Two-Stroke dirtbikes question
 

so I bought my first two stroke - an RM 250 twostroke that was blown up, and rebuilt it just for fun.

Runs/idles good, but the power delivery is not consistent at all, particularly at partial throttle. Is this just the nature of the two stroke bikes or do I need to play with the jetting? It's basically slightly richer than stock jetting that I believe the previous owner richened up after he put the aftermarket exhaust on.

Carb is clean and freshly rebuilt, brand new throttle cable so it's not hanging up at all.

Any thoughts? Just got done with break-in so trying to figure it out

First thing to do with any 2 stroke is put a new plug in it. JMHO

Brand new plug and wire. Gapped to OEM specs

u got an old school motorcross bike
they we made for wide open throttle at high rpms
very doggish rolling throttle loading motor,,,,,,
when they get to the powerband u hold on,,,,lol
I still have my old honda cr250 sitting in my garage from when I used to motorcross
till I broke my collar bone on it ,,,,so sat it up in the corner for time out been there since

That is my opinion and I am sticking with it.

My first dirt bike was in 1960

Could be you so you will need someone that nows dirt bikes to drive it and see if an issue or not.

2 strokes have an extremely narrow power band. Especially motocross engines. If you are in the wrong gear it ill be a dog.
Correct gear and you punch it...hold on and keep shifting :)

Pretend you have a 302cid smallblock with 270/280 at .050" cam and 102 LSA. :)

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woo hoo

I like the 69 Z28 # matching 350 car.............:popcorn:

I owned an RM250 for several years.
Mine ran just fine at part throttle. A 250 should pull well even if it's not in the ideal gear. Not like a 125 that will fall on its face if you're a gear to high.

I'd replace the reeds. Also, check to see if the power valve is carboned up.

Reeds are between the carb and jug.
power valve is in the jug in the exhaust port.

Haha ! Bringing out the crazy frog Axle huh ? The original :
https://www.youtube.com/watch?v=Vju2pyqXBLg&feature=youtu.be

Also, I always re-ringed and replaced the piston in the engine every two years. Big difference when a fresh set of rings and piston went in.

You didn't say how old it is. The real early ones did not have power valves and were lightswitches (pre-1983 or about if my memory serves me). If newer with power valve, check the linkage and take the cover off and spray with carb cleaner. Try to turn in over by hand on the kick starter (with plug wire off). It should be almost impossible to get past TDC or really hard to do. If not rings at least. We would re-ring every 20 hours and pistons and rings every 60 hours on our race bikes. A 250 should be a rocket - able to wheely in any gear up and into 4th. I raced from the early 80's thru 2008. Broke more than my share of bones!

Eric, take notes:

YZ490 here.
Fookin crazy chit.

with the wacky rear swing arm shock setup?
Those were like light switches!
Friend had an old '77' or so YZ490

My first bike was a '78' RM80.
Bought it for 50 bucks in milk crates and boxes when my parents were away on a cruise and my aunt was watching us.
The old man was less than happy!
I put it together myself (old boy didn't think I could) and terrorized the neighborhood
for years on that thing.

YZ490, that was my last dirt bike. At 5'4" 165# that thing was CRAZY. Biggest problem was starting from atop a milk crate (and keeping front wheel on the ground). The seat was 4' off the ground, just like the front wheel when riding.

ok....so it was just rebuilt?? and a new plug doesnt help...
before you take anything apart, do a leakdown test, on a 2 stroke if you have the slightest airleak anything else you do is going to be chasing your tail
next make sure the powervalve is not gummed up and the airfilter is clean and properly oiled.
once you have done that, you can either mount an egt or become good at plug chops. once you get used to it you can get pretty close by ear.

Is it stock?? If some bubba with a dremel decided to port it, it may not run for chit no matter what you do. a well built and tuned power valved 2 stroke 250 should pull fairly strong all the way thru.

Got a CR 250 that I've made a pretty good woods bike out of. That thing spooled up to quickly for me so I put a Stealthy flywheel weight on it and the biggest rear sprocket i could find. Now it has smoother low end with good midrange punch. Helped tame the quirky throttle down a bit to.

Alright - I omitted some details.

Its a 2000 RM250. I rebuilt it from the ground up - pulled the motor down to bare cases and replaced everything -bearings, tranny bearings, seals, piston, rings, etc...... powervalve was completely dissassembled and put back together spotless. the only thing I didn't replace is the reeds, but you can't even see a flashlight through them so I don't see that as the issue.

Ill ride it it a couple times tomorrow and better understand it. It just honestly feels like it's missing, I guess I've just never ridden a two stroke so I don't know - my fourwheeler is 4-stroke.

Even without a power valve, way back we had Harry Klemm build a 250r motor for baja. all low and mid power to pull tall gears, by 7500 rpm it was done, but hitting 6th gear was like making the jump to light speed. bonus points as it was an easy motor to ride, very smooth, didnt wear you out and ran forever.

