UPDATE: new to 2 strokes / dirt bike and have a few questions...

ok guys got a freshly rebuilt 91 cr 250 at a bargin..... kid told me it hasnt been run since the rebuild but he knows the carb needs to be cleaned.... took it all apart and unblocked the secondary jet and cleaned, reassembled and put in new plug, 30gap, dont know correct spec.... anyway long story short it is breaking up at the higher rev end and isnt anywhere near as powerful as people tell me it should be.... not sure of proper plug gap or air / gas screw, now i am 1.5 out from close..... there is also 2 big dents in the expansion pipe, people tell me this could also cause this.... someone please help.... i am new to 2 strokes....

I'm no expert but it almost sounds like you're running too rich. try lowering the pin for the main jet one notch at a time to see if that helps. The kid before you might have raised the pin to compensate for the blocked secondaries.

Originally Posted by nedro

I'm no expert but it almost sounds like you're running too rich. try lowering the pin for the main jet one notch at a time to see if that helps. The kid before you might have raised the pin to compensate for the blocked secondaries.

please explain, ive had the carb out and apart and i am familiar with the main jet but please explain the "pin" i can take it all apart again and take pics if i need to.... im no carb genious either i might add...

and btw: does anyone know the proper plug gap, ive been searching and cant seem to find it

[ConqSoft]

If you haven't yet, check out dirtrider.net.

I *believe* that when nedro says "pin" he is talking about the needle jet. If you pull the top of the carb off (if it was like my old Fourtrax 250R the top just screws off) then the needle jet is the pointed thing sticking out from the bottom of the slide. It should have a little clip and several grooves which allow you to move it up and down. It is responsible for controlling the flow of fuel through the main jet. If you drop the needle jet one groove at a time then it should cut off some of the fuel flow as you open up the throttle, thus leaning out your mixture.

I will add that if the top does screw off then there is no need to actually remove the carb to do any of this. Just pull the slide assembly out, adjust the needle jet, put things back in, and test.

Originally Posted by stopsign

please explain, ive had the carb out and apart and i am familiar with the main jet but please explain the "pin" i can take it all apart again and take pics if i need to.... im no carb genious either i might add...

Was there a brass rod/pin hanging down from the throttle slide when you took apart the carb? That's the piece I'm talking about. Sorry I don't have the terminology down. It is held in place by a "C" clip and adjusts the amount of gas that comes out of the jet. It moves up with the slide, the more you open the throttle the more gas comes out of the jet.

Originally Posted by lfg929

I *believe* that when nedro says "pin" he is talking about the needle jet. If you pull the top of the carb off (if it was like my old Fourtrax 250R the top just screws off) then the needle jet is the pointed thing sticking out from the bottom of the slide. It should have a little clip and several grooves which allow you to move it up and down. It is responsible for controlling the flow of fuel through the main jet. If you drop the needle jet one groove at a time then it should cut off some of the fuel flow as you open up the throttle, thus leaning out your mixture.

YES! This is what I am talking about!
Also, make sure there are no holes in you exhaust pipe (exept, of course the inlet and outlet).

My Dad has a 91 CR250 in mint condition. Those dents in the expansion chamber can cause the rough running, especially at high rpms. Cleaing the carb out was a good first step. Make sure you've drained out any old gas from the tank too. Check the air filter and make sure it's cleaned and oiled properly. Clean it in kerosene and oil it with air filter oil (liquid kind). Pour the oil over the filter until it's covered and squeeze out the excess.

I don't have the service manual in front of me but here are the stock settings for the carb.

Main jet: #175
Slow jet: #55
Needle: R1369NS
Needle Clip Position: 3rd Groove from Top
Air Screw: 2 turns out
Float Level: 16mm (0.63 in)
Spark Plug: NGK BR8EG

Check the carb and see what you're got in there now. Set the carb back to stock. Replace (or repair) the exhaust pipe. That expansion chamber is how 2 strokes work. Just changing the size and shape of that affects the engine's performance. Once you've done that see how it runs. If you buy an aftermarket pipe (like FMF), they will have recommended carb settings.

Also you'll need to check the power valves. At high RPMS two "Finger" like valves open (they are under that HPP cover on the side of the head) to allow more air flow through the exhaust port. If they are sticking, that kills top end (assuming they are stuck closed and not open).

Try to find a shop manual. You will need it to clean those power valves. Helm Inc should have one or your local Honda dealer can order you one. They are about $30 last time I checked.

One last thing. What ratio are you mixing the gas at? We always ran Golden Spectro oil and mixed it 40:1. I think the manual says 32:1, which would also be fine. Those power valves can get very caked in oil, so if after setting the carb and replacing the pipe it's still running funky, clean those.

And also check the ried valves (forgot about this one). If they are weak or a bit cracked. That will also cause this problem.

Originally Posted by nedro

And also check the ried valves (forgot about this one). If they are weak or a bit cracked. That will also cause this problem.

Yep, forgot about that one too. Even if the reeds look fine and aren't cracked, if they are still original or have set for a few years they dry out and don't work properly.

