1/2 range. Woudlnt that mean i need a leaner main?

Maybe, IF the problem is jetting. How does it run on the top end?

Generally, if your WOT is rich is the mid-rpm range, it's really rich in the upper rpms.

One of the problems with simple mechanisms like this, they aren't perfectly linear. For example, 6000 rpms pulls twice as much air through the carb than 3000rpms, but twice the airflow ends up pulling about three times as much fuel. You don't just double everything, you get double the air, but triple the fuel.

Now, that's just a general analogy, we haven't nailed down your problem yet.

So, before we can nail down your problem, we need a little more information. Again, how does it run at the top end?

[Honda 250r 001]

It runs awsome on the top end. When im 2nd i pin it and run it till the motor stops revvving up and it is running real smooth and crisp.

[wilkin250r]

Try the same test you were having problems with, 4th gear, low rpms, and pin it.

EXCEPT, this time, do it in in 3rd gear, and don't pin it completely wide open. Stick it a little over half throttle, but not quite 3/4 throttle. And then the same test (just over half throttle) in 4th gear.

Here's my train of thought: We already know your motor can't spin the tires in 4th gear, even if it was in top shape. (because if it could, you wouldn't need 1-3rd gear) But you SHOULD be able to accelerate into the powerband and beyond. Since your motor cannot, it seems to be having a problem in the midrange.

You can do it in 2nd, because the gearing makes it easier, so the problem isn't as obvious.

What we want to do now is determine if the problem is jetting, or something else. The 1/2 throttle test I described above will move you out of the main-jet arena, and into the arena of the needle jet. If it runs great, and behaves just like it's supposed to, then we've nailed the problem to your main jet.