Odd problems with a 1995 RMX250

[rebelpacket]

Hello Guys!

Been browsing this forum for a week or so now, and I finally joined up so I could search too. Unfortunately, I didnt find what I needed. But if there is a thread that could answer my question, please point me there.

Little background... I've been riding dirtbikes for about 2 years, but in total (as a little tyke) I've ridden them for about 6. Always two-strokes. Just something about the smell of burning castor-bean oil and mud that is synnonmous with dirtbikes in my head.

Anyways, I have a '95 RMX250 that has a very weird problem, that I've stumped on. The bike fires right up, and runs very well. At idle it'll revv fast, and loud with no problems. Once I throw it in gear and start off, it runs great as well.. until I get to the powerband.

I get about 1/4-1/2 of the way into the powerband, and the bike just seems to sputter and bog. I go from super amounts of power lofting the front wheel (or spinning the rear) to almost nothing. I suspected it was running rich, so I did a quick test. Rode it for 2-3 minutes, throttle wide open in 3rd gear (with it bogging) and then killed it, and checked the sparkplug. It was the perfect Tan color.

So if its jetted correctly (specifically on the main jet), why in the world is it sputtering halfway through the powerband? My next step is to pull apart the carbs, and check the float height. But I just dont know why a motor would act so very differently when underload, and opposed to at idle.

Any ideas?

Thanks again, for the great forum. I'm digging it here. Y'all have t-shirts?

[6thWide]

almost 100% sure thats your reeds. they are either chipped or frayed or they are worn out and dont sit flush against your reed cage. buy Boyesen Power Reeds. they are under $30 and just scrap the old ones and put on the new ones. the reed cage is infront of the carb. move the carb aside and remove the 4 bolts and the whole thing will pop right out. the screws holding in the reeds are phillups, so use a good scewdriver. you will also get more power across the whole rpm range. its the most effective upgrade for the price and will replace stock reeds

[MX86]

the reeds deal more with bottom end "responce" not exactly mid range... when was the last time your powervalve was cleaned? i have a suspition that your powervalve is sticking... it's common on bikes now-a-days... i doubt it would be the carb since it runs fine on the bottom end... just pull the powervalve cover off and turn it by hand... see if it turns/ moves freely... then worry about the carb..

[88lt250racer]

i'm guessing main jetting or needle height.

[rebelpacket]

Originally Posted by VMX_Slowpoke2

the reeds deal more with bottom end "responce" not exactly mid range... when was the last time your powervalve was cleaned? i have a suspition that your powervalve is sticking... it's common on bikes now-a-days... i doubt it would be the carb since it runs fine on the bottom end... just pull the powervalve cover off and turn it by hand... see if it turns/ moves freely... then worry about the carb..

Thanks for the responses guys. I'll check my reed valves again tonight under closer inspection. I took a look at them before, and they looked to be in good shape.

I think you might be on to something with the powervalve problem VMX... As just yesterday, I was having the issue, and trying to run through some more tests, whacked the throttle open (expecting it to die out like it has before) and it DIDNT. Nearly flipped me on my rear. I'll be checking that out tonight. Out of curiosity, whats a good way to clean the powervalve? Reading the Clymers breifly, it looks pretty involved to take it out and apart.

Its possible its a jetting issue as well, so I've got the carb off and in peices. Float height looks right. I think I'll lean the main jet out a bit and see if it helps.

[diesel-mech]

That bike should have a "camel toe" style power valve. You will have to pull your exhaust pipe off to see if it is working correctly, I would bet it is. I had a 95 rm250 with your same problem. My 1st question is, Is it a moddified engine? Are you running high enough ocatin fuel? If you float hight were the problem than unless fuel it running out of the carb its not over fueling and it would ping and knock like you just ran out of fuel if it were not enough fuel. I think you have jetting problems. I say this because I know how sensitive that engine is on jetting. Mine would change from day to day depending on the weather. I agree that your reeds may be worn out but I will bet you solve the problem in the jetting.

[rebelpacket]

Diesel-mech,

Thanks for the response. Nope, its is not a modded engine. It is very possible its a fuel octane problem, as I'm only using 87. But I would suspect to hear some pinging (especially on a motor thats so close to my... giblets). But its nothing. Just sputtering. I cant exactly explain why the bike worked perfectly one day, and not another. Thats whats so annoying about this problem. Is sometimes its good, and other-times, its absolute crap.

I'll try things 1 degree leaner all around. But the plug was a nice tan color when I last pulled it.

[diesel-mech]

My 250 was doing the same thing. The plug looked great but it ran terrible some times and great other times during the same day. Its odd sure but it was a jetting problem. Also if I ran pump gas in my engine it would be fine until I actually put it in gear then it would bog if I laid into it (My engine was highly modded) . Here is something to try, when your riding try accelerating slowly to wide open throttle a couple times, if it seems to run to its full rpm range without bogging baddly than you are too rich on your main jet. (if it still boggs when you nail it.) Like I said before that engine is very finicky about jetting. I would be sure to add the highest octain fuel you can get right off the bat, that way you are not making solution harder than it should be, even if your engine is stock 87 is to low of octain for it (not to mention the quality of fuel you get at the pumps today).

[MX86]

i just can't see it being a carb problem, or gas... but it could be

[rebelpacket]

Diesel-mech, VMX,

I performed a little test today to try and rule out the jetting. I already have the throttle marked in 1/4 incrments (1/4, 1/2, 3/4, full). I fired the bike up, and on a straight away, shifted into 2nd, and held the throttle between 1/4 and 1/2 throttle, trying to eliminate the main jet. Each time, its bogging down in the exact same way, no matter how much throttle i'm using. And each time, the plug is coming back tan.

Considering the bike runs real well everywhere else but the powerband, I'm really starting to think it has to be the powervalve, (or a leak in the exhaust system somewhere).

Thoughts? (i'm going to start pulling apart the powervalve assembly tommrow).