250r jetting question

[mattpiston]

i just bought an 86 250r and i was woundering if my jets where to big...the engine is ported and polished +4mm stroker hot rods, 37mm keihin carborator,fmf fatty, i think the head was milled by sparks also...it says spark on the cylinder but im not sure....but any way i was woundering what would be a good jet size for this? it has a 142 in it right now but i think thats kinda small but when i put a 158 in it, it runs good untill it revs up high then it startes cutting out and getting choppy. i adjusted my air and fuel screw but nothing seems to change. ....any sugestion?

[x.system]

What makes you think the 142 is to small? Have you done plug chops? Gap your plug at .018 and run the 158 and see if that clears up the top end. I would start with a new plug then check it after you do a run. Check out the jetting thread, lots of good info there.

[mattpiston]

well lol idk...there's a tech guy and he said i should be no smaller than 150 with my setup....he may not know what he is talking about? any reason why my 250r stops smoking when i put the bigger jet in??? it smokes with the 142 jet but stops when i put the 158 in.

[Yamahondaman]

If it say's "Spark's" .. it's a NASTY Motor !! i dunno why the Main is so small ?
Like x.system said ... gap at .018-.02 .. BR9ES..
also .... was that thing a MX Bike ? prolly so cause of the small carb.
the PWK 39 you will LIKE ! :naughty:

[mattpiston]

im not sure if it was an mx bike or not. the carb i have now is a round slide 37...i do alot of dune riding and trail riding....but not slow trail riding we ride hard lol. would the 39 pwk be *better suited for my engine? i was told to upgrade to a pwk but i didnt know if i would be able to tell a diffrence. yeah the motor is runing pretty strong...ran it on the dyno and it was 55.7hp but idk what jets he had in it....he said he pulled his jets out and stuck in these small ones to save on gas. but never would say what size they was.......

[x.system]

You don't change jets to save gas. He may have changed jets so you don't blow it up but who knows. Did you get a copy of the dyno output? I'd like to see it. Were you present for the dyno run? I would have been taking notes on jet changes. Ask him what he had in it and go from there. A PWK will be an improvement over that 37mm round slide? You will see improvements up top and is a good carb for dunes.

Depending on your elevation (I'm at 600ft) My 39 pwk is set up with a 45 pilot and 180 to 190 main for summer riding depending on temp, usually between 70 and 90 degree's. Winter riding I set it at 50 pilot and 225 main for 0 to 10 degree's.

The first thing you need to learn with a 2 stroke that has alot of motor work is how to jet and read plugs, otherwise you will be selling or rebuilding often. A ported motor needs more attention with the temp changes if you want it to stay running crisp, a stock motor you can usually jet it for winter and summer and be done if you don't care to get the full potential out of it.

[mattpiston]

yeah i was present for the dyno run but i didnt get a copy of it...i asked him what jets he used and it was a 160...he said them are the jets he ran at the dunes...but i put in the 160 and my engine cuts out on top end...runs good everywhere else though...how come it worked for him and not me lol.? maybe need a new plug? i might try to plug chop it this weekend and see what that tells me.

[x.system]

Its probably your plug gap, with these old electrical systems they get weak with age, thats why I said to close the gap to .018 and run it again. You will probably see it clear up the top end. Try it and let us know what your outcome is.

[mattpiston]

well i regaped my plug it still cuts out bad with the 168 jet in it. but runs good with the 145 in it..i put the 142 in it and plug came out white. put the 145 and was darker... put in the 152 and it was brown but cut out on top end...but puting in a new plug and gaping it i was able to run the 145. should i just go with a new carb?

[x.system]

What needle do you have and what position is the clip in? The bike starting up good? Can you get some pics of that carb? I've never seen a 37mm round slide so I'm wondering what you actually have on it.

[Yamahondaman]

Back in the day we used to Bore out the Oval 34mm Carb's to 36mm .. i tried to go to 37mm and it didn't work .. you could see daylight on each side of the slide .. the jetting was WAY OFF to compensate for the extra air seeping thru ..also i used to use the "86" Needle (1468) in the "88-89" Carb's that had the (1471).
it gave it more of a Lean mixture in the mid .. kinda like useing the larger cutaway in a diff. slide ... but as soon as those PWK'S came out... we NEVER used the round or Oval carb's again

[mattpiston]

do you think if i drop my needle down i would be able to run a bigger jet? or would that get me any where? yeah i will take some pictures of it when i take it off and post them on here.

[mattpiston]

well i just ordered a 39mm keihin pwk carb i hope its not to big...whats a rough guess on what my jetting should start at? 1,300 feet above sea level. and my motor specs are above just put in fmf torque reeds....my clutch isnt holding my power anymore so i bought a new clutch with carb also hope it holds lol!