As previously suggested, reset all jetting back to stock if it isn't already. Find a long firm straight and run the bike throught the gears to clean it out. It may sputter/smoke as it comes onto the needle (~1/4-1/2 throttle) and finally the main (~3/4 throttle and greater) but once there it should "clean up" and "come onto the pipe" aka power band. The dented expansion chamber on the pipe will affect the power within this time but it should still start pulling. If the motor revs hard but isn't pulling/low power, your clutch probably needs to be replaced. Check the cable tension adjusters to see how much adjustment has been used.

Also check the engines compression once it is warmed up and it should be 140psi minimum.

you guys are a huge help....

heres what we got so far....

reeds are new, air filter is new

found and adjusted needle from 2 from top to stock 3 from top... moved air/gas from 1.5 out to 2 out. correctly gapped plug....

bike is definitely stronger and better but still not there. in 1st, 2nd there is no high.... the higher the gear i get into and the faster i am going the smoother it seems to get although i really havent been able to get out to open it all the way up, i wont be able to do this until tomorrow.

at this point should i wait to blast thru all the gears and clean it out as mavic suggested or do i make somemore small adjustments and guage by quick jaunts up the street as i have been doing ? if this is good then what should i go for next, another needle more ? in/out on the screw ???

I had a similar problem with my CR500. No amount of fiddling with the jetting or needle position seemed to help. I called these guys and they explained to me how the "choke" circuit wears out in the carburetor and it's like always having the choke part way on. The solution, unfortunately, was a new carburetor. But I have to say the thing rocked after that. It was well worth the money.

Your sparkplug gap sounds a little wide it should be 0.5-0.6mm (I think ~20-24 thou) This can cause a miss at high rpm if the ignition system is struggling with the bigger gap as more potential has to be generated before the spark can bridge the gap, it also places an extra strain on you ignition coil. Also as EatnAsphalt mentioned above with the power valves, you bike has the older style HPP valves as opposed to the CRV valve which I think came in in 92 or 93, the Hpp valves work fine when maintained but they are more prone to seizing due to carbon buildup than the CRV valve, Ive had them seize up on my 86 CR250, a quick clean and presto it ran like a new bike!

Originally Posted by EatnAsphalt

I don't have the service manual in front of me but here are the stock settings for the carb.

Main jet: #175
Slow jet: #55
Needle: R1369NS
Needle Clip Position: 3rd Groove from Top
Air Screw: 2 turns out
Float Level: 16mm (0.63 in)
Spark Plug: NGK BR8EG

special thanks to you EnA !!!!!!!!!!!! You saved my bike and my sanity !!!

UPDATE : I dont know what i would do without you guys !!!

here goes, well finally after forever and a vacation i got around to playing with the bike yesterday, set everything back to stock and added a new stock expansion pipe i picked up from ebay in mint condition for $21 and it was a ton better but not great, got a new plug cuz i noticed the wrong one was in there and made another 1/2 turn out on the air/gas and presto a stupidly fast honda dirt bike !!! amazingly clean power throughout the rev range and the powerband i have heard stories of but never felt, well lets just say it doesnt want my 170lb ass on the seat when it hits....

this morning i went out with a neighbor kid (yz125 but this kid can ride, i cant, lol) and he took me out on a whole variety of terrain from wide open flats (OMFG!!!) and jumps (uhh, we have to work on this one), to trails (bike sucks here unless it screaming), to mud (woohoo unless ya faceplant in one like i did ) anyway long story short i have alot of learning to do but i have this strange feeling this can make me a better street rider (any input on this ?) i for the first time ever jumped a dirtbike and its a great feeling as well as working on mastering the controlled power slide, this thing is amazing... if i sound ampped up its because i am, that was 3hrs of super intense fun... i cant believe i waited 27 years to experience this..

all thru my childhood i have ridden quads but they just dont compare.. i never realised how much of a workout these things are, the guys on tv make it look sooo easy, its not.... i have a ton of practicing to do to even be semi decent but im willing and ready to learn... id currently describe my riding style as semi controlled sloppy drunkin off roading.... with a touch of scardis hillieitis....

oh ya did i metion we have hills here that i bearly make up on quads and i was nailing them on the bike fishtailing the whole way !!!!!

DAMN AM I EXCITED !!!!!!! WOO HOO !!!!! I think im going to run around the block now and chew on a cinder block to come down, lol !!!!!

You don't sound excited you little chatter box..... Glad to hear the good news. Now get out there and get some dirt!!!

Glad you got it sorted out! Usually when I ride at different elevations, the first thing I try adjusting is that air screw. It will help with throttle response right off the bottom end. So if one day it's running great and the next it isn't, try adjusting that. Just keep the stock numbers so you know where to go back to.

There are a couple different things you can do to make the bike more fun on trails. You could try a different expansion chamber. FMF makes ones for more mid range, or low end torque compared to the stock one. If you try that, get their jetting recommendations for that specific pipe.

Another thing you could try would be a heavier flywheel. My Dad did that to his 91 CR250 and it made it more trackable in tight trails.

Have fun!