Check jetting 1st. Make sure your gas mixture is about 40:1 upto 20:1. Your jetting will change with this. A simple way to check the 1st step of jetting is to get in 3rd gear on a long smooth section of road. Hold the throttle at 1/3 opening and see what it does after a few seconds. But keep it there longer. If it starts to surge your probably lean. If it sound very high pitch and feels wrung out. It's very lean. If it's deep sounding and doesn't instantly respond to more throttle it's rich. At this point I would check the needle valve and see where the clip is. Raising the needle/lower the clip with richer the midrange. Lower the needle will lean it out. If it's better but not good. You can go up or down on the main jet. If it idle and responds off idle you pilot jet is close. All the jets are marked. Bigger the #. Richer/larger jet. Hope this helps.

plug the exhaust port, get a pvc fitting that fits the intake manifold with a tube, then a low pressure gauge then a shrader valve.. pressurize it to about 5-6 psi. it should hold that for at least 5 minutes. If it doesnt. use a squirt bottle and soapy water to find the leak. Air leaks are the enemy of 2 strokes and they are easy to get. If you have an air leak you will never get the jetting right and you will melt it down.

When jetting, get the main jet pretty much spot on, but run the needle a clip rich.. A ,lot of people seize a motor and blame the main jet, but the truth is the main jet was good, the needle was ahair lean. You run it long and hard, then back off, if the needle is the least bit lean, the minute you back off the temp spikes and it sticks.

When I rebuilt the carb and was reassembling I noticed they were running the needle either up or down one notch (5 setting with number 3 being the middle it was off one notch). I think I will start with that and see where that is set.....

I raced for a long time and worked at a bike shop for about 7 years was always tuning bikes, you can call the pipe manufacture and they will give the jetting specs to you, id start there and then set the needle in the middle clip position and set the low speed air screw usually set to 1 1/2 turns out when first starting to tune it. you can read the plug by running wide open and shutting it down the lower the clip position will give it more fuel and vice versa for less fuel. two strokes like to be wide open and blown out often i always ran motoil and mixed 32 to 1

What year is it? Are you coming from a 4 stroke? "Partial" throttle on a 4 stroke vs a 2 stroke is a big difference. 1/4 throttle on a 4 stroke just purrs, 1/4 throttle on a 2 stroke just bucks and farts...

Run it about 1/2 throttle from mid to high RPM, the powerband should consistantly pull you into higher RPM.

Was it ported? If somebody got there hands on it and did more of top end porting that could make it even worse.

Take it out, run the pizz out of it like you stole it and report back... Then we can properly diagnose.

The newer style carbs with the needle clip position is probably your best bet. The owner's manual should tell you how to do this. Back when I was riding this was almost a weekly adjustment. Have fun!

I used to ride an old CZ. you had to wind it up just to get it rolling..........but when it hit the power band you better be holding on.

It's probably carb jetting. Make sure you have the stock jets, etc, and go from there.

If everything was done correctly mechanically, i would say you're looking at jetting issues. Does it start and run at low rpm ok? Something as simple as you're float height could be too high. Pull all of your jets out and see what they are.

Unfortunately I didn't get to mess with it today. Actually spent the day riding 4-wheeelers with the kids. Will probably give the neighbors the next two days off since it's Christmas Eve and Day - the FMF is loud as ****. Makes the boat sound like a Prius!

Jetting is essentially stock. I recall putting them in that either the pilot or main jet was richened up "2". I think I need to go back and play with the needle. Again, it was adjusted off the middle position and now I can't recall if I left it like that or returned it to the middle clip position.

Also - anyone see anything in this chart that doesn't make sense?

https://www.fixyourdirtbike.com/fix/...e-carb-tuning/

seemse like Ike a nice guide for the newbie.....


Somewhat of a new hobby for us so we are feeling it out. Naturally my 4 and 6 year olds love it.

Or throw something like an optak (egt) on it and take all of the guess work out of it.

Clip was curiously up a notch from stock (needle down) which doesn't make sense to me since it has an aftermarket FMF pipe on it. I figured it would need to be richened up a little given the hopefully better flow of an aftermarket pipe....

ill probably try try it Saturday since the weather was way colder here than I would ever run the bike in.

That carb also has a Power Jet. Its an electric solenoid that opens up between 1/4 and 3/4 throttle for more mid-range pull. I cant say that i ever had one go bad but it's something else to check. Unfortunately, i don't remember if the PJ solenoids are normally open or closed. If its stuck open the bike would run rich from idle all the way up but if its stuck closed or the passage is blocked, the bike will have a big lean spot in the mid throttle region. I used to race a 96, 97, and 98 RM250 before switching to Hondas and KTMs. In 2000 Suzuki finally figured out their porting, that bike should rip.

on a PJ? isnt a pj just a basic oval slide?? (ive been out of bikes since 05) though I liked pwk's over the pj's

Yes, it has an enrichment solenoid that operates from 3000-8000 rpm if I remember correctly. I didnt test it, but if this doesn't work maybe I'll pull that off and check it. I think it's an easy on/off 12v

Once upon a time long long ago a CZ taught me how to ride 2 strokes. Moved to Spanish stuff later, OSSA. The lurking monster was MAICO.

Did the piston have numbers on it/was it bored? Was it ported? So many things could have been done to a 18 year old bike.

If you really think its missing check the gap on the mag/flywheel.

New piston. Bone stock, put in the proper way with the arrow facing the exhaust port.

I miss my two strokes. I loved my big bore ktm 550 until it bucked me about 5 times climbing a hill one saturday and I had to get a more tame 380.